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. 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Redox control of arsenic mobilization in Bangladesh groundwater. Applied Geochemistry, 19, 201-214. 84 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 85 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) 86 [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]) 88 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')