Natural arsenic contamination in alluvial aquifers
of Chianan Plain, Taiwan
by
Brian Keith Woodall
A thesis submitted to the Graduate Faculty of
Auburn University
in partial fulfillment of the
requirements for the Degree of
Master of Science in Geology
Auburn, Alabama
August 3, 2013
Approved by
Ming-Kuo Lee, Chair, Professor of Geology
James Saunders, Professor of Geology
Charles E. Savrda, Professor of Geology
Lorraine W. Wolf, Professor of Geophysics
ii
Abstract
Geological and geochemical analyses were performed on groundwater and
sediment samples collected from Budai and Yichu deep drilling sites in the Chianan Plain
of Taiwan to examine the distribution and mobilization of arsenic in alluvial aquifers.
The Chianan Plain is an area historically known for the unique occurrence of As-related
endemic cases of Blackfoot disease (BFD). Groundwater at these drilling sites is mainly
HCO3-Na-Cl or Na-Cl type and contains high levels of Sr and Br, indicating the influence
of saltwater intrusion. However, the concentrations of As, Mn, Fe, Ba, and Si are
significantly higher than those in seawater, implying that possible sources of As are either
the reductive dissolution of Fe- and Mn-oxyhydroxides or chemical breakdown of silicate
minerals such as biotite. Lower concentrations of PO43-, NO22-, and NO3- (<1 mg/kg) in
groundwater suggest that ionic competition of nitrate and phosphate for sorbing sites
likely is not the major mechanism for As mobilization. Sediment arsenic concentrations
exhibit large depth dependence and range from 1.9 to 32.3 mg/kg and 1.7 to 23 mg/kg at
the Budai and Yichu sites respectively. Arsenic levels in these sediments are notably
higher than those in Bangladesh and West Bengal (generally < 10 mg/kg). Surface
hydrologic transport modeling suggests that arsenic derived from dewatering fluids from
mud volcanoes or weathering of potential source rocks in the headwaters may be routed
through stream-channel networks and deposited in the Chianan Plain. Results of grain
size and total organic carbon analyses indicate that high levels of arsenic are likely
iii
associated with fine-grained (silt and clay fraction) sediments and organic matter.
Sequential extraction analysis reveals that Mn- and Fe-hydroxides and organic matter are
the major leachable solid phases that host arsenic. More than 60% of arsenic is
incorporated in silicates and other recalcitrant solid phases. The poor correlation between
As and Fe in Na3P2O7-extracted fractions suggest that As may be associated in part with
organic matter or humic substances in addition to Fe-bearing phases in sediments.
Results of sediment bulk geochemistry and sequential leaching analysis indicate that
dissolved As loads in groundwater represent a relatively small fraction of total As
sources in sediments. The results of aquifer flushing models indicate that most initial
mobile arsenic in a considerable part of alluvial aquifers, without new inputs from
sediments, may be flushed out in a few thousand years. The modeling results are
consistent with the young ages (<10,000 years) of most As-rich groundwater estimated
for Holocene floodplain aquifers in Bangladesh and Taiwan.
iv
Acknowledgments
I would first like to thank Dr. Ming-Kuo Lee for welcoming me to his homeland
of Taiwan and their rich culture. It has been one of the greatest personal and professional
experiences of my life to work with Dr. Lee on this project. This thesis would not have
been feasible without Dr. Lee?s vast knowledge, willingness to help and endless patience.
I also want to thank my wife Jessica Woodall, my mother Nancy Pressley, and children
Abby and Alex. They all made huge sacrifices so that this would be possible.
v
Table of Contents
Abstract .......................................................................................................................... ii
Acknowledgments ......................................................................................................... iv
List of Tables............................................................................................................... viii
List of Figures ............................................................................................................... ix
Introduction .................................................................................................................... 1
Background and Geologic Settings ................................................................................. 5
The Coastal Range ............................................................................................. 7
The Central Range ............................................................................................. 7
The Western Foothills ........................................................................................ 8
The Coastal Plain ............................................................................................... 8
The Chianan Plain ............................................................................................ 10
Mud Volcanoes in Southwest Taiwan ............................................................... 13
Previous Work on As Geochemistry ................................................................. 15
Arsenic Occurrence and Speciation ............................................................ 15
Arsenic Mobility ........................................................................................ 17
Mechanisms of As Release ......................................................................... 17
Methods ....................................................................................................................... 19
Drilling ............................................................................................................. 19
Grain-Size Analysis of Sediments ..................................................................... 22
vi
Compositional Analysis .................................................................................... 22
Bulk-Geochemical Analysis of Core Sediments ................................................ 23
Total Organic Carbon Analysis ......................................................................... 23
Sequential Leaching of Arsenic-Hosting Solids ................................................. 24
Groundwater Geochemistry ............................................................................... 25
Overland Flow Model ....................................................................................... 25
Aquifer Flushing Model .................................................................................... 26
Results and Discussion ................................................................................................ 28
Chemical and Physical Characteristics of Sediments ......................................... 28
Sediment Color ........................................................................................... 28
Grain-Size Analysis ..................................................................................... 30
Mineralogy and Petrology ........................................................................... 32
Sediment Geochemistry ............................................................................... 34
Total Organic Carbon .................................................................................. 48
Sequential Extraction .................................................................................. 52
Groundwater Chemistry and Hydrogeology ....................................................... 58
Groundwater Geochemistry ......................................................................... 58
Overland Flow and Transport Model ........................................................... 63
Aquifer-Flushing Model .............................................................................. 66
Conclusions ................................................................................................................. 69
References ................................................................................................................... 72
Appendix 1 .................................................................................................................. 85
Appendix 2 .................................................................................................................. 86
vii
Appendix 3 .................................................................................................................. 87
viii
List of Tables
Table 1. Data from the grain-size analysis of Yichu and Budai sediments .................... 31
Table 2. Composition of major and trace elements in Budai sediment Samples from
southwest Taiwan ................................................................................................... 35
Table 3. Composition of major and trace elements in Yichu sediment Samples from
southwest Taiwan ................................................................................................... 37
Table 4. Correlation coefficients for Yichu grain size, TOC, and elemental analysis .... 40
Table 5. Correlation coefficients for Budai grain size, TOC, and elemental analysis .... 43
Table 6. Total organic carbon (TOC) in Budai sediments ............................................. 49
Table 7. Total organic carbon (TOC) in Yichu Sediments ............................................ 50
Table 8. Compositions of selected ions and pH-ORP values of Budai groundwater ....... 59
ix
List of Figures
Figure 1. Geologic map showing the four major geological provinces of Taiwan ........... 6
Figure 2. West to east cross section of Coastal Plain ...................................................... 9
Figure 3. Generalized geologic map showing location of Chianan Plain ........................ 11
Figure 4. Geologic map of Taiwan ............................................................................... 12
Figure 5. Redox-pH diagrams for arsenic calculated at 25?C and fixed As and SO4 2-
activities of 10-2 ...................................................................................................... 16
Figure 6. Map showing locations of the Budai and Yichu drilling sites in Chianan Plain,
southwest Taiwan ................................................................................................... 20
Figure 7. Photograph of nested peizometers installed at the Budai drilling site ............. 21
Figure 8. Simplified stratigraphic columns from Chianan Plain showing sand, silt, clay
and sediment color .................................................................................................. 29
Figure 9. Representative photomicrographs of coarse-grained sediments (fine sand)
from Chianan Plain ................................................................................................ 33
Figure 10. Plot comparing Budai and Yichu stratigraphic columns and showing general
positive correlation of clay and silt with As ............................................................ 39
Figure 11. Line charts showing As correlation with Fe and Mn at both well sites ........ 47
Figure 12. Total organic carbon (TOC) correlated with As in Yichu and Budai well sites
................................................................................................................................ 51
Figure 13. Graph showing variations of leachable As concentrations in different host
materials in the Budai well ..................................................................................... 53
Figure 14. Graph showing variations of As fraction (including both leachable and non-
leachable form) associated with different host materials in the Budai well ............. 54
Figure 15. Graph showing variations of leachable Fe concentrations in different host
materials in Budai well .......................................................................................... 55
x
Figure 16. Graph showing variations of Fe fraction (including both leachable and non-
leachable form) associated with different host materials in the Budai well ............. 56
Figure 17. Piper diagram illustrating the main hydro-chemical facies of Chianan Plain
groundwater ........................................................................................................... 60
Figure 18. ?D and ?18O ratios of Chianan Plain groundwater plotted along with seawater,
mud volcano fluids, and local meteoric water line (Wang et al., 2001) .................... 62
Figure 19. Overland transport model for the Chianan Plain and Pingtung Plain watersheds
................................................................................................................................ 65
Figure 20. Aquifer flushing model showing breakthrough curve for As at 50, 1000, 2000,
and 4000 years ....................................................................................................... 68
1
INTRODUCTION
Natural As contamination of groundwater occurs around the world by a variety of
biogeochemical processes (e.g., Welch et al., 2000; Smedley and Kinniburgh 2002;
Saunders et al., 2005). Many hypotheses have been proposed for the sources of As in
alluvial sediments. The most widely accepted model is the bacterial reductive dissolution
of As-sorbed hydrous ferric oxides (HFO) in association with the breakdown of natural
organic matter in river-floodplain alluvium (e.g., Nickson et al., 1998, 2000; McArthur et
al., 2004; Islam et al., 2004). In the early 1990?s, it became apparent that natural arsenic
contamination of groundwater in young (Holocene) alluvial floodplain aquifers was an
important health problem, particularly in southeast Asia (e.g., Chatterjee et al., 1995;
Nickson et al., 1998; Archaryya et al., 1999, 2000; Ahmed et al., 2004; Polya et al., 2008;
Saunders et al., 2008). Elevated concentrations of arsenic in groundwater also have been
reported in many other areas of the world, including Europe, Africa, South America, as
well as states of the former Soviet Union (Bundschuh et al., 2009). Global estimates of
the number of people exposed to elevated levels of arsenic range from 125 to 150 million
(Bhattacharya et al., 2002). Arsenic is a suspected carcinogen and listed as a hazardous
material (National Research Council et al., 1999). Around the world, people are using
As-tainted groundwater for drinking, cooking, and irrigation. Groundwater used in the
aquaculture business also can result in high As concentrations that bioaccumulate in
certain fish such as tilapia (Han et al., 1998). Bioaccumulation in rice irrigated with As-
2
contaminated groundwater can occur as well (Bhattachary et al., 2012). Arsenic can
affect unborn children by crossing the placental membrane (Karim et al., 1999). Many
human diseases are linked to arsenic in groundwater. These diseases include arsenical
skin lesions, arsenic neuropathy, increased rates of spontaneous abortions, preterm birth,
stillbirth, low birth weight (Chakraborti et al., 2004), and cancers of the nasal cavity,
lung, liver, bladder, kidney and prostate (Wu et al., 1989). In areas of northeastern
Taiwan, erectile dysfunction has been found to be prevalent in men who have consumed
As contaminated groundwater (Hsieh et al., 2008). The carcinogenic effects of arsenic
pose a major health hazard that needs to be addressed in many areas around the world.
This project, completed in collaboration with Taiwanese colleagues at the National
Cheng Kung University (NCKU), addresses As in sediments and groundwater in the
Chianan Plain of southwest Taiwan. With funding provided by the National Research
Council of Taiwan, two deep boreholes were drilled in Budai and Yichu townships of the
Chianan Plain in 2008 and 2009. The two drilling sites are located in the area recognized
as the home of the world's only well-documented Blackfoot disease (BFD) occurrence
(Kao et al., 1954), where extremely high concentrations of arsenic (hundreds of ppb)
have been found in the artesian wells used for drinking-water supply. The island of
Taiwan is characterized by very high rates of tectonic uplift (up to 10 mm/yr) (Shin and
Teng 2001), rapid erosion (2-8 mm/yr) (Fuller et al., 2003), high riverine transport of
organic carbon (Lyons et al., 2002; Carey et al., 2006), frequent seismic activity (Ng et
al., 2009), and intense precipitation events (average four typhoons per year) (Wu et al.,
2005). Because of this unique high-standing island setting, transport and mobilization of
3
As in the Chianan Plain may be affected by a variety of tectonic, hydrologic, weathering
and biogeochemical processes.
Certain iron-bearing minerals (Fe oxides, silicates, sulfides) and organic matter
can contain high levels of natural-occurring arsenic, which can be mobilized by either
chemical weathering or biogeochemical processes (e.g., bacterial Fe reduction). The
source of arsenic contamination in the Chianan Plain may come from the weathering of
iron-rich silicate minerals such as biotite (e.g., Saunders et al., 2005). Active tectonic
uplift of the Taiwan Central Mountain Range can enhance mechanical weathering of As-
rich bedrock and set the stage for chemical weathering and release of arsenic to surface
waters. Arsenic is strongly adsorbed by Fe- and Mn-oxides in stream sediments under
oxidizing conditions. Migration of river channels leads to deposition of As-sorbing Fe-
oxides along with organic matter in alluvial plains. Subsequently, anaerobic conditions
may develop in alluvial aquifers as bacteria utilize organic matter to grow. Specifically,
Fe-reducing bacteria can cause the reductive dissolution of As-bearing Fe- or Mn-oxides,
causing As release to groundwater.
Other possible As sources, such as mud volcanoes in southwestern Taiwan, have
released arsenic-rich mud associated with their eruptions (Lewis et al., 2007). Changes in
pH associated with the intrusion of seawater (high pH) in shallow alluvial aquifers also
may lead to the desorption of As from surfaces of hydrous oxides.
This study aims to test various hypotheses on the sources, mobilization, and
transport of As in the environment. Experiments have been conducted to: (1) characterize
the vertical variation of sediment geochemistry, (2) characterize the relationships among
various elevated concentrations of trace elements, organic matter (TOC content), and
4
grain size in alluvial sediments, (3) quantify the concentration of As bound to various
hosts mineral phases (clay, carbonates, Fe- and Mn oxides, silicate minerals) and organic
matter via bulk chemical analysis and sequential leaching experiments, and (4) simulate
aquifer flushing and stream hydrologic transport processes and arsenic mobility in
groundwater aquifers. The overarching objective of this study is to assess the possible
sources, transport, distribution and accumulation of arsenic in the Chianan Plain. This
research, supported by a full suite of geochemistry/mineralogy analyses as well as
hydrologic transport modeling, provides a wealth of observational, experimental and
modeling data on the sources and reactive transport of As in alluvial aquifers in Taiwan.
5
BACKGROUND AND GEOLOGIC SETTINGS
Taiwan is a mountainous island that formed from the collision of the Luzon
Volcanic arc of the Philippine Sea plate with the continental margin of the Eurasian plate
(Lallemand and Tsien 1997). The island?s formation began around 6.5 Ma when the
continuing subduction of the oceanic crust of the South China Sea led to the collision of
the Eurasian continental margin with the Luzon Arc (Lin et al., 2003). South of Taiwan,
this plate boundary leaves the continental shelf and runs into the South China Sea where
oceanic crust subducts under the Philippine Sea plate along the Manila trench. East of the
trench, the outer non-volcanic arc expands tremendously (Suppe and Wittke 1977),
forming the backbone of the island. Taiwan?s tectonic and orogenic history, along with
subsequent erosion and sediment deposition, has led to the formation of four major
geological provinces: the Coastal Range; the Central Range; the Western Foothills; and
the Coastal Plain (Figure 1).
6
Figure 1. Geologic map showing the four major geological provinces of Taiwan (Shin
and Teng 2001).
7
The Coastal Range
The Coastal Range, which forms the easternmost part of Taiwan, is characterized
by the abundant volcanic derivatives, poorly sorted volcaniclastic sediments, turbiditic
clastic rocks and chaotic melange. The Coastal Range is separated from the Central
Range by the Taitung Longitudinal Valley. The Taitung Longitudinal Valley is an
important tectonic feature and is considered an independent sub-province. The Coastal
range, about 135 km long, averaging 6 km in width, is the remnant of a westward-facing
Neogene island arc the Luzon Island Arc on the leading edge of the Philippine Sea
plate. The Coastal Range is thus considered the northern continuation of the Luzon Island
Arc and the Luzon Trough to the south (Biq 1972).
The Central Range
The Central Range forms the backbone ridge of the island and rises over 3,000 m.
This region includes the Paleogene sub-metamorphic rocks and the pre-Paleogene
metamorphic complex east of a major boundary fault, the Chuchih fault, which further to
the south is called the Laonungchi or Chaochow fault. The Central Range is bounded on
the east by the Eastern Longitudinal Valley. This geologic province can be subdivided
geologically into two sub-provinces, eastern and western. The western Central Range belt
is a broad Paleogene sub-metamorphic belt that is exposed along the western flank, the
crest zone, and the southern part of the Central Range. The lithologies of the western
central range are slates, phyllites and argillites. Some of the arsenic-bearing minerals
associated with these metamorphic rocks are considered as the ultimate source of arsenic
in the Chianan Coastal Plain (Smedley and Kinniburgh 2013). The eastern Central Range
belt is underlain by a pre-Paleogene metamorphic complex exposed largely on the eastern
8
flank of the Central Range. Rocks exposed in this area are mainly gneiss, marble,
amphibolite and schist (Chi et al., 1981).
The Western Foothills
The western foothills province is characterized by a thick (? 8,000 m) sequence of
Paloeogene, Neogene and Quaternary sediments. The rocks are mainly sandstones and
shales with locally interspersed lenses of limestone, tuff, coal and peat. The western
foothills are the site of a Late Cenozoic sedimentary basin west of the Central Range.
Recent paleontologic studies indicate that sedimentation in this basin began in Oligocene
time and continued during early Pleistocene. A major orogenesis began during early
Pleistocene and all the sedimentary units in this western portion of Taiwan were folded
and faulted to a series of mountains and rolling hills flanking the western margin of the
Central Range. The Chuchih fault is a tectonic line that separates the western foothills
from the upthrust argillite-slate series of the Central Range to the east. The western
foothills gradually merge westward into the tablelands and coastal plains bordering the
Taiwan Strait.
The Coastal Plain
The sediments of the Coastal Plain are dominated by alluvial gravel, sand and
clay. The physical character of Taiwan changes dramatically from the foothills zone to
the alluvial plain. The source of all major rivers running through the plains is in the high
mountains. Flowing out of the western foothills, these rivers diverge into a number of
channels and meander to the ocean, forming large alluvial deltas. Groundwater flow also
mimics this trend of east to westward flow through several main aquifers (Figure 2). The
western edge of the plain, where it meets the Taiwan Strait, is marked by wide tidal flats
9
Figure 2. West to east cross section of Coastal Plain (Wang et al., 2007). Groundwater
generally moves westward through main aquifer units confined by clay- and silt-rich
sediments (gray).
10
and swamps. Shore currents have built up a series of spits and offshore bars, with many
lagoons formed by shoreward shifting of the sandbars (Huh et al., 2011).
The Chianan Plain
In southwestern portion of the Coastal Plain, there is a 2400 km? area known as
the Chianan Plain, which hosts the most As-contaminated area in Taiwan. This area is the
focus of this study (Figure 3). The Chianan Coastal Plain is located in a tectonically
active, subsiding depression created at the foreland of an actively uplifting mountain belt
to the east (Nath et al., 2008). The Chianan Plain is bounded by the Erngen River in the
south and the Peikang River in the north. The east and west are bounded by the Western
Foothills and Taiwan Strait, respectively. The Chianan Plain extends 60 km north and
south and 40 km east and west. Two major rivers, the Tsengwen River and the Pachang
River, flow west to southwest across the Chianan Plain. Holocene sediments comprise the
upper 60 m of the Chianan plain (Figure 4). These overlie 60 to 300 m of Miocene to
Pleistocene sediments (Wang et al., 2007). Strata beneath the Chianan Plain are
composed of poorly sorted, alternating beds of fine-grained clastic sediments; i.e, fine
sand, silt and clay. The depositional environments for these clastics range from fluvial,
lagoonal, estuarine and shallow marine settings (Chen and Liu et al., 2007). Sediments in
the Chianan Plain were deposited at a relatively high rate (1 cm/yr) (Nath et al., 2008)
and the present ground surface slopes gently westward with a gradient of less than 3 m/10
km.
11
Figure 3. Generalized geologic map showing location of Chianan Plain (Fisher et al.,
2007).
Chianan
Plain
12
Figure 4. Geologic Map of Taiwan (British Geological Survey, 2001).
13
Many alluvial aquifers (mostly sands and gravels) in the Chianan Plain provide
freshwater for domestic and irrigation uses in Southwest Taiwan (Liu et al., 2006). Some
of these aquifers contain elevated levels of As (hundreds of ppb). Naturally occurring
high concentrations of dissolved As in the Chianan Plain alluvial aquifers caused serious
health problems (e.g., endemic Blackfoot disease and cancers) in the 1960?s (Tseng et al.,
1997). The As-contaminated wells are in Holocene alluvial aquifers. Many possible
primary sources of arsenic in the Chianan Plain have been proposed, including: (1)
chemical weathering of arsenic-rich minerals (biotite/arsenopyrite) in the metamorphic
rocks of the Central Range; (2) discharge of arsenic-rich fluids from dewatering of mud
volcanoes in southwestern Taiwan; (3) pH-induced As desorption associated with
saltwater intrusion; (4) weathering of black shale in the foothills of southwestern Taiwan;
and (5) weathering of As-bearing coal and peat beds of the foothills (Jean et al., 2009).
Mud Volcanoes in Southwest Taiwan
The collision the Eurasian plate with the Luzon Volcanic arc has resulted in an
accretionary prism. This accretionary prism is considered as the source of mud volcanoes
distributed in the southwestern part of Taiwan (Jean et al., 2007). Occurrences of at least
ten active mud volcanoes were reported in this area. Their erupted muds and fluids
frequently have high trace metal contents (Shih et al., 1967; Wang et al., 1988). Mud
volcanoes are located in tectonically active areas where mud and fluids are permitted to
transfer from deep geological layers to the Earth?s surface, forming a cone-shaped
landscape with a central vent. The formation of mud volcanoes is often linked to a
tectonic compression, dehydration of clay minerals, and rapid deposition of mass flows
such as slumps or turbidites (Kopf et al., 2002; Huguen et al., 2004). Mud-volcano fluids
14
in Taiwan are characterized by high Cl contents, suggesting a marine origin from actively
de-watering sedimentary pore waters along major structures on land (You et al., 2004).
None of previous studies have investigated the link between mud volcanic
activities and high As concentration in the downstream Chianan Plain. This study aims at
elucidating the possible linkage of mud volcanic activity to As-enriched sediments in the
Chianan Plain by tracking overland flow and transport processes.
15
Previous Work on As Geochemistry
Arsenic Occurrence and Speciation
Arsenic is the 20th most common element in the earth?s crust, and is commonly
associated with igneous and sedimentary rocks, particularly sulfidic ores. Arsenic-bearing
compounds are found in rock, soil, water and air as well as in plant and animal tissues.
Although elemental arsenic is not soluble in water, arsenic salts exhibit a wide range of
solubilities depending on pH and the geochemical environment. Arsenic can exist in four
valency states: ?3, 0, +3 and +5. Under reducing conditions, the +3 valency state as
arsenite (i.e., As(III)) is the dominant chemical; the +5 valency state as arsenate (i.e.,
As(V)) is generally the more stable form in oxidizing environments (Boyle and Jonasson
et al., 1973). The most important factors controlling As speciation and mobility are redox
potential (Eh), pH, and Fe activity. Arsenic will transform into different aqueous or solid
phases under varying Eh-pH conditions (Lee et al., 2005; Root et al., 2007; Nath et al.,
2008). Figure 5 shows the phase diagram for As speciation in the presence of sulfur. The
diagram was generated using the ACT2 sub-program of Geochemist?s Workbench
(GWB). Under oxidizing conditions, the As(V) species H2AsO4- and H3AsO4 are
dominant at relatively low pH (< 7), while HAsO42- and AsO43- become dominant at
neutral or higher pH (Figure 5). Under weakly reducing conditions, As(III) species
As(OH)3 is predominate over a wide range of pH values (Yang et al., 2006). Under
highly reducing conditions, solid arsenic sulfides (orpiment) or thioarsenite
16
Figure 5. Redox-pH diagrams for arsenic calculated at 25?C and fixed As and SO42-
activities of 10-2. Dashed lines show stability limits of water at 1 bar pressure. Realgar,
As(SH)4-, As(OH)2(SH), and As(OH)22- are suppressed (not considered in the
calculation).
17
aqueous complexes become the dominant phases in S-rich systems (Figure 5). Most
Holocene alluvial aquifers in Taiwan and Bangladesh are under moderately reducing
conditions, implying that As(III) is the dominant species in these groundwaters (Chen et
al., 1994).
Arsenic Mobility
As(III) is more mobile than As(V) because H3AsO3 is a non-ionic neutral species.
By contrast, As(V) exists as anions that can be strongly adsorbed by minerals with
positive mineral surface charges. Under neutral pH and oxidized conditions, As is
strongly sorbed by Fe(III) and Mn(IV) oxyhydroxides. However, significant desorption
of As(V) occurs as pH increases to about 8.5 when anions of As(V) are expelled by iron
oxides with negative surface charge (Lee et al., 2005). Under Fe-reducing conditions, As
can be mobilized by bacterial reduction or dissolution of Fe(III) and Mn(IV)
oxyhydroxides. In very reducing conditions, sulfate-reducing bacteria can immobilize
dissolved As by incorporating it into Fe-sulfide solids (amorphous and crystalline
sulfides) by adsorption or co-precipitation (Saunders et al., 2008).
Mechanisms of As Release
The main natural geologic sources of As in groundwater are: (1) oxidation of
metal sulfide solids (e.g., pyrite or other metal sulfides); (2) weathering of As-bearing
silicate minerals (e.g., biotite); and (3) bacterial reduction and dissolution of Fe(III) and
Mn(IV) hydroxides. There is now a general consensus that biogenic reductive dissolution
of arsenic-bearing Fe(III) and Mn(IV) oxyhydroxides is the principal mechanism leading
to arsenic release in groundwaters hosted by Holocene river flood-plain deposits in such
places as Bangladesh and West Bengal (Zheng et al., 2004). Recent research also has
18
shown that the presence of organic matter and inorganic solutes (e.g., phosphates,
bicarbonate, and silicate) in groundwater can compete with As for limited sorption sites
on Fe oxides, further leading to As mobilization (Acharyya et al., 1999). Moreover, the
intrusion of seawater in shallow alluvial aquifers also may lead to the desorption of As
from surfaces of Fe- and Mn-hydrous oxides due to pH effects and ionic competition for
HFO-sorbing sites. All of these mechanisms have been proposed to account for the
elevated As contents in the Chianan Plain groundwater (Polizzotto et al., 2005; Nath et
al., 2008).
19
METHODOLOGY
Drilling
Two boreholes were drilled (funded by the National Research Council of Taiwan)
in the Budai and Yichu townships of Chianan Plain for collection and analysis of solid
aquifer materials and groundwater. The two drilling sites are located near the worst
Blackfoot disease (BFD) affected areas (Figure 6). The length of Budai and Yichu drilled
cores are about 150 and 200 m, respectively. The semi-continuous sediment core samples
of both wells were collected by a split-spoon sampler with a rotary core-drilling system.
In this drilling method, steel augers penetrate unconsolidated silts, sands and gravels. A
sampling device is passed through the hollow center of the steel augers and is pounded
into the undisturbed samples below the lead auger. Once extracted, the hollow tube is
split and sediment is retained inside the split spoon. Samples that needed to be stored
under anaerobic conditions were immediately transferred into a portable glove bag for
retrieval and then heat-sealed in pouches under argon for transport back to the laboratory.
Ten nested peizometers were installed near each of the boreholes for collecting
groundwater from different depths (Figure 7). Water samples were collected at depths of
59, 101, and 145 m from the Budai well site.
20
Figure 6. Map showing locations of the Budai (green triangle) and Yichu (red triangle)
drilling sites in Chianan Plain, southwest Taiwan. Also shown are groundwater
monitoring wells (orange dots) used in this study.
21
Figure 7. Photograph of nested piezometers installed at the Budai drilling site. Ten
stainless steel piezometers are screened at depths of 13, 30, 45, 59, 77.5, 90, 101, 116,
129, and 145 m.
22
Grain-Size Analysis of Sediments
Sediment samples from both drilling sites were selected at 5-m intervals for grain-
size analysis. Percent sand, silt and clay fractions were determined using a combination
of wet sieve and pipette techniques. Samples were dried in oven for 24 hours at 80?C.
Once dried, between 5 and 6 grams of the sediment sample was placed in a 50-ml beaker
with distilled water and 30 ml of dispersant solution. After disaggregation, the sand
fraction was removed by wet sieving the sample through a 4? sieve using distilled water.
The sand fraction was then transferred to a pre-weighed beaker, dried at 80?C for 24
hours, and then weighed. During wet-sieving, the finer-than-sand fraction (mud) was
transferred to a 1000-ml graduated cylinder. The volume of mud suspensate was brought
to 1000 ml via addition of distilled water. The suspensate was then agitated thoroughly
for 1 minute and, following standard pipette techniques, 20-ml aliquots were drawn at 20-
cm depth after 20 seconds and at 10-cm depth after 2 hours 3 minutes. Dry weights of the
20-second aliquot were used to determine total weight of silt + clay in each sample. Dry
weights of the 2 hour 3 minute aliquot were used to determine the relative contributions
of clay.
Compositional Analysis
Well-cutting samples were chosen from intervals where As- and Fe-rich
sediments occur. Coarse-grained sediments were used to produce grain-mount thin
sections, which were examined under petrographic microscope to identify major mineral
phases and to search for authigenic minerals precipitated from As-, Fe-, and Mn-rich
groundwater. The latter minerals may serve as the sinks for trace metals in alluvial
sediments.
23
Fine-grained portions of sediments were subjected to x-ray diffraction analysis by
our collaborator Dr. Zhaohui Li (University of Wisconsin, Parkside). Oriented samples
were prepared by depositing clay-sized suspension onto glass slides and dried naturally.
Analyses were performed using a Rigaku Powder Diffractometer with Ni-filtered Cu Ku
radiation at 35 kV and 20 mA.
Bulk Geochemical Analysis of Core Sediments
Core sediments recovered from both Budai and Yichu drilling sites were selected
at 5-meter intervals for total digestion analysis using inductively coupled plasma mass
spectrometry (ICP-MS). Selected sediments were dried in an oven at 60?C for 24 hours.
The low drying temperature prevented the loss of volatiles. After drying, sediments were
allowed to cool and were ground to a fine powder using a mortar and pestle. The samples
were then sent to ACME Analytical Laboratories Ltd. for group 1DX elemental analysis.
About 0.5 grams of selected sediments were digested by 3-mL 2-2-2 HCL-HNO3-H2O
(Aqua Regia) at 95?C and diluted to 10-mL for ICP-MS elemental analysis. The ICP-MS
measures the element concentrations by counting the atoms for each element present in
solution. The results of total digestion elemental analyses were correlated with results of
grain-size and total organic carbon analysis (see below) to test the hypothesis that trace
metals such as arsenic tend to concentrate in fine-grained sediments and organic matter
with high-surface sorbing areas.
Total Organic Carbon Analysis
Core sediments were selected at 5-meter intervals (0.25-0.5 grams) from both
drilling sites for analysis of total organic carbon content using a LECO carbon analyzer.
Sediments were first dried 24 hours in oven at 80?C. Once the samples were dried, they
24
were ground to a fine powder by mortar and pestle. Approximately 0.25 to 0.30 grams of
each powdered sample was carefully weighed in a tared 50-ml beaker before digesting
samples in dilute hydrochloric acid to remove carbonate carbon prior to instrument
analysis. After acid digestion, the samples were filtered through pre-weighed carbon-free
filters. Filters plus residues were dried in an oven at 80?C for 24 hours and then
reweighed. The filters with residues were then placed into a ceramic crucible along with
LECOCEL II and iron chip accelerators and then combusted in the LECO carbon
analyzer. Total organic carbon was measured from CO2 yield as detected by an infrared
(IR) cell.
Sequential Leaching of Arsenic-Hosting Solids
Sequential leaching experiments (similar to those by Keon et al., 2001; Swartz et
al., 2004) were conducted to determine the concentrations of As bounded up in different
solid phases in sediments recovered from As-rich zones of the Budai well (at 6, 60, 105,
110, and 150 m of depth). Selective or sequential extractions can target elements held by
a specific soil phase or range of phases thus allowing better interpretation of As-hosting
minerals in sediments. Used sequentially, the leaches can determine elements occurring
in surface soils as salts or adsorbed ions on clays, carbonate, organic compounds,
amorphous and crystalline Mn- and Fe-oxyhydroxides, and silicate minerals.
Sediments were first dried for 24 hours at 80?C. Samples were then ground to a
fine powder with mortar and pestle and sent to ACME Laboratories for sequential
leaching analysis. The sequence of extraction include: (1) extracting water-soluble As by
distilled water, (2) leaching of exchangeable As adsorbed by clay and co-precipitated
with carbonates using 1 M ammonium-acetate (NH4C2H3O2), (3) extraction of As
25
adsorbed by organic matter or arsenic oxides by 0.1 M Na-pyrophosphate (Na3P2O7), (4)
leaching of strongly sorbed As from amorphous Mn oxyhydroxides by 0.1 M
hydroxylamine (NH2OH) solutions, and (5) leaching of strongly sorbed As from
amorphous Fe oxyhydroxides and crystalline Mn by 0.25 M hydroxylamine solutions.
Amounts of As co-precipitated in silicate minerals and other recalcitrant solid phases also
were calculated as the difference between bulk concentration (determined by Aqua Regia
total digestion) and the sum of leachable fractions.
Groundwater Geochemistry
To procure a sample that is representative of groundwater, all piezometers were
pumped continuously for at least one hour before sampling. A raw and an acidified
sample from each piezometer and their replicates were sent to ACTLABS for major ion
and trace element analysis using ion chromatography (IC) and inductively coupled
plasma mass spectrometry (ICP-MS). Samples for trace element and cation analyses were
passed through a 0.45-?m filter, acidified with concentrated HNO3, and stored in
polyethylene bottles. Temperature, pH, and electrical conductivity were measured in the
field using hand-held, water-quality probes. The stable O and H isotopic ratios of
groundwater were also analyzed at the National High Magnetic Field Lab (Florida State
University) to characterize the sources of As-rich groundwater.
Overland Flow Model
ARC/INFO software and digital elevation model (DEM) were used to predict
where the sediments will be transported downstream into the Chianan alluvial plain.
Calculations are made using a physics-based computational watershed model. The
watershed model is simulated as flow occurring overland. Stream channels are routed
26
into rivers and flow until they reach sea level (elevation = 0 m). Modeling sediment and
adsorbed contaminant transport within the contributing watersheds can help to identify
possible arsenic sources. Two Digital Elevation Models (DEMs) were downloaded into
Arc GIS 9.2 from the CGIAR-CSI SRTM Database to generate a drainage map of the
watershed. This served as a base map for the study area in Taiwan. Locations of
groundwater wells used in this study were plotted onto the map. The simulations included
8 major mud volcanoes as potential point sources in Chiayi (Kuanzeling and Chunlung),
Tainan (Yenshuikeng) and Kaohsiung (Kunshuiping, Wushanting, Hsiaokunshui,
Dakunshui, Yannue Lake) counties in southwest Taiwan. Locations of mud volcanoes
were entered onto the DEM using coordinates from Appendix 1. The Watershed
Delineation Tool of Arc GIS9.2 creates a stream network onto the DEM based on a
threshold and delineates watersheds for each stream link. After a watershed tool raster
was created, major rivers were labeled. The iRainDrop function from Watershed
Delineation Tools.tbx was then used to create overland flow paths from eight mud
volcanoes. The iRainDrop tool creates flow paths from any point based on flow direction
and flow-accumulation rasters. The iWatershed tool creates a watershed for any pour
point (outlet) and was not used in this study.
Aquifer Flushing Model
A Matlab?-based model was built to simulate the effects of titration of
freshwater through the As-bearing alluvial aquifer in the Chianan Plain. The simulations
provided insight into some of the important parameters controlling reactive transport of
arsenic. In the model, an alluvial aquifer containing 300 ?g/kg of arsenic was infiltrated
by dilute rainwater at 25?C. Assuming a Darcian flow rate of 0.1 m/day, the distribution
27
of As along a 1000-meter flow path was calculated. The modeling results were used to
assess the time required to flush out initial mobile arsenic loads in the aquifer. The
modeling results are compared to the average Chianan Plain 14CDIC groundwater ages (or
residence times) reported by Chen and Liu (2007). The modeling results shed light on
how hydrologic transport processes control groundwater As concentrations in alluvial
aquifers under various hydrogeologic and climatic conditions.
28
RESULTS AND DISCUSSION
Chemical and Physical Characteristics of Sediments
Sediment Color
Core samples ranged from dark grey and grey to brown and yellow brown. The
yellow brown and brown layers suggest relatively oxidizing conditions while the grey
and dark grey strata indicate more reducing conditions. At the Budai well site, various
shades of grey were found at depths between 0 and 65 m with the exception of the sample
at 45 m, which was black (probably due to high organic content) and the sample at 0 m,
which was brown (Figure 8). Core sediments from 65 to 140 m were various shades of
brownish grey, with one layer of very light grey at 75 m. Sediments below 140 m were
mostly brown. Sediments in the Yichu core samples follow a similar pattern. Shades of
brown dominate from 0 to 10 m, while various shades of brownish grey characterize
intermediate depths of 10 to 165 m. Sediments from depths below 165 m ranged from
various shades of brown to brownish grey. Patterns in both cores suggest widespread
reducing conditions at shallower depths (except for surface layers) and a transition from
reducing to oxidizing conditions through intermediate to deeper levels.
29
Figure 8. Simplified stratigraphic columns from Chianan Plain showing sand, silt, clay
and sediment color.
Depth
(m)
Depth
(m)
30
Grain-size analysis
Sediment grain sizes in the 71 samples collected from the two well sites in the
study area range from fine sand to clay (Table 1). Results of grain-size analyses from the
Yichu well-site samples show that sand, silt and clay contents range from 0.2 to 70.16%
(average = 26.77%), 2.74 to 62.48% (average = 33.25%) and 26.26 to 74.28% (Average
39.97%), respectively. These results indicate that sediments at the Yichu well site are
dominated by silt and clay, although fine sands occur at 20-25, 55, 95-115, 150, and 165-
170 m (Figure 8).
The Budai well site had an average of 35.41% sand, 29.27% silt and 35.32% clay.
Sand-sized particles in each sample ranged from 1.11% to 79.35%. Silt-sized particles
ranged from 0.65% to 64.59%. The range of clay-sized particles ranged from 19.99% to
63.06%. The section of the core from 45 to 75 m was dominated by silt-sized sediments.
31
Table 1. Data from the grain-size analysis of Yichu and Budai sediments.
Yichu and Budai sediment grain-size distribution
Location Depth Sand Silt Clay Location Depth Sand Silt Clay
(m) (%) (%) (%) (m) (%) (%) (%)
Yichu 5 36.43 31.14 32.43 Budai 0 49.39 26.34 24.26
Yichu 10 25.28 29.08 45.65 Budai 5 28.11 29.63 42.26
Yichu 15 9.49 32.60 57.91 Budai 10 79.35 0.65 19.99
Yichu 20 65.38 4.81 29.82 Budai 15 63.61 7.87 28.52
Yichu 25 57.92 12.72 29.36 Budai 20 65.47 11.98 22.55
Yichu 30 44.60 26.02 29.38 Budai 25 45.32 23.65 31.03
Yichu 35 38.68 30.36 30.97 Budai 30 3.35 45.40 51.26
Yichu 40 0.69 49.99 49.32 Budai 35 67.58 10.26 22.15
Yichu 45 9.85 47.98 42.17 Budai 40 65.50 13.21 21.29
Yichu 50 0.60 61.46 37.94 Budai 45 72.88 6.47 20.64
Yichu 55 52.26 16.72 31.01 Budai 50 3.39 59.85 36.76
Yichu 60 22.39 43.46 34.16 Budai 55 2.77 58.41 38.82
Yichu 65 7.47 58.08 34.45 Budai 60 1.11 64.59 34.30
Yichu 70 29.52 37.68 32.81 Budai 65 20.90 57.56 21.54
Yichu 75 2.01 54.40 43.59 Budai 70 25.75 43.61 30.64
Yichu 80 0.63 54.68 44.69 Budai 75 2.19 55.36 42.45
Yichu 85 0.82 24.90 74.28 Budai 80 58.40 10.24 31.36
Yichu 90 24.00 38.71 37.29 Budai 85 7.14 35.37 57.48
Yichu 95 63.26 7.63 29.12 Budai 90 16.80 48.88 34.32
Yichu 100 47.07 15.09 37.83 Budai 95 64.09 6.84 29.07
Yichu 105 63.47 5.69 30.84 Budai 100 45.80 14.07 40.12
Yichu 110 46.87 21.40 31.73 Budai 105 15.96 47.76 36.28
Yichu 115 55.45 12.16 32.40 Budai 110 67.71 1.94 30.35
Yichu 120 22.30 39.88 37.83 Budai 115 26.08 33.18 40.74
Yichu 125 2.48 62.48 35.04 Budai 120 4.35 39.88 55.77
Yichu 130 8.76 41.55 49.69 Budai 125 63.33 6.09 30.58
Yichu 135 3.95 43.46 52.59 Budai 130 35.62 22.85 41.53
Yichu 140 35.10 30.40 34.50 Budai 135 65.57 5.13 29.30
Yichu 145 0.20 36.40 63.40 Budai 140 1.45 51.61 46.94
Yichu 150 59.11 12.31 28.58 Budai 145 21.84 15.10 63.06
Yichu 155 0.36 35.13 64.51 Budai 150 6.93 53.61 39.45
Yichu 160 0.57 32.59 66.84
Yichu 165 70.16 2.74 27.09
Yichu 170 68.96 4.78 26.26
Yichu 175 16.19 44.04 39.76
Yichu 180 11.07 52.29 36.64
Yichu 185 13.21 47.48 39.30
Yichu 190 14.99 39.90 45.11
Yichu 195 9.65 55.24 35.11
Yichu 200 29.80 32.72 37.49
32
Mineralogy and Petrology
Petrographic analysis shows that most sands from the Chianan study area are
classified as lithic arkose or feldspathic litharenite. The most common detrital mineral
grains were quartz (65-75%) (Figure 9a, b). Feldspars and lithic fragments comprise 10-
20% and 5-10% of samples, respectively. Mica is a common accessory (2-5%). Most
quartz grains were monocrystalline (90%) and exhibited various degrees of undulose
extinction. Quartz content generally increased with well depth. Most lithic fragments
were sedimentary, but some igneous clasts also were identified. Siderite concretions,
which may potentially serve as a sink for As, also are observed (Figure 9c). Several types
of foraminifera were found in the Budai well site at 95 m (Figure 9d). The fossil content,
coupled with the grain size (fine sand), at this depth may be indicative of an estuarine/bay
environment. XRD analysis performed by Dr. Zhaohui Li showed that the fine-grained
minerals were mostly kaolinite, illite, smectite, chlorite, and mixed layer clays. Surfaces
of these clay minerals can serve as sorption sites for aqueous arsenic species (Mohapatra
et al., 2007).
33
Figure 9. Representative photomicrographs of coarse-grained sediments (fine sand) from
Chianan Plain. Key: Qm ? monocrystalline quartz; Qp ? polycrystalline quartz; F ?
feldspar; Ls ? sedimentary lithic; Sil ? sillimanite; Sr ? siderite concreation; Fa -
foraminifera.
34
Sediment Geochemistry
Arsenic concentrations in the Budai and Yichu sediments ranged from 1.5 to 37.2
ppm and 0.8 to 23 ppm, respectively (Tables 2 and 3), which are notably higher than
those in sediments tested from the Chapai-Nawabganj District in northwest Bangladesh
(Reza et al., 2012). The three well sites tested in northwest Bangladesh had
concentrations that ranged from 1.0 to 4.4 ppm (Rajaran well), 0.9 to 7.2 ppm (Ghat
well), and 1.4 to 17.1 (Jorgachi well). The average concentrations of As at the Budai and
Yichu well sites were 11.2 and 9.9 ppm, respectively. In comparison, the average
concentration of As in the 31 sediment samples from northwestern Bangladesh was
about 3.4 ppm. Highest As concentrations at the Budai well site were found at depths of
60, 105, 110, and 150 meters with concentrations of 32.3, 28.0, 29.9, and 37.2 ppm,
respectively. Levels of As in all other sediments at the Budai well site were 15.1 ppm or
less. Arsenic concentrations correlate with the clay and silt contents in Yichu sediments
(Figure 10 and Appendix 2). Clay-sized particles in Yichu positively correlate with Fe (r
= 0.65) and Mn (r = 0.81) (Table 4). Silt also correlated weakly with Fe and Mn (0.22
and 0.41, respectively).
The highest levels of As in the Yichu well were 18.8, 21.5, 22.8, and 23.0 ppm at
depths of 180, 100, 90, and 15 m. Fe concentrations in sediments from the Budai well had
a range of 1.1 to 5.12%. The Yichu well site had Fe concentrations in sediments that
ranged from 1.0 to 5.6%. The average concentration of Fe in the Budai and Yichu well
sites was 2.4% and 2.6%, respectively. Mn contents in Budai sediments range from 88 to
1292 ppm with an average concentration of 421.9 ppm. Both silt- and clay-sized particles
correlate positively with As in Budai core sediments (Table 5). Correlations
35
Table 2. Compositions of major and trace elements in Budai sediment samples from
southwest Taiwan.
Budai core sediment geochemistry
Depth As Fe Mn Ba Ni Co Cu Zn Pb Sr Mo Cr La V
(m) mg/kg % mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg
0 10.3 2.30 420 52 23.3 10.2 11.2 58 15.3 39 0.2 21 14 23
5 14.4 3.25 491 24 28.7 12.5 19.2 79 21.6 38 0.4 25 8 24
10 11.2 1.62 328 21 13.2 6.1 4.7 40 7.6 43 0.2 10 16 13
15 8.1 1.76 255 30 16.9 7.7 5.8 51 8.5 31 0.2 15 10 16
20 7.3 2.00 277 22 18.9 7.7 6.1 52 11.0 30 0.3 15 10 16
25 5.9 2.27 294 23 21.7 8.2 8.2 59 11.8 36 0.2 17 9 15
30 10.5 2.75 394 32 25.9 10.3 12.7 73 17.7 43 0.2 22 9 22
35 9.1 2.04 266 31 18.3 7.6 5.6 56 10.8 31 0.2 16 11 17
40 4.7 2.01 279 35 19.1 7.2 5.2 55 9.6 39 0.2 17 10 17
45 7.9 1.99 318 26 17.9 7.8 6.4 48 10.6 39 0.2 15 12 17
50 9.8 3.28 498 28 32.9 13.8 15.8 80 21.7 47 0.3 27 10 25
55 12.8 3.22 459 21 28.6 12.0 16.3 74 22.4 38 0.3 24 9 23
60 32.3 4.14 699 74 41.1 17.8 27.0 102 31.2 46 0.4 33 12 32
65 13.0 2.64 536 40 26.4 11.9 16.3 64 12.4 47 0.6 25 13 24
70 9.9 2.18 427 39 23.0 10.1 12.0 57 14.5 39 0.3 19 12 21
75 15.1 3.47 570 45 32.5 14.5 18.3 84 22.4 51 0.2 29 12 28
80 8.7 2.35 319 27 22.2 8.5 9.2 58 12.6 34 0.3 18 10 17
85 10.5 5.12 930 76 45.1 18.8 29.9 104 35.1 35 0.3 35 16 29
90 7.0 1.96 291 35 20.9 9.1 9.3 54 13.1 24 0.2 17 12 17
95 3.4 1.52 541 29 16.9 8.4 5.2 43 10.9 52 0.1 12 10 12
100 6.4 1.72 548 66 19.3 8.6 12.9 48 13.4 10 0.3 17 16 19
105 28.0 3.68 1292 40 25.0 14.2 10.0 58 13.7 15 0.2 18 10 19
110 29.9 2.21 409 50 25.6 13.1 11.1 62 21.6 12 1.0 19 10 18
115 10.9 2.40 617 41 24.6 12.2 10.4 58 17.7 49 0.2 20 13 21
120 12.5 2.54 514 72 27.9 12.9 11.8 65 20.1 17 0.4 25 16 25
125 2.5 1.32 337 33 12.4 6.8 4.8 34 8.3 9 0.2 10 13 12
130 1.5 1.31 99 34 15.2 7.6 5.9 41 11.2 8 0.1 13 12 16
135 2.6 1.20 88 26 14.0 6.9 5.2 39 8.5 9 0.2 11 13 11
140 1.9 2.28 112 49 26.3 10.1 11.7 65 14.3 13 0.1 24 15 22
145 2.3 1.10 126 31 10.6 5.4 4.5 29 7.4 9 0.2 10 13 11
150 37.2 2.26 345 70 23.7 9.7 11.1 59 14.5 20 0.3 19 13 19
36
Table 2. Continued.
Budai core sediment geochemistry
Depth Ca Mg Na K Al P S
(m) % % % % % % %
0 0.75 0.52 0.190 0.19 1.22 0.038 <0.05
5 0.77 0.91 1.238 0.23 1.55 0.047 0.32
10 0.95 0.37 0.213 0.11 0.74 0.029 <0.05
15 0.56 0.50 0.739 0.16 0.92 0.036 0.06
20 0.59 0.52 0.328 0.14 0.94 0.037 <0.05
25 0.72 0.62 0.352 0.14 1.09 0.043 <0.05
30 0.89 0.74 0.400 0.21 1.37 0.052 0.10
35 0.57 0.51 0.181 0.18 1.08 0.036 <0.05
40 0.64 0.53 0.352 0.19 1.11 0.035 <0.05
45 0.72 0.52 0.319 0.15 0.94 0.040 <0.05
50 1.02 0.87 0.276 0.21 1.53 0.057 0.10
55 0.87 0.82 0.185 0.17 1.45 0.049 0.11
60 0.98 1.00 0.262 0.24 1.89 0.050 0.18
65 1.13 0.75 0.138 0.17 1.18 0.054 0.16
70 0.82 0.61 0.099 0.18 1.13 0.042 0.09
75 0.91 0.84 0.176 0.21 1.57 0.052 0.16
80 0.67 0.60 0.229 0.16 1.14 0.041 <0.05
85 0.76 0.87 0.090 0.16 2.17 0.062 <0.05
90 0.45 0.42 0.056 0.14 1.01 0.034 <0.05
95 1.13 0.33 0.038 0.10 0.76 0.026 <0.05
100 0.07 0.23 0.044 0.10 1.01 0.013 <0.05
105 0.24 0.38 0.039 0.16 1.11 0.041 0.20
110 0.11 0.37 0.034 0.13 1.21 0.012 0.17
115 1.11 0.52 0.037 0.15 1.12 0.036 0.06
120 0.19 0.41 0.036 0.15 1.51 0.016 <0.05
125 0.07 0.21 0.033 0.10 0.73 0.020 <0.05
130 0.05 0.24 0.032 0.14 0.89 0.010 <0.05
135 0.06 0.23 0.035 0.10 0.67 0.015 <0.05
140 0.11 0.39 0.034 0.15 1.28 0.019 <0.05
145 0.06 0.21 0.042 0.11 0.64 0.015 <0.05
150 0.13 0.37 0.040 0.15 1.20 0.038 <0.05
37
Table 3. Compositions of major and trace elements in Yichu sediment samples from
southwest Taiwan.
Yichu core sediment geochemistry
Depth As Fe Mn Ba Ni Co Cu Zn Pb Sr Mo Cr La V
(m) mg/kg % mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg
5 6.0 1.83 234 32 18.8 8.4 8.0 49 9.1 16 <0.1 19 13 21
10 13.5 3.19 500 24 29.6 13.2 15.5 75 18.1 31 0.2 26 10 28
15 23.0 3.77 742 22 35.1 15.7 18.9 89 22.3 34 0.2 30 11 33
20 10.9 2.07 337 21 18.7 8.0 9.5 53 12.0 26 0.3 16 11 17
25 6.5 1.90 294 21 17.7 7.9 5.3 52 8.3 26 0.1 15 10 17
30 8.4 2.35 334 22 23.8 9.9 8.1 60 11.1 29 0.2 18 9 17
35 6.8 2.26 351 21 20.4 9.3 7.6 54 11.0 29 0.1 16 10 19
40 18.0 3.81 672 25 38.1 16.0 28.8 102 28.2 40 0.4 29 8 27
45 6.6 2.03 351 21 21.6 9.5 7.7 51 12.2 29 0.1 19 19 20
50 14.0 3.92 808 60 38.9 18.0 19.5 86 21.9 46 0.2 35 16 38
55 12.0 2.10 431 36 22.1 10.2 9.2 55 11.5 37 0.1 12 12 23
60 2.7 1.96 354 39 19.4 8.6 9.5 51 10.4 29 <0.1 18 13 20
65 2.7 1.92 262 30 20.2 8.2 7.9 67 10.3 26 <0.1 18 11 18
70 3.6 1.86 99 31 19.6 8.1 9.3 54 8.6 10 <0.1 17 13 19
75 6.9 2.36 136 45 34.0 17.4 16.6 78 18.4 16 0.2 26 14 27
80 6.4 3.69 815 45 31.6 14.0 17.7 82 18.8 21 <0.1 28 15 28
85 13.0 4.21 531 55 41.1 16.6 22.0 97 21.8 38 0.2 36 14 36
90 22.8 2.46 504 51 21.9 10.4 10.0 57 12.1 12 0.2 18 11 20
95 0.8 1.04 69 24 14.2 7.7 5.1 41 8.4 6 <0.1 13 17 12
100 21.5 2.35 411 69 21.1 9.8 8.5 52 12.0 13 0.2 20 15 24
105 3.4 2.55 622 30 17.4 8.9 6.6 43 9.9 8 0.1 14 9 15
110 13.4 1.98 448 23 18.7 8.4 7.7 45 10.3 16 0.3 15 8 15
115 4.0 1.42 181 24 15.4 7.3 5.8 42 8.1 15 0.1 13 10 13
120 7.9 2.17 129 25 23.4 9.2 7.3 53 11.3 11 0.1 18 12 18
125 8.4 2.48 391 28 25.5 11.1 22.4 62 12.3 23 0.2 21 11 21
130 5.9 2.72 164 43 25.5 11.2 14.2 71 13.4 12 0.2 22 13 21
135 1.7 2.43 157 33 26.9 10.0 13.3 85 16.1 14 <0.1 22 12 20
140 7.6 1.91 203 29 19.9 9.2 7.4 49 12.0 13 0.2 17 9 16
145 17.3 3.47 805 24 32.9 15.3 16.0 83 19.1 40 0.4 29 11 27
150 4.0 1.39 112 22 14.6 7.1 4.4 37 8.0 8 0.2 12 11 12
155 17.5 5.60 1511 43 38.8 16.8 22.7 101 24.5 51 0.4 33 13 35
160 10.3 4.78 1248 38 36.2 15.3 20.4 92 26.3 37 0.5 30 10 32
165 5.2 1.24 82 18 12.9 6.5 5.0 37 6.5 5 0.1 11 9 11
170 3.9 1.58 146 19 14.4 6.5 5.3 45 7.4 11 <0.1 13 11 13
175 16.3 3.27 478 20 28.5 13.0 18.7 78 20.1 34 0.3 23 8 24
180 18.8 3.08 422 19 30.8 14.0 15.7 79 18.0 30 0.3 22 7 25
185 4.7 2.57 338 24 23.0 8.3 7.0 65 11.6 31 0.1 20 9 18
190 18.4 4.05 694 24 34.4 15.6 21.2 102 33.5 41 0.4 30 11 30
195 13.2 2.95 400 23 27.4 10.9 11.0 74 17.4 35 0.1 22 10 23
200 9.3 2.70 398 24 25.8 10.3 10.0 73 15.7 31 0.1 20 9 19
38
Table 3. Continued.
Yichu core sediment geochemistry
Depth Ca Mg Na K Al P S
(m) % % % % % % %
5 0.41 0.43 0.016 0.18 1.09 0.036 <0.05
10 0.93 0.80 0.025 0.20 1.56 0.045 0.12
15 1.05 0.92 0.023 0.20 1.71 0.049 0.19
20 0.74 0.48 0.021 0.12 0.95 0.032 <0.05
25 0.75 0.52 0.028 0.12 0.98 0.031 <0.05
30 0.82 0.62 0.024 0.13 1.16 0.038 <0.05
35 0.90 0.62 0.056 0.13 1.08 0.042 <0.05
40 1.23 1.05 0.096 0.17 1.80 0.049 0.36
45 1.01 0.62 0.059 0.13 0.99 0.069 <0.05
50 1.20 1.07 0.092 0.21 1.83 0.052 0.27
55 1.09 0.63 0.030 0.13 1.00 0.035 0.15
60 0.93 0.55 0.030 0.17 1.04 0.043 <0.05
65 0.80 0.52 0.014 0.12 0.98 0.036 <0.05
70 0.12 0.37 0.021 0.14 1.05 0.036 <0.05
75 0.18 0.51 0.020 0.13 1.44 0.043 0.05
80 0.57 0.71 0.019 0.14 1.68 0.055 <0.05
85 0.67 0.96 0.029 0.18 2.03 0.046 0.13
90 0.10 0.36 0.018 0.15 1.25 0.038 <0.05
95 0.07 0.19 0.017 0.10 0.58 0.013 <0.05
100 0.14 0.33 0.017 0.14 1.22 0.051 <0.05
105 0.17 0.29 0.021 0.12 0.88 0.026 <0.05
110 0.45 0.38 0.016 0.11 0.85 0.025 0.13
115 0.36 0.33 0.019 0.12 0.78 0.022 <0.05
120 0.11 0.42 0.017 0.13 0.99 0.032 <0.05
125 0.82 0.56 0.021 0.14 1.22 0.045 <0.05
130 0.08 0.47 0.018 0.17 1.54 0.038 <0.05
135 0.12 0.55 0.017 0.15 1.45 0.026 <0.05
140 0.12 0.37 0.020 0.12 0.95 0.034 <0.05
145 1.12 0.83 0.025 0.14 1.59 0.048 0.18
150 0.08 0.27 0.021 0.10 0.69 0.027 <0.05
155 1.73 0.91 0.028 0.17 1.90 0.125 <0.05
160 0.98 0.88 0.026 0.17 1.81 0.102 0.14
165 0.05 0.25 0.015 0.10 0.69 0.013 0.05
170 0.22 0.34 0.013 0.10 0.83 0.020 <0.05
175 0.95 0.80 0.022 0.14 1.50 0.044 0.18
180 0.91 0.80 0.018 0.13 1.44 0.042 0.15
185 0.75 0.70 0.029 0.15 1.33 0.038 <0.05
190 1.07 1.08 0.038 0.19 1.97 0.050 0.18
195 0.87 0.77 0.032 0.17 1.46 0.047 0.12
200 0.69 0.69 0.032 0.16 1.46 0.035 0.08
39
Figure 10. Plot comparing Budai and Yichu stratigraphic columns and showing general
positive correlation of clay and silt with As.
40
Table 4. Correlation coefficients for Yichu grain size, TOC, and elemental analysis.
Correlation coefficients of sediment elemental composition, grain size,
and total organic carbon (TOC) content at Yichu well site.
As Cu Pb Zn Ni Co Mn Fe U Th
As 1 0.55 0.62 0.53 0.58 0.6 0.56 0.6 0.4 0.36
Cu 0.55 1 0.88 0.89 0.91 0.89 0.66 0.85 0.52 0.61
Pb 0.62 0.88 1 0.93 0.91 0.89 0.72 0.89 0.59 0.66
Zn 0.53 0.89 0.93 1 0.95 0.89 0.67 0.9 0.56 0.68
Ni 0.58 0.91 0.91 0.95 1 0.97 0.7 0.91 0.63 0.75
Co 0.6 0.89 0.89 0.89 0.97 1 0.7 0.87 0.66 0.74
Mn 0.56 0.66 0.72 0.67 0.7 0.7 1 0.89 0.5 0.53
Fe 0.6 0.85 0.89 0.9 0.91 0.87 0.89 1 0.54 0.66
U 0.4 0.52 0.59 0.56 0.63 0.66 0.5 0.54 1 0.82
Th 0.36 0.61 0.66 0.68 0.75 0.74 0.53 0.66 0.82 1
Sr 0.52 0.65 0.73 0.74 0.75 0.69 0.72 0.77 0.56 0.56
Sb 0.21 0.43 0.34 0.31 0.27 0.24 0.13 0.19 -0.01 0.01
Bi 0.64 0.91 0.94 0.92 0.9 0.87 0.69 0.88 0.56 0.64
V 0.62 0.84 0.84 0.87 0.94 0.93 0.74 0.89 0.71 0.79
Ca 0.46 0.58 0.61 0.62 0.61 0.57 0.71 0.67 0.53 0.45
P 0.41 0.6 0.63 0.62 0.66 0.63 0.84 0.8 0.59 0.65
La -0.17 0 -0.03 -0.01 0.07 0.12 -0.01 -0.02 0.45 0.63
Cr 0.54 0.87 0.89 0.92 0.96 0.93 0.71 0.9 0.65 0.8
Mg 0.56 0.81 0.88 0.9 0.89 0.83 0.69 0.86 0.59 0.64
Ba 0.24 0.29 0.21 0.26 0.37 0.4 0.29 0.33 0.24 0.51
Ti -0.37 -0.58 -0.63 -0.61 -0.61 -0.6 -0.54 -0.66 -0.3 -0.36
Al 0.58 0.88 0.91 0.96 0.96 0.9 0.69 0.92 0.55 0.72
Na 0.23 0.42 0.44 0.37 0.43 0.42 0.31 0.35 0.35 0.4
K 0.47 0.66 0.69 0.71 0.73 0.67 0.52 0.7 0.53 0.63
Sc 0.54 0.82 0.8 0.83 0.91 0.9 0.8 0.9 0.7 0.8
clay 0.41 0.72 0.73 0.8 0.81 0.75 0.65 0.81 0.61 0.7
sand -0.33 -0.76 -0.7 -0.81 -0.82 -0.75 -0.49 -0.7 -0.59 -0.72
silt 0.18 0.55 0.45 0.56 0.57 0.52 0.22 0.41 0.4 0.52
TOC 0.51 0.75 0.82 0.79 0.84 0.84 0.73 0.82 0.54 0.51
41
Table 4. Continued
Correlation coefficients of sediment elemental composition, grain size
and total organic carbon (TOC) content at Yichu well site.
Sr Sb Bi V Ca P La Cr Mg Ba
As 0.52 0.21 0.64 0.62 0.46 0.41 -0.17 0.54 0.56 0.24
Cu 0.65 0.43 0.91 0.84 0.58 0.6 0 0.87 0.81 0.29
Pb 0.73 0.34 0.94 0.84 0.61 0.63 -0.03 0.89 0.88 0.21
Zn 0.74 0.31 0.92 0.87 0.62 0.62 -0.01 0.92 0.9 0.26
Ni 0.75 0.27 0.9 0.94 0.61 0.66 0.07 0.96 0.89 0.37
Co 0.69 0.24 0.87 0.93 0.57 0.63 0.12 0.93 0.83 0.4
Mn 0.72 0.13 0.69 0.74 0.71 0.84 -0.01 0.71 0.69 0.29
Fe 0.77 0.19 0.88 0.89 0.67 0.8 -0.02 0.9 0.86 0.33
U 0.56 -0.01 0.56 0.71 0.53 0.59 0.45 0.65 0.59 0.24
Th 0.56 0.01 0.64 0.79 0.45 0.65 0.63 0.8 0.64 0.51
Sr 1 0.21 0.72 0.75 0.95 0.66 -0.06 0.7 0.92 0.07
Sb 0.21 1 0.35 0.12 0.23 0.06 -0.21 0.2 0.3 -0.08
Bi 0.72 0.35 1 0.84 0.64 0.66 -0.02 0.86 0.86 0.18
V 0.75 0.12 0.84 1 0.63 0.71 0.23 0.95 0.85 0.51
Ca 0.95 0.23 0.64 0.63 1 0.66 -0.08 0.57 0.83 -0.07
P 0.66 0.06 0.66 0.71 0.66 1 0.2 0.67 0.6 0.34
La -0.06 -0.21 -0.02 0.23 -0.08 0.2 1 0.19 -0.06 0.52
Cr 0.7 0.2 0.86 0.95 0.57 0.67 0.19 1 0.86 0.43
Mg 0.92 0.3 0.86 0.85 0.83 0.6 -0.06 0.86 1 0.11
Ba 0.07 -0.08 0.18 0.51 -0.07 0.34 0.52 0.43 0.11 1
Ti -0.42 -0.04 -0.63 -0.58 -0.32 -0.46 0.12 -0.62 -0.53 -0.19
Al 0.73 0.22 0.9 0.92 0.57 0.64 0.04 0.95 0.89 0.39
Na 0.56 0.61 0.38 0.41 0.53 0.27 0.14 0.4 0.58 0.1
K 0.62 0.14 0.67 0.8 0.51 0.49 0.13 0.78 0.75 0.38
Sc 0.68 0.1 0.8 0.96 0.57 0.78 0.28 0.93 0.76 0.57
clay 0.54 0.13 0.74 0.77 0.41 0.66 0.16 0.84 0.65 0.35
sand -0.57 -0.18 -0.7 -0.74 -0.47 -0.6 -0.17 -0.82 -0.71 -0.32
silt 0.41 0.16 0.46 0.49 0.37 0.38 0.12 0.55 0.53 0.21
TOC 0.75 0.25 0.75 0.8 0.67 0.53 -0.17 0.79 0.85 0.12
42
Table 4. Continued.
Correlation coefficients of sediment elemental composition, grain size,
and total organic carbon (TOC) content at Yichu well site.
Ti Al Na K Sc clay sand silt TOC h
As -0.37 0.58 0.23 0.47 0.54 0.41 -0.33 0.18 0.51 g
Cu -0.58 0.88 0.42 0.66 0.82 0.72 -0.76 0.55 0.75 g
Pb -0.63 0.91 0.44 0.69 0.8 0.73 -0.7 0.45 0.82 g
Zn -0.61 0.96 0.37 0.71 0.83 0.8 -0.81 0.56 0.79 g
Ni -0.61 0.96 0.43 0.73 0.91 0.81 -0.82 0.57 0.84 g
Co -0.6 0.9 0.42 0.67 0.9 0.75 -0.75 0.52 0.84 g
Mn -0.54 0.69 0.31 0.52 0.8 0.65 -0.49 0.22 0.73 g
Fe -0.66 0.92 0.35 0.7 0.9 0.81 -0.7 0.41 0.82 g
U -0.3 0.55 0.35 0.53 0.7 0.61 -0.59 0.4 0.54 g
Th -0.36 0.72 0.4 0.63 0.8 0.7 -0.72 0.52 0.51 g
Sr -0.42 0.73 0.56 0.62 0.68 0.54 -0.57 0.41 0.75 g
Sb -0.04 0.22 0.61 0.14 0.1 0.13 -0.18 0.16 0.25 g
Bi -0.63 0.9 0.38 0.67 0.8 0.74 -0.7 0.46 0.75 g
V -0.58 0.92 0.41 0.8 0.96 0.77 -0.74 0.49 0.8 g
Ca -0.32 0.57 0.53 0.51 0.57 0.41 -0.47 0.37 0.67 g
P -0.46 0.64 0.27 0.49 0.78 0.66 -0.6 0.38 0.53 g
La 0.12 0.04 0.14 0.13 0.28 0.16 -0.17 0.12 -0.17 g
Cr -0.62 0.95 0.4 0.78 0.93 0.84 -0.82 0.55 0.79 g
Mg -0.53 0.89 0.58 0.75 0.76 0.65 -0.71 0.53 0.85 g
Ba -0.19 0.39 0.1 0.38 0.57 0.35 -0.32 0.21 0.12 g
Ti 1 -0.65 -0.13 -0.42 -0.61 -0.6 0.35 -0.07 -0.58 g
Al -0.65 1 0.38 0.81 0.87 0.81 -0.8 0.55 0.79 g
Na -0.13 0.38 1 0.42 0.32 0.14 -0.32 0.35 0.42 g
K -0.42 0.81 0.42 1 0.72 0.59 -0.64 0.48 0.59 g
Sc -0.61 0.87 0.32 0.72 1 0.85 -0.72 0.41 0.74 g
clay -0.6 0.81 0.14 0.59 0.85 1 -0.72 0.3 0.6 g
sand 0.35 -0.8 -0.32 -0.64 -0.72 -0.72 1 -0.88 -0.59 g
silt -0.07 0.55 0.35 0.48 0.41 0.3 -0.88 1 0.4 g
TOC -0.58 0.79 0.42 0.59 0.74 0.6 -0.59 0.4 1 g
43
Table 5. Correlation coefficients for Budai grain size, TOC, and elemental analysis.
Correlation coefficients of sediment elemental composition, grain size,
and total organic carbon (TOC) content at Budai well site.
As Cu Pb Zn Ni Co Mn Fe U Th
As 1 0.44 0.48 0.46 0.49 0.57 0.51 0.5 -0.01 0.29
Cu 0.44 1 0.93 0.93 0.95 0.9 0.57 0.9 0.45 0.83
Pb 0.48 0.93 1 0.92 0.95 0.93 0.56 0.88 0.52 0.77
Zn 0.46 0.93 0.92 1 0.98 0.9 0.52 0.94 0.33 0.75
Ni 0.49 0.95 0.95 0.98 1 0.95 0.61 0.95 0.39 0.82
Co 0.57 0.9 0.93 0.9 0.95 1 0.74 0.92 0.46 0.77
Mn 0.51 0.57 0.56 0.52 0.61 0.74 1 0.74 0.27 0.4
Fe 0.5 0.9 0.88 0.94 0.95 0.92 0.74 1 0.31 0.7
U -0.01 0.45 0.52 0.33 0.39 0.46 0.27 0.31 1 0.54
Th 0.29 0.83 0.77 0.75 0.82 0.77 0.4 0.7 0.54 1
Sr 0.08 0.34 0.3 0.47 0.4 0.31 0.26 0.41 -0.17 0.17
Sb 0.34 0.55 0.57 0.48 0.49 0.5 0.54 0.57 0.34 0.23
Bi 0.37 0.94 0.91 0.93 0.94 0.88 0.48 0.88 0.41 0.83
V 0.45 0.9 0.87 0.92 0.93 0.88 0.53 0.86 0.44 0.87
Ca 0.03 0.36 0.31 0.45 0.39 0.32 0.29 0.42 -0.15 0.16
P 0.28 0.62 0.52 0.72 0.66 0.55 0.47 0.75 -0.18 0.38
La -0.1 0.11 0.04 -0.09 0.02 0.04 0.02 -0.06 0.33 0.43
Cr 0.41 0.94 0.91 0.97 0.98 0.91 0.52 0.9 0.4 0.87
Mg 0.29 0.77 0.72 0.87 0.8 0.68 0.37 0.8 0.08 0.53
Ba 0.49 0.58 0.58 0.47 0.56 0.59 0.41 0.44 0.46 0.71
Ti -0.47 -0.61 -0.73 -0.64 -0.67 -0.67 -0.42 -0.6 -0.28 -0.47
Al 0.44 0.93 0.94 0.98 0.97 0.9 0.53 0.92 0.42 0.8
Na 0 0.15 0.08 0.25 0.1 0.02 -0.06 0.18 -0.08 -0.19
K 0.36 0.6 0.57 0.76 0.66 0.57 0.28 0.66 0.14 0.45
Sc 0.51 0.91 0.9 0.92 0.95 0.94 0.6 0.88 0.47 0.87
clay 0.01 0.39 0.44 0.3 0.35 0.36 0.17 0.31 0.57 0.46
sand -0.32 -0.67 -0.62 -0.63 -0.68 -0.66 -0.38 -0.62 -0.39 -0.72
silt 0.41 0.66 0.57 0.67 0.69 0.66 0.4 0.64 0.2 0.69
TOC 0.42 0.86 0.88 0.91 0.91 0.9 0.64 0.91 0.37 0.73
44
Table 5. Continued.
Correlation coefficients of sediment elemental composition, grain size,
and total organic carbon (TOC) content at Budai well site.
Sr Sb Bi V Ca P La Cr Mg Ba
As 0.08 0.34 0.37 0.45 0.03 0.28 -0.1 0.41 0.29 0.49
Cu 0.34 0.55 0.94 0.9 0.36 0.62 0.11 0.94 0.77 0.58
Pb 0.3 0.57 0.91 0.87 0.31 0.52 0.04 0.91 0.72 0.58
Zn 0.47 0.48 0.93 0.92 0.45 0.72 -0.09 0.97 0.87 0.47
Ni 0.4 0.49 0.94 0.93 0.39 0.66 0.02 0.98 0.8 0.56
Co 0.31 0.5 0.87 0.88 0.32 0.55 0.04 0.91 0.68 0.59
Mn 0.26 0.54 0.48 0.53 0.29 0.47 0.02 0.52 0.37 0.41
Fe 0.41 0.57 0.88 0.86 0.42 0.75 -0.06 0.9 0.8 0.44
U -0.17 0.34 0.41 0.44 -0.15 -0.18 0.33 0.4 0.08 0.46
Th 0.17 0.23 0.83 0.87 0.16 0.38 0.43 0.87 0.53 0.71
Sr 1 0.2 0.32 0.42 0.98 0.75 -0.28 0.41 0.72 -0.17
Sb 0.2 1 0.41 0.31 0.23 0.34 0.01 0.38 0.35 0.27
Bi 0.32 0.41 1 0.87 0.34 0.61 0.09 0.93 0.77 0.5
V 0.42 0.31 0.87 1 0.4 0.62 0.11 0.97 0.79 0.58
Ca 0.98 0.23 0.34 0.4 1 0.75 -0.27 0.39 0.72 -0.21
P 0.75 0.34 0.61 0.62 0.75 1 -0.33 0.65 0.89 -0.02
La -0.28 0.01 0.09 0.11 -0.27 -0.33 1 0.06 -0.34 0.59
Cr 0.41 0.38 0.93 0.97 0.39 0.65 0.06 1 0.81 0.56
Mg 0.72 0.35 0.77 0.79 0.72 0.89 -0.34 0.81 1 0.07
Ba -0.17 0.27 0.5 0.58 -0.21 -0.02 0.59 0.56 0.07 1
Ti -0.2 -0.51 -0.59 -0.54 -0.18 -0.28 0.03 -0.61 -0.46 -0.4
Al 0.36 0.49 0.92 0.93 0.35 0.64 0.02 0.97 0.8 0.56
Na 0.34 0.24 0.05 0.15 0.32 0.38 -0.53 0.15 0.49 -0.35
K 0.56 0.09 0.59 0.77 0.51 0.72 -0.38 0.72 0.85 0.13
Sc 0.36 0.36 0.89 0.97 0.35 0.56 0.13 0.97 0.72 0.63
clay -0.28 0.28 0.4 0.32 -0.27 -0.06 0.24 0.37 0.06 0.41
sand -0.09 -0.16 -0.69 -0.71 -0.1 -0.4 -0.1 -0.72 -0.47 -0.46
silt 0.27 0.05 0.69 0.75 0.27 0.55 0 0.74 0.58 0.37
TOC 0.42 0.35 0.88 0.88 0.42 0.65 -0.07 0.89 0.8 0.41
45
Table 5. Continued.
Correlation coefficients of sediment elemental composition, grain size,
and total organic carbon (TOC) content at Budai well site.
Ti Al Na K Sc clay sand silt TOC
As -0.47 0.44 0 0.36 0.51 0.01 -0.32 0.41 0.42
Cu -0.61 0.93 0.15 0.6 0.91 0.39 -0.67 0.66 0.86
Pb -0.73 0.94 0.08 0.57 0.9 0.44 -0.62 0.57 0.88
Zn -0.64 0.98 0.25 0.76 0.92 0.3 -0.63 0.67 0.91
Ni -0.67 0.97 0.1 0.66 0.95 0.35 -0.68 0.69 0.91
Co -0.67 0.9 0.02 0.57 0.94 0.36 -0.66 0.66 0.9
Mn -0.42 0.53 -0.06 0.28 0.6 0.17 -0.38 0.4 0.64
Fe -0.6 0.92 0.18 0.66 0.88 0.31 -0.62 0.64 0.91
U -0.28 0.42 -0.08 0.14 0.47 0.57 -0.39 0.2 0.37
Th -0.47 0.8 -0.19 0.45 0.87 0.46 -0.72 0.69 0.73
Sr -0.2 0.36 0.34 0.56 0.36 -0.28 -0.09 0.27 0.42
Sb -0.51 0.49 0.24 0.09 0.36 0.28 -0.16 0.05 0.35
Bi -0.59 0.92 0.05 0.59 0.89 0.4 -0.69 0.69 0.88
V -0.54 0.93 0.15 0.77 0.97 0.32 -0.71 0.75 0.88
Ca -0.18 0.35 0.32 0.51 0.35 -0.27 -0.1 0.27 0.42
P -0.28 0.64 0.38 0.72 0.56 -0.06 -0.4 0.55 0.65
La 0.03 0.02 -0.53 -0.38 0.13 0.24 -0.1 0 -0.07
Cr -0.61 0.97 0.15 0.72 0.97 0.37 -0.72 0.74 0.89
Mg -0.46 0.8 0.49 0.85 0.72 0.06 -0.47 0.58 0.8
Ba -0.4 0.56 -0.35 0.13 0.63 0.41 -0.46 0.37 0.41
Ti 1 -0.68 -0.12 -0.29 -0.61 -0.23 0.28 -0.24 -0.56
Al -0.68 1 0.19 0.71 0.93 0.4 -0.66 0.64 0.89
Na -0.12 0.19 1 0.52 0.05 -0.12 0.11 -0.07 0.13
K -0.29 0.71 0.52 1 0.68 0.05 -0.48 0.6 0.72
Sc -0.61 0.93 0.05 0.68 1 0.39 -0.75 0.77 0.89
clay -0.23 0.4 -0.12 0.05 0.39 1 -0.7 0.37 0.32
sand 0.28 -0.66 0.11 -0.48 -0.75 -0.7 1 -0.92 -0.64
silt -0.24 0.64 -0.07 0.6 0.77 0.37 -0.92 1 0.66
TOC -0.56 0.89 0.13 0.72 0.89 0.32 -0.64 0.66 1
46
between clay contents and Fe (r = 0.31) and Mn (r = 0.17) were also good. Silt contents
correlate with Fe (r = 0.64) and Mn (r = 0.40). Arsenic correlates strongly with Fe and
Mn at both well sites (Figure 11).
Trace-element analyses also indicate that levels of Sc, V, Ba, Pb, Co, Ni, and Zn
were elevated in many of the As-rich zones. Concentrations of these elements correlate
with As (Table 3 and 4): Fe (r = 0.50), Mn (r = 0.51), Co (r= 0.57), Ni (r = 0.49), Zn (r =
0.46), Sc (r = 0.51), V (r = 0.45), Ba (r = 0.49) and Pb (r = 0.48). Elements that correlate
with As in the Yichu well site are Fe (r = 0.60), Mn (r = 0.56), Co (r = 0.60), Ni (r =
0.58), Zn (r = 0.53), Sc (r = 0.54), V (r = 0.62), Ba (r = 0.24) and Pb (r = 0.62), Cu (r =
0.55), Sr (r = 0.52), and Bi (r = 0.64). Arsenic contents also correlate well with major
ions. At the Budai well site, As correlates with K (r = 0.36), Mg (r = 0.29), Al (r = 0.44),
and P (r = 0.28). In the Yichu sediment, As correlates with Fe (r = 0.60), K (r = 0.47), Mg
(r = 0.56), Ca (r = 0.46), Al (r = 0.58), and P (r = 0.41).
These correlations suggest that bacterial reduction or dissolution of Fe- or Mn-
oxides is involved in mobilization of Fe, Mn and As under moderately reducing
conditions. Arsenic in these sediments also correlates with major ions K, Mg, Ca, Al, and
P, suggesting that Fe- or K-rich silicate minerals such as biotite and muscovite or even
Ca-rich minerals such as apatite may locally host As in sediments.
47
Figure 11. Line charts showing As correlation with Fe and Mn at both well sites.
48
Total Organic Carbon
Data collected from LECO organic carbon analysis show that TOC in sediments
ranges from 0.21 to 0.72% in sediments sampled from Budai (Table 6). TOC levels in
sediments from Yichu range from 0.15 to 0.61% (Table 7). The mean concentrations of
TOC in sediments are 0.39% and 0.36% in the Budai and Yichu wells, respectively.
Levels of As correlate with TOC at both sites (Figure 12). The correlation coefficients for
As with TOC in the Budai and Yichu well sites are 0.42 and 0.51, respectively. At the
Budai well site, high TOC contents occur at two major depth intervals: 50 to 90 meters
and 105 to 120 meters. Some of the highest levels of As in the Budai sediment also are
contained within these two intervals. Arsenics levels of 28.0, 29.9, and 32.3 ppm were
found at depths of 105, 110, and 60 meters, respectively. The highest concentrations of
TOC in the Budai samples are found at depths of 50, 55, 60, 75, and 85 meters. TOC
contents at these depths are 0.53, 0.53, 0.76, 0.67, and 0.68%, respectively. TOC also
correlated positively with Fe and Mn, with correlation coefficients of 0.91 and 0.64,
respectively. TOC also correlates well with other trace elements in the Budai core
samples. Correlation coefficients for TOC with trace elements in Budai samples are as
follows: Cu (r = 0.86), Pb (r = 0.88), Zn (r = 0.91), Ni (r = 0.91), Co (r = 0.90), Ti (r =
0.88), Th (r = 0.73), Bi (r = 0.88), V (r = 0.88), P (r = 0.65), Cr (r = 0.89), Mg (r = 0.80),
Al ( r = 0.89), K (r = 0.72), and Sc (r = 0.89). High TOC contents are linked to silt- and
clay-dominated portions of core samples. The silt content of core samples from Budai
had the strongest correlation with TOC (r = 0.66). Results of sequential leaching
experiments on selected sediments (see section below) from the Budai well also confirm
a strong correlation of organic carbon with As.
49
Table 6. Total organic carbon (TOC) in Budai sediments.
Budai
Depth
Meters
Initial
Sample
Weight
Filter
Weight
Filter +
Sample
Weight
Digested
Sample
Weight Yield
%
Carbonate
%
Organic
Carbon
0 0.2645 0.1301 0.3848 0.2547 0.962949 3.705104 0.393
5 0.2627 0.1326 0.3755 0.2429 0.924629 7.537115 0.482
10 0.2638 0.1269 0.3695 0.2426 0.919636 8.036391 0.218
15 0.279 0.1255 0.3899 0.2644 0.94767 5.232975 0.273
20 0.2612 0.1277 0.3782 0.2505 0.959035 4.096478 0.316
25 0.2617 0.1271 0.3781 0.251 0.959113 4.088651 0.348
30 0.2599 0.1263 0.371 0.2447 0.941516 5.848403 0.473
35 0.2516 0.1276 0.3661 0.2385 0.947933 5.206677 0.318
40 0.2517 0.1284 0.3668 0.2384 0.947159 5.284068 0.338
45 0.2806 0.1273 0.3933 0.266 0.947969 5.203136 0.319
50 0.2538 0.1274 0.3637 0.2363 0.931048 6.895193 0.525
55 0.2546 0.129 0.3669 0.2379 0.934407 6.559309 0.525
60 0.2577 0.1296 0.3683 0.2387 0.926271 7.372914 0.761
65 0.255 0.1301 0.3706 0.2405 0.943137 5.686275 0.371
70 0.2601 0.1305 0.3778 0.2473 0.950788 4.921184 0.458
75 0.2558 0.1295 0.3723 0.2428 0.949179 5.082095 0.67
80 0.2663 0.1312 0.3888 0.2576 0.96733 3.266992 0.374
85 0.2586 0.1291 0.3753 0.2462 0.952049 4.79505 0.679
90 0.2615 0.129 0.3844 0.2554 0.976673 2.332696 0.413
95 0.2569 0.1311 0.3762 0.2451 0.954068 4.593227 0.227
100 0.2609 0.1294 0.3847 0.2553 0.978536 2.146416 0.295
105 0.2598 0.1311 0.3756 0.2445 0.941109 5.889145 0.518
110 0.2522 0.1273 0.3751 0.2478 0.982554 1.744647 0.385
115 0.2541 0.1286 0.3709 0.2423 0.953562 4.643841 0.41
120 0.2534 0.126 0.3728 0.2468 0.973954 2.604578 0.514
125 0.2537 0.1279 0.3779 0.25 0.985416 1.458415 0.229
130 0.2536 0.1287 0.3771 0.2484 0.979495 2.050473 0.213
135 0.2775 0.1289 0.4026 0.2737 0.986306 1.369369 0.246
140 0.2638 0.1275 0.3884 0.2609 0.989007 1.099318 0.335
145 0.2753 0.1282 0.4004 0.2722 0.98874 1.126044 0.217
150 0.2532 0.1292 0.3782 0.249 0.983412 1.658768 0.217
Table 7. Total organic carbon (TOC) in Yichu sediments.
50
Yichu
Depth
Meters
Initial
Sample
Weight
Filter
Weight
Filter +
Sample
Weight
Digested
Sample
Weight Yield
%
Carbonate
%
Organic
Carbon
5 0.2576 0.1239 0.3732 0.2493 0.96778 3.22205 0.274
10 0.2774 0.1239 0.3778 0.2539 0.915285 8.471521 0.465
15 0.2726 0.1274 0.3843 0.2569 0.942406 5.759354 0.549
20 0.264 0.1254 0.3816 0.2562 0.970455 2.954545 0.237
25 0.2528 0.124 0.3697 0.2457 0.971915 2.808544 0.288
30 0.2796 0.1244 0.3961 0.2717 0.971745 2.825465 0.328
35 0.2712 0.1252 0.3875 0.2623 0.967183 3.281711 0.321
40 0.2578 0.1277 0.3717 0.244 0.94647 5.352987 0.573
45 0.2533 0.1255 0.3692 0.2437 0.9621 3.789972 0.261
50 0.2541 0.1263 0.3664 0.2401 0.944904 5.509642 0.588
55 0.2661 0.1267 0.3845 0.2578 0.968809 3.119128 0.339
60 0.2657 0.1253 0.3775 0.2522 0.949191 5.080918 0.291
65 0.2653 0.1248 0.3805 0.2557 0.963815 3.618545 0.284
70 0.2778 0.1256 0.3986 0.273 0.982721 1.727862 0.253
75 0.2622 0.1278 0.3861 0.2583 0.985126 1.487414 0.455
80 0.2632 0.1252 0.3778 0.2526 0.959726 4.027356 0.518
85 0.2545 0.1244 0.3685 0.2441 0.959136 4.086444 0.559
90 0.2526 0.1258 0.3741 0.2483 0.982977 1.702296 0.185
95 0.2547 0.1251 0.3778 0.2527 0.992148 0.785238 0.151
100 0.2568 0.1268 0.3793 0.2525 0.983255 1.674455 0.201
105 0.251 0.125 0.3684 0.2434 0.969721 3.027888 0.432
110 0.26 0.1248 0.3776 0.2528 0.972308 2.769231 0.418
115 0.254 0.1263 0.3751 0.2488 0.979528 2.047244 0.258
120 0.2506 0.1307 0.3794 0.2487 0.992418 0.75818 0.211
125 0.2594 0.1304 0.3822 0.2518 0.970702 2.929838 0.273
130 0.2546 0.1304 0.3812 0.2508 0.985075 1.492537 0.2
135 0.2536 0.1303 0.3796 0.2493 0.983044 1.695584 0.232
140 0.2525 0.1293 0.3773 0.248 0.982178 1.782178 0.192
145 0.2736 0.1292 0.3867 0.2575 0.941155 5.884503 0.478
150 0.261 0.1275 0.3882 0.2607 0.998851 0.114943 0.234
155 0.2521 0.1279 0.3616 0.2337 0.927013 7.298691 0.482
160 0.2653 0.1287 0.3707 0.242 0.912175 8.78251 0.611
165 0.2733 0.1293 0.3989 0.2696 0.986462 1.353824 0.15
170 0.2573 0.1266 0.3791 0.2525 0.981345 1.865527 0.184
175 0.2622 0.1268 0.3753 0.2485 0.94775 5.225019 0.493
180 0.2597 0.1288 0.3762 0.2474 0.952638 4.736234 0.545
185 0.2535 0.13 0.3739 0.2439 0.96213 3.786982 0.35
190 0.2565 0.1292 0.3707 0.2415 0.94152 5.847953 0.532
195 0.2627 0.1269 0.3777 0.2508 0.954701 4.529882 0.418
200 0.2704 0.1269 0.3869 0.26 0.961538 3.846154 0.42
150a 0.2592 0.1277 0.3861 0.2584 0.996914 0.308642 0.221
51
Figure 12. Total organic carbon (TOC) correlated with As in Yichu and Budai well sites.
52
Sequential Extraction
In order to determine the relative amounts of As contained in different host pools
(clay, carbonate, organic matter, amorphous and crystalline Mn-oxides, Fe-oxides,
silicate minerals), sequential extraction procedures were conducted on the Budai
sediment samples collected from As-rich zones (at 6, 60, 105, 110, and 150 m of depth).
Figures 13 and 14 show the fraction of leachable and non-leachable As-bearing solid
phases at different depths. Only a small fraction of As (0.07 to 4.72%) was present in the
water soluble/leachable form, implying the relatively high affinity of As with sediments.
Among various leachable host pools, organic matter and exchangeable portions of oxides
appear to be the major binding phases (53.3-78.2%). Mn- and Fe-oxyhydroxides, account
for 19.9-42.4% of total leachable As. Arsenic bound in clay and carbonate solids
represent relatively small percentages (1.6 to 4.2%). The relative proportions of different
As pools do not show much depth variation. Amounts of non-leachable As were
calculated as the difference between the total digested As and the sum of total leachable
As from various extraction steps. The results show that most As (>60%) was associated
with recalcitrant solids (crystalline silicates and sulfides, organic matter, etc.) as
?residual? phase. These results imply that dissolved As in groundwater is a relatively
small fraction of As pools in sediments.
The amounts of Fe in hydroxylamine extracts (53-82%) are the highest among
various extractable pools, suggesting that Fe(III) oxyhydroxides are the dominant
leachable Fe solid phase (Figure 15). Percentages of non-leachable Fe, constituting
approximately 53 to 70% of total solid Fe, are fairly consistent with those of As (Figure
16). The ?residual? solid Fe is likely incorporated in silicate and sulfide minerals. The
53
Figure 13. Graph showing variations of leachable As concentrations in different host
materials in the Budai well.
0% 20% 40% 60% 80% 100%
Budai 5m
Budai 60m
Budai 105m
Budai 110m
Budai 150m
% As in Each Leachable Fraction
Water Leachable %
Carbonate & Clay %
Organic Matter & Exchangeable
Oxides %
Amorphous Mn Oxyhydroxides
%
Amorphous Fe Oxyhydroxides &
Crystalline Mn Oxyhydroxides %
54
Figure 14. Graph showing variations of As fraction (including both leachable and non-
leachable form) associated with different host materials in the Budai well.
0% 20% 40% 60% 80% 100%
Budai 5m
Budai 60m
Budai 105m
Budai 110m
Budai 150m
% As in Each Fraction
Water Leachable %
Carbonate & Clay %
Organic Matter & Exchangeable
Oxides %
Amorphous Mn Oxyhydroxides
%
Amorphous Fe Oxyhydroxides &
Crystalline Mn Oxyhydroxides %
Non-Leachable %
55
Figure 15. Graph showing variations of leachable Fe concentrations in different host
materials in Budai well.
0% 20% 40% 60% 80% 100%
Budai 5m
Budai 60m
Budai 105m
Budai 110m
Budai 150m
% Fe in Each Leachable Fraction
Water Leachable %
Carbonate & Clay %
Organic Matter & Exchangeable
Oxides %
Amorphous Mn Oxyhydroxides
%
Amorphous Fe Oxyhydroxides &
Crystalline Mn Oxyhydroxides %
56
Figure 16. Graph showing variations of Fe fraction (including both leachable and non-
leachable form) associated with different host materials in the Budai well.
0% 20% 40% 60% 80% 100%
Budai 5m
Budai 60m
Budai 105m
Budai 110m
Budai 150m
% Fe in Each Fraction
Water Leachable %
Carbonate & Clay %
Organic Matter & Exchangeable
Oxides %
Amorphous Mn Oxyhydroxides
%
Amorphous Fe Oxyhydroxides &
Crystalline Mn Oxyhydroxides %
Non-Leachable %
57
fraction of Fe in Na3P2O7 extracts (13-30%) from all five depths (Figure 15) are
significantly smaller than those of As extracted from phosphate solution, indicating that a
substantial portion of As extracted in the Na3P2O7 step is liberated from non-Fe-bearing
phases such as organic matter or As oxides. Percentages of hydroxylamine-extractable Fe
(53-82%) in the sediments are higher than the fraction of As in hydroxylamine extracts
(20-43%), suggesting that organic matter and other non-Fe-bearing solids may constitute
another important arsenic pool in addition to Fe(III) oxyhydroxides.
58
Groundwater Chemistry and Hydrogeology
Groundwater Geochemistry
Major ion compositions of groundwater samples from the Budai site at different
depths (59, 101, 145 m) are presented in Table 8. The major ions in the sampled
groundwaters are Na (156-5070 mg/kg) and Cl (53.5-9430 mg/kg), and the Piper diagram
indicates that the groundwater is mostly mixed Ca-HCO3-Na-Cl or Na-Cl type (Figure
17). The Budai groundwater shows extreme salinization and Na and Cl enrichment as
compared to most Chianan Plain groundwater. Na/Cl molar ratios in two shallow
groundwater samples from 59 and 101 m (0.70-0.83) are comparable with that of
seawater (0.86), indicating strong influence from saltwater intrusion. By contrast, the
deep groundwater (145 m) has much lower salinity and higher Na/Cl ratios compared to
those of seawater.
Occurrence of high alkalinity or dissolved HCO3- (210-370 mg/kg) in
groundwater is probably due to active biodegradation of organic matter or bacterial
reduction of Fe- and Mn-oxyhydroxides in the aquifers.
Lower concentrations of PO43-, NO2-, and NO3-(<1 mg/kg) in groundwater suggest
that ionic competition of nitrate and phosphate for sorbing sites is not likely the major
mechanism for As mobilization (Nath et al., 2008). Such processes have been proposed
as a major mechanism for maintaining high As levels in Bangladesh groundwater (Swartz
et al., 2004).
59
Table 8. Compositions of selected ions and pH-ORP values of Budai groundwater.
Depth Ca Mg Na K Cl SO4 Alkalinity pH ORP Eh1
(m) mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mV mV
59 nnnnnn144 266 5070 145 9430 104 210 7.53 -71 149
101 ddd 385 232 1840 385 4030 1 280 7.10 -127 93
145 22.9 9.7 156 22.9 53.5 20.5 370 7.64 -56 164
Seawater2 411 1290 10760 399 19350 2710 8.07
Depth Br Sr Rb Mn As Fe Ba Si Pb I
(m) ?g/kg ?g/kg ?g/kg ?g/kg ?g/kg ?g/kg ?g/kg ?g/kg ?g/kg ?g/kg
59 32600 4160 46 232 60 <1000 1110 <20000 9.6 914
101 14300 3810 17 287 35 <100 2610 <20000 1.8 512
145 273 299 5 1 48 <100 64 7600 4.7 72
Seawater2 67300 8100 120 0.2 3 2 10 5000 0.03 64
1Eh = ORP + 220 (at 20?C, Weight, 2008)
2Drever (1997)
60
Figure 17. Piper diagram illustrating the main hydro-chemical facies of Chianan Plain
groundwater (data from Nath et al., 2008 and this study). In Budai well (shown in black
circles), the overall groundwater type is a mixed Ca-HCO3-Na-Cl or Na-Cl type.
61
Field oxidation reduction potential (ORP) values of groundwaters collected from
varying depths range from -56 to -172 mV (Table 8). Such moderately reducing
conditions are favorable for bacterial iron reduction.
Elevated concentrations of Br, Sr, Rb, Fe, Mn, As, I, Ba, and Si are noted in two
shallow groundwater samples from 59 and 101 m (Table 8). Elevated Sr, Br, and Rb
contents indicate the influence of saltwater intrusion. However, the concentrations of As,
Mn, Fe, Ba (common in Mn-oxyhydroxides), and Si are significantly higher than those in
seawater, which implies that possible sources of As are the reductive dissolution of Fe-
and Mn-oxyhydroxides or rapid chemical break down of silicate and phyllosilicate
minerals such as biotite ( Shamsudduha et al., 2008; Itai et al., 2008). The composition of
the most rapidly weathering Fe- and Mg-rich silicate mineral (e.g., biotite) in the host
sediments may control the trace-element composition of groundwaters.
The ?D and ?18O values of the groundwater samples from the Budai well are in
the range of -62.1 to -31.8 ? and -8.5 and -4.1 ? relative to SMOW, respectively.
Figure 18 shows ?D and ?18O ratios of groundwater in the Chianan Plain and mud
volcano fluids relative to standard mean ocean water (SMOW), mud volcanoes fluids
(Nath et al., 2008), and the local meteoric water line (LMWL). The isotopic compositions
of groundwater and LMWL were taken from data compiled by Wang et al. (2001). Most
groundwater samples have isotopic compositions resembling that of LMWL. However,
several groundwater samples bear close relationship to the isotopic composition of
seawater (Figure 18), apparently reflecting the influence of saltwater intrusion. The mud
volcano fluids display significant oxygen isotope enrichment relative to LMWL, as
characterized by most geothermal or hot spring fluids (Craig et al., 1966). Apparently the
62
Figure 18. ?D and ?18O ratios of Chianan Plain groundwater plotted along with seawater,
mud volcano fluids, and local meteoric water line (Wang et al., 2001). The Budai
groundwater samples have isotopic compositions resembling that of local meteoric water
with slightly elevated 18O composition.
63
isotopic composition of mud volcano fluids has been modified by either (1) exchange
with 18O-rich silicate and carbonate rocks in the crust, or (2) mixing with deep
isotopically heavier brines. The ?D-?18O trends of mud volcano fluids do not converge
toward seawater, but instead intersect the LMWL with compositions close to those of
present-day precipitation in southwest Taiwan. Thus, the mud volcano fluids did not
evolve solely from seawater but likely originated from mixing of local meteoric sources
with deep brines. The ?D and ?18O ratios of Budai groundwater are similar or slightly
enriched in 18O compared to local meteoric water, indicating that their origin may be
from local surface water. The positive 18O shifts may be caused by evaporation before
infiltration or exchange with 18O-rich fluids or sediments.
Overland Flow and Transport Modeling
Arsenic derived from weathering of source rocks and dewatering fluids from mud
volcanoes may be carried by the streams and channel networks as suspended particles
and dissolved loads in sediment and eventually be deposited in the Chianan Plain.
ARC/INFO software and digital elevation models (DEM) were used to predict where the
sediments will be transported downstream into the alluvial plain. The calculations are
based on a physics-based computational watershed model. The watershed model is
simulated as flow occurring on overland flow strips that are routed into channel networks
once they reach river and stream channels. A map of the Taiwan watershed was
generated based on DEM using hydrology tools of ArcMap. Modeling sediment and
adsorbed contaminant transport within the contributing watersheds can help to identify
possible arsenic sources. The simulations included 8 major mud volcanoes as potential
point sources in Chiayi (Kuanzeling and Chunlung), Tainan (Yenshuikeng), and
64
Kaohsiung (Kunshuiping, Wushanting, Hsiaokunshui, Dakunshui, Yannue Lake)
counties in southwest Taiwan (Figure 19). The modeling results show that there seems to
be a geographic correlation between As hot-spots and the mud-volcano outflow areas in
southwest Taiwan. Fluids and muds released from seven (out of 8) active mud volcanoes
are traveling overland and routed through major river channels in the Chianan Plain. By
contrast, only one mud volcano (Yannue Lake in Kaohsiung County) has outflows
terminated in the Pingtung alluvial plain, where sediments are largely As-free.
Future modeling efforts should include other As- and Fe-rich bedrock (e.g., metamorphic
slate, black shales, coal/peat beds etc.) exposed in Western Foothills and Central
Mountain Range as potential sources of As. For example, the Chuko black shale exposed
upstream of Pachang River (Figure 19) is a potential As source. Erosion of the Chuko
black shale in the headwaters may result in arsenic-bearing shale fragments that are
carried as particulate matter by the Pachang River and deposited in the Chianan Plain.
65
Figure 19. Overland transport model for the Chianan Plain and Pingtung Plain
watersheds. Blue triangles show the locations of active mud volcanoes in the area. The
results show that As-rich fluids and muds released from Kuanzeling and Chunlung mud
volcanoes are transported (green lines) via Pachang and Shishui rivers into the arsenic
and BFD hot spots (yellow circles) in the Chianan Plain. As-rich bedrock such as black
shale also crop-out in the headwaters of Pachang River.
Groundwater
As Levels
?g/kg
66
Aquifer-Flushing Model
To gain insight into how sea level and hydraulic gradient may control
concentrations of groundwater arsenic in alluvial aquifers, 1-D simulations of freshwater
flushing of an As-contaminated aquifer were conducted. The following solute transport
equation takes into account the effect of advection, hydrodynamic dispersion, adsorption,
and chemical reactions on plume migration:
(1)
Where Ci, D, and Ri represent concentration, dispersion coefficient, and chemical reaction
rate (by production and consumption) of solute i in a flow system with velocity v. The
terms ? and ? are grain density and porosity, respectively. Traditional Freundlich and
Langmuir sorption theories use distribution coefficients Kd to calculate the ratios of
sorbed to dissolved ions. The length along the aquifer is represented by x. The reaction
term Ri is for all possible mineral precipitation-dissolution, speciation, redox, and surface
complexation reactions (Dzombak and Morel et al., 1990; Stumm et al., 1992). In the
model, an alluvial aquifer containing 300 ?g/kg of arsenic is infiltrated by dilute
rainwater at 25?C. Assuming a Darcian flow rate of 0.1 m/day, the program (Appendix
3) calculates the distribution of arsenic along the flowpath of 1000 m. To calculate the
minimum time required to flush out existing arsenic loads dissolved in groundwater, the
reaction term Ri is set to zero (i.e., no arsenic is released from chemical reactions). The
modeling results show that most initial mobile arsenic loads in a considerable part of the
aquifer can be flushed out in a few thousand years (Figure 20). The modeling results are
consistent with the ages of As-rich groundwater estimated for Bangladesh Holocene
iiidi RxCvxCDKtC ?????????? 2
2)1(
??
67
floodplain aquifers (Van Geen et al., 2008). However, longer groundwater residence
times (with 14C groundwater ages of 10-40 ka) have been reported in the alluvial plain of
Taiwan near the study sites (Chen and Liu et al., 2007). It should be noted that the time
required to flush the aquifer would be considerably longer for slow flow rates and higher
Kd values (not shown), or as long as the geochemical conditions allow continuous arsenic
release from solid phases in a reaction-dominated system. Slow arsenic flushing in
alluvial aquifers will likely continue if the acceleration of sea-level rise (Church and
White et al., 2006) is maintained.
68
Figure 20. Aquifer flushing model showing breakthrough curve for As at 50, 1000,
2000, and 4000 years.
0 100 200 300 400 500 600 700 800 900 1000
0
0 . 2
0 . 4
0 . 6
0 . 8
1
t = 5 0 y e a r
t = 1 0 0 0 y e a r s
t = 2 0 0 0 y e a r s
t = 4 0 0 0 y e a r s
T r a ve l d i st a n ce ( m )
R
e
l
a
t
i
ve
co
n
ce
n
t
r
a
t
i
o
n
69
CONCLUSIONS
Groundwater at the Budai and Yichu drilling sites is mostly of a mixed Ca-HCO3-
Na-Cl or Na-Cl type. Enrichment of Na and Cl and elevated Sr, Br, and Rb contents
indicate the influence from saltwater intrusion. However, concentrations of As, Mn, Fe,
Ba, and Si in groundwater are significantly higher than those in seawater, implying that
reductive dissolution of Fe- and Mn-oxyhydroxides and/or rapid chemical break down of
silicate and phyllosilicate minerals such as biotite are potential sources of As. Low ORP
values of groundwaters (-56 to -172 mV) indicate slightly reducing conditions, which are
favorable for bacterial iron reduction. Occurrence of high alkalinity or dissolved HCO3
(210-370 mg/kg) in groundwater is probably due to active biodegradation of organic
matter or bacterial reduction of Fe- and Mn-oxides in the aquifers. Lower concentrations
of PO43-, NO22-, and NO3- (<1 mg/kg) may suggest that ionic competition of nitrate and
phosphate for sorbing sites is not the major mechanism for As mobilization.
Arsenic concentrations in Budai and Yichu sediments are correlated well with
both siderophile (e.g., Fe, Ni, Co) and chalcophile (e.g., Cu, Zn) elements.
Concentrations of major ions K, Mg, Ca, Al, and P are also higher in the As- and Fe-rich
zones. These correlations suggest that high As levels in sediment are likely associated
with Fe-oxyhydroxides, silicate minerals (e.g., biotite, amphibole), apatite, and sulfide
solids. Solid phase As levels in the Chianan Plain sediments are strongly depth-
70
dependent. Arsenic concentrations in fine-grained sediments are much higher than the
average crustal levels, and mobilization of As under reducing conditions might lead to
elevated aqueous As concentrations. The poor correlation between As and Fe in Na-
pyrophosphate (Na3P2O7) extracted fractions suggests that As may be associated in part
with organic matter in addition to Fe-bearing phases in sediments. Levels of As in
sediments correlate well with total organic carbon content at both sites (r = 0.42 and
0.51), furthering supporting the strong link between arsenic and organic matter in alluvial
sediments. A significant fraction of the As (>60%) is associated with recalcitrant solids
(crystalline silicates and sulfides, organic matter, etc.) as ?residual? phase. This result
may imply that dissolved As loads in groundwater only represent a relatively small
fraction of total As pools in the sediments. The results of aquifer flushing models indicate
that most initial mobile arsenic loads in a considerable part of alluvial aquifers may be
flushed out in a few thousand years. The modeling results are consistent with the young
ages of As-rich groundwater estimated for Holocene floodplain aquifers in Bangladesh
and Taiwan (Van Geen et al., 2008; Chen and Liu et al., 2007). Levels of As and humic
substances in the Chianan Plain sediments are notably higher than those in Bangladesh
and West Bengal (generally < 10 mg/kg of As). Overland flow modeling suggests that
arsenic derived from dewatering fluids from mud volcanoes and weathering of potential
source rocks in the headwaters may be carried by the streams and eventually deposited in
the Chianan Plain. Humic substances along with silty sands and clays were deposited
during the Holocene. Because of the biological productivity of the wetlands in Chianan
Plain, abundant humus material has been deposited with periodic flooding events. It is
possible that elevated As in combination with high levels of humic compounds may be
71
the primary etiological factors for unique occurrence of Blackfoot disease in
southwestern Taiwan.
72
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Appendix 1. Locations of mud volcanoes Chianan Plain
Mud volcanoes Located in Chianan Plain
Latitude Longitude
Kuanzeling, Chiayi County 23.339884 120.504725
Yenshuikeng, Tainan County 23.174894 120.297420
Chunlung, Chiayi County 23.373895 120.562417
Kunshuiping, Kaohsiung County 22.769922 120.338082
Wushanting, Kaohsiung County 22.796510 120.406530
Hsiaokunshui, Kaohsiung County 22.886012 120.390085
Yannue Lake, Kaohsiung County 23.010871 120.666004
Dakunshui, Kaohsiung County 22.886012 120.390085
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Appendix 2. Matlab? code for correlation coefficients.
fid=fopen('Budai.txt', 'r')
metals = textscan(fid, '%f %f %f %f %f %f %f %f %f %f %f %f %f %f %f %f %f
%f %f %f %f %f %f %f %f %f', 'headerlines', 1)
As=metals{1}
Cu=metals{2}
Pb=metals{3}
Zn=metals{4}
Ni=metals{5}
Co=metals{6}
Mn=metals{7}
Fe=metals{8}
U=metals{9}
Th=metals{10}
Sr=metals{11}
Sb=metals{12}
Bi=metals{13}
V=metals{14}
Ca=metals{15}
P=metals{16}
La=metals{17}
Cr=metals{18}
Mg=metals{19}
Ba=metals{20}
Ti=metals{21}
Al=metals{22}
Na=metals{23}
K=metals{24}
Sc=metals{25}
[rCu,pCu]=corrcoef(As,Cu)
[rPb,pPb]=corrcoef(As,Pb)
[rZn,pZn]=corrcoef(As,Zn)
[rNi,pNi]=corrcoef(As,Ni)
[rCo,pCo]=corrcoef(As,Co)
[rMn,pMn]=corrcoef(As,Mn)
[rFe,pFe]=corrcoef(As,Fe)
[rU,pFe]=corrcoef(As,U)
[rTh,pTh]=corrcoef(As,Th)
[rSr,pSr]=corrcoef(As,Sr)
[rSb,pSb]=corrcoef(As,Sb)
[rBi,pBi]=corrcoef(As,Bi)
[rV,pV]=corrcoef(As,V)
[rCa,pCa]=corrcoef(As,Ca)
[rP,pP]=corrcoef(As,P)
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[rLa,pLa]=corrcoef(As,La)
[rCr,pCr]=corrcoef(As,Cr)
[rMg,pMg]=corrcoef(As,Mg)
[rBa,pBa]=corrcoef(As,Ba)
[rTi,pTi]=corrcoef(As,Ti)
[rAl,pFe]=corrcoef(As,Al)
[rNa,pNa]=corrcoef(As,Na)
[rK,pK]=corrcoef(As,K)
[rSc,pSc]=corrcoef(As,Sc)
meanAs=mean(As)
stdAs=std(As)
87
Appendix 3. Matlab? codes for simulating 1-D aquifer flushing
% transport parameters
ntimes=500 %
dt_year = 10
dt = dt_year*365 % days
dx = 25 % meters
c0 = 200 % initial plume concentrations
nx = 40
rho_rock = 2.2 % sediment density, gm/cm^3
porosity = 0.25
kd = 5 % distribution coefficient, cm3/g
rf = 1+(rho_rock*kd/porosity) % retardation factor
alpha = 10 % dispersivity in meter
phi = 0.15 % porosity
vx= 0.1 % flow velocity m/day
Dcoef = vx*alpha % m^2/day
% initial plume concentration, initial plume at nx=2
for i=1:nx
conc(i)=200
rel_conc(i)=conc(i)/c0
disx(i)=dx*(i-1)
end
conc(1)= 0
rel_conc(1) = 0
% calculate colute concentration with time
for n=1:ntimes
for i=2:nx-1
disp=Dcoef*(conc(i-1)-2*conc(i)+conc(i+1))/dx/dx
adv= -vx*(conc(i)-conc(i-1))/rf
concnew(i)=dt*(adv+disp)/rf+conc(i)
end
% update concentration at each node
for i=2:nx-1
conc(i)=concnew(i)
rel_conc(i)=conc(i)/c0
end
% post-processing
if n==5
ylim([0,1.10])
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plot(disx,rel_conc,'g', 'Linewidth',3)
text(50, 0.60, 't = 50 year','fontsize',10)
hold on
end
if n==100
ylim([0,1.10])
plot(disx,rel_conc,'r','Linewidth',3)
text(250, 0.48, 't = 1000 years','fontsize',10)
hold on
end
if n==200
ylim([0,1.10])
plot(disx,rel_conc,'b', 'Linewidth',3)
text(600, 0.28, 't = 2000 years','fontsize',10)
end
if n==400
ylim([0,1.10])
plot(disx,rel_conc,'k', 'Linewidth',3)
text(710, 0.12, 't = 4000 years')
xlabel('Travel distance (m)','fontsize',12)
ylabel('Relative concentration','fontsize',12)
end
end
% title ('alpha=10 m, retardation factor = 10')