HYDROLOGY AND WATER CHEMISTRY IN WEEKS BAY, ALABAMA: IMPLICATIONS FOR MERCURY BIOACCUMULATION Except where reference is made to the work of others, the work described in this thesis is my own or was done in collaboration with my advisory committee. This thesis does not include proprietary or classified information. ________________________________ Robert Horvath Monrreal Certificate of Approval: ________________________ ________________________ Lorraine W. Wolf Ming-Kuo Lee, Chair Professor Professor Geology Geology ________________________ ________________________ James A. Saunders George T. Flowers Professor Interim Dean Geology Graduate School HYDROLOGY AND WATER CHEMISTRY IN WEEKS BAY, ALABAMA: IMPLICATIONS FOR MERCURY BIOACCUMULATION Robert Horvath Monrreal A Thesis Submitted to the Graduate Faculty of Auburn University in Partial Fulfillment of the Requirement for the Degree of Masters of Science Auburn, Alabama August 4, 2007 iii HYDROLOGY AND WATER CHEMISTRY IN WEEKS BAY, ALABAMA: IMPLICATIONS FOR MERCURY BIOACCUMULATION Robert Horvath Monrreal Permission is granted to Auburn University to make copies of this thesis at its discretion, upon request of individuals or institutions at their expense. The author reserves all publications rights. ______________________________ Signature of Author ______________________________ Date of Graduation iv VITA Robert Horvath Monrreal, son of John Monrreal and Mary Horvath, was born on April 25, 1981, in Detroit, Michigan. He attended Pope Paul VI Catholic High School in Fairfax, Virginia, and graduated in the spring of 1999. After high school he attended Old Dominion University in Norfolk, Virginia, and graduated in December 2003 with a Bachelors of Science in Geology. Shortly after receiving his undergraduate degree, he started graduate school at Auburn University in the fall of 2004. v THESIS ABSTRACT HYDROLOGY AND WATER CHEMISTRY IN WEEKS BAY, ALABAMA: IMPLICATIONS FOR MERCURY BIOACCUMULATION Robert H. Monrreal Master of Science, August 4, 2007 (B.S. Old Dominion University, 2003) 78 Typed Pages Directed by Ming-Kuo Lee Recent studies within Weeks Bay, an estuary of Mobile Bay, have revealed that both water and fish are contaminated by mercury (Hg), an element known to be extremely toxic to wildlife and humans. Seasonal variations of total mercury in precipitation were analyzed using data collected at two Mercury Deposition Network stations. The results showed that the most likely source for mercury is from atmospheric deposition. Once in an aqueous environment, inorganic mercury can methylate to toxic methylmercury (CH3Hg). To understand the water chemistry in which mercury methylates, seasonal measurements of temperature, pH, dissolved oxygen (DO), oxidation-reduction potential (ORP), specific conductance (SpC), and dissolved organic carbon (DOC) were conducted in water samples taken from Weeks Bay, from groundwater wells, and from a major surface tributary. Correlations of these constituents indicate that high salinity and pH seawater invade below acidic, low salinity water in the vi bay to form a wedge interface. The mixing of warm, acidic, and low-salinity waters in the upper bay (near the mouth of the Fish River) provide a favorable conditions for Hg methylation. Low DO and ORP values observed in this mixing zone indicate high microbial activities that may initialize Hg methylation. Geochemical analyses show that most major ions exhibit conservative behavior while sulfate shows slight depletion during water mixing. River and bay water are enriched in 18O and 2H relative to groundwater, indicating they have undergone greater evaporation or mixing with isotopically heavier seawater. In summary, water chemistries can vary both spatially and seasonally in the bay based on environmental conditions. Storms, stream discharge, and seasonal climate changes can affect the conditions within the bay. Seasonal data indicate that high temperature, low pH, low conductivity, low DO, conditions ideal for mercury methylation are found in the bay especially during summer months. vii ACKNOWLEDGMENTS This research was supported by funds from the Southeast Section of the Geological Society of America, Vulcan Materials, and the Alabama Geological Society. I would like to thank Dr. Ming-Kuo Lee, Dr. James Saunders, and Dr. Lorraine Wolf for their guidance and assistance throughout this project. The author would also like to thank Scott Phipps and Mike Shelton of the Weeks Bay National Estuary Research Reserve and James Robinson of the United States Geological Survey for contributing their knowledge and allowing the use of facilities and equipment as well as Dr. Yang Wang of Florida State University for completing isotope analysis. Also thanks to Scott Caruso and Ian Oleschig for their assistance in collecting field data. Finally, I would like to thank my family and friends for their support during my time spent at Auburn University. viii Style manual or journal used: Geology Computer software used: Microsoft Word, Microsoft Excel, Microsoft PowerPoint, Tecplot, Geochemist Workbench, Surfer, AcrGIS, Adobe Illustrator ix TABLE OF CONTENTS LIST OF FIGURES............................................................................................................xi LIST OF TABLES...........................................................................................................xiii INTRODUCTION...............................................................................................................1 MERCURY CONTAMINATION......................................................................................5 Mercury sources......................................................................................................5 Mercury speciation..................................................................................................6 Atmospheric deposition...........................................................................................6 Mercury methylation...............................................................................................9 Biomagnification and Bioaccumulation................................................................11 Weeks Bay geology...............................................................................................11 Hydrological Effects on Mercury Cycling............................................................12 METHODOLOGY............................................................................................................13 Mercury deposition analysis..................................................................................13 Surface water and groundwater sampling.............................................................15 Laboratory geochemical analyses.........................................................................17 Computer modeling and visualization...................................................................18 MERCURY DEPOSITION AND WATER CHEMSITRY IN THE WEEKS BAY WATERSHED..................................................................................................................19 Mercury deposition...............................................................................................19 Surface water chemistry........................................................................................24 Groundwater chemistry.........................................................................................24 Oxygen, hydrogen, carbon isotopic signatures.....................................................28 x DISCUSSION OF CHEMICAL AND HYDROLOGICAL DATA FROM THE WEEKS BAY WATERSHED.................................................................................31 Mercury deposition and precipitation ...................................................................31 Surface water chemistry and seawater intrusion...................................................34 Groundwater movement........................................................................................54 Groundwater influences........................................................................................56 CONCLUSIONS...............................................................................................................59 REFERENCES..................................................................................................................62 APPENDICES..................................................................................................................CD xi LIST OF FIGURES Figure 1. Southwest Alabama location map.......................................................................3 Figure 2. Eh ? pH diagram of mercury..............................................................................7 Figure 3. Eh ? pH diagram of mercury with the addition of chlorine................................8 Figure 4. Location map of MDN sites..............................................................................14 Figure 5. Weeks Bay sampling locations.........................................................................16 Figure 6. Mercury deposition vs. precipitation within the Fish River.............................32 Figure 7. Mercury deposition vs. precipitation within the Magnolia River.....................33 Figure 8. pH vs. conductivity...........................................................................................35 Figure 9. Conductivity vs. DO.........................................................................................36 Figure 10. Temperature vs. pH.........................................................................................37 Figure 11. Temperature vs. conductivity.........................................................................38 Figure 12. North ? South transect of pH values at different depths within Weeks Bay.................................................................................................39 Figure 13. North ? South transect of conductivity at different depths within Weeks Bay.................................................................................................40 Figure 14. Surface and 1 meter depth contour maps of Weeks Bay temperature............42 Figure 15. Surface and 1 meter depth contour maps of Weeks Bay pH..........................43 Figure 16. Surface and 1 meter depth contour maps of Weeks Bay conductivity...........44 Figure 17. Surface and 1 meter depth contour maps of Weeks Bay DO.........................45 Figure 18. Surface and 1 meter depth contour maps of Weeks Bay ORP.......................46 xii Figure 19. Piper Diagram showing signatures of different types of water ......................48 Figure 20. Plot of Cl vs. Br..............................................................................................50 Figure 21. Plot of Cl vs. Ca..............................................................................................51 Figure 22. Plot of Cl vs. Mg.............................................................................................52 Figure 23. Plot of Cl vs. Na..............................................................................................53 Figure 24. Groundwater-flow model showing direction of water flow...........................55 Figure 25. Plot of Oxygen-hydrogen isotope plot showing groupings of water..............57 xiii LIST OF TABLES Table 1. MDN site AL02 deposition and concentration data...........................................20 Table 2. MDN site AL24 deposition and concentration data...........................................22 Table 3. Weeks Bay surface and 1-meter depth water-quality data.................................25 Table 4. Selected major ions from sampled surface water...............................................26 Table 5. Groundwater water-quality data from USGS wells...........................................27 Table 6. Selected major ions from samples collected from USGS wells.........................29 Table 7. Oxygen, hydrogen, and carbon isotope data......................................................30 1 INTRODUCTION In recent years there has been an increase in the amount of mercury (Hg) found in the waters in and around the Gulf Coast region of the United States. Mercury is an element known to be extremely toxic to wildlife and humans. The amount of mercury detected in these waters exceeds the EPA drinking water standards of 2 parts per billion (ppb) and poses a threat to human health. It is from these waters that mercury can be introduced into the human body, either from drinking water contaminated by mercury or by eating fish that live in mercury contaminated waters. Mercury contamination can result from both natural and anthropogenic sources. Mercury exists as numerous species in natural aqueous environments, some of which are more stable than others. These different species of mercury include elemental mercury (Hg), oxidized, reduced, or ionic mercury (Hg2+), and methylmercury (CH3Hg). The unstable forms of mercury are the most hazardous to human health because they are easily broken down and placed into solution. The speciation and transportation of mercury in aquatic environments are controlled mainly by the water chemistry. The common factors that determine Hg speciation include dissolved organic matter, chloride, sulfide, redox conditions, and pH (Ravichandran et al., 1998). Mercury can be found as many types of trace constituents and its principal minerals cinnabar (HgS), corderoite (Hg3S2Cl2), and livingstonite (HgSb4S8). Major anthropogenic sources of mercury are coal burning power plants. When burned, mercury 2 turns into a vapor phase and is transported through the atmosphere as elemental and oxidized mercury. Elemental mercury has a residence time of up to a year in the atmosphere, whereas oxidized mercury has a residence time of only a few days. This is due to the higher solubility of Hg2+ in water which allows for mercury to return to the surface during rain falls. The longer the residence time, the farther mercury can be transported through the atmosphere. As a result, much of the world?s mercury pollution is mainly a result of atmospheric deposition (Senior et al., 2000). Health concerns from fish consumption have forced advisories against eating fish portions of nearly every state. The most susceptible areas to mercury contamination appear to be coastal waterways and estuaries. Of the United States coastal waters, 65 % contain advisories; this includes 100 % of the Gulf Coast and 92 % of the Atlantic Coast (EPA, 2005). Due to the contamination issues studies have been performed in some of the United States? larger bays and estuaries, these include; the San Francisco Bay Estuary, the Chesapeake Bay, and the Mobile Bay (Benoit et al., 1998; Mason et al., 1999; Lawson et al., 2001; Conaway et al., 2003; and Warner et al., 2005). Weeks Bay is an estuary located in eastern Mobile Bay and is economically important to Alabama due to its large seafood industry (Fig. 1). In 1992 Weeks Bay designated as an ?Outstanding national Water Resources? by the EPA (1999). Both the Fish and Magnolia Rivers drain into Weeks Bay before reaching Mobile Bay and eventually the Gulf of Mexico. Weeks Bay consists of 126,000 acres of watersheds with a diverse suite of depositional environments and ecosystems in Baldwin County. Several watersheds in the region are surrounded by urban centers that host large industrial and agricultural activities, all of which are potential point and non-point sources of Hg 3 Figure 1. Weeks Bay, an estuary of Mobile Bay, is located in southwestern Alabama?s Baldwin County and has a watershed of 126,000 acres. Fish, such as Largemouth Bass, caught within the Weeks Bay watershed have been found to contain mercury levels above Federal Food and Drug Administration standards of 1 mg/kg. 0 50 km 4 pollution. The ecosystems, depositional environments, and stratigraphy of Weeks Bay have been studied by various investigators (Brannon et al., 1977; Haywich et al., 1998; U.S. Fish & Wildlife Service, 1996; Fisher et al., 2006). Largemouth Bass in Weeks Bay have been found to contain Hg greater than 1 ?g/L (M. Shelton personal communication). A health standard issued by the Federal Food and Drug Administration requires that fish contain less than 1 ?g/L. Despite many previous studies, the sources, sinks, and cycling of Hg and general hydrology in this estuary remain poorly understood. Estuaries and bays are believed to be the primary traps for mercury contamination due to the low percentage of mercury exported to the ocean (Mason et al., 1999, Kim et al., 2004). This study initialized field and laboratory analysis to collect general hydrology and water chemistry data of the bay. The results will assist in the understanding of how the water chemistries within Weeks Bay change throughout the year and the effects of these changes may have on the methylation of mercury. 5 MERCURY CONTAMINATION Mercury Sources Mercury can be found as trace constituents in many types of rocks within the Earth?s crust. At room temperature mercury is a liquid and can be volatized at low temperatures. This allows mercury to become a vapor and can lead to worldwide contamination problems through atmospheric circulation. Mercury sources are both natural and anthropogenic. Natural occurrences of volatized mercury include volcanoes and associated geothermal systems, fires, and the evaporation of seawater. Anthropogenic sources of mercury are more common and have a relatively steady rate of expulsion since the industrial revolution. Anthropogenic sources include numerous industrial processes and waste incineration. As coal is burned, mercury sequestered in the organic matter is volatized and released. The mercury that exists in the coal is the result of organic matter bonding with mercury from atmospheric deposition and from mercury enriched waters. As the organic matter is compacted and transformed into coal mercury levels are concentrated. (Senior et al., 2000; Yudovich and Ketris, 2005). 6 Mercury Speciation Mercury exists as numerous species, some of which are more stable than others under various Eh-pH conditions (Fig. 2 and 3). All forms of mercury can create health problems in humans and have a variety of affects based on the species that is introduced into the body. Which species of mercury exists in aquatic environments is controlled by water chemistry. The common factors that determine speciation include dissolved organic matter, chloride, sulfide, and pH (Suzuki et al., 1992; Ravichandran et al., 1998). Ionic Hg is dominant under oxidizing conditions (Fig. 2). Solid elemental Hg and Hg sulfide (cinnabar) become the dominant phases under reducing conditions. In Cl-rich environments, the formation of aqueous HgCl complex could enhance the solubility and mobility of Hg (Fig. 3). Grassi and Netti (2000) showed similar results between increased Cl- levels and an increase in the amount of mercury in solution. Atmospheric Deposition As mercury vapors enter the atmosphere a wide variety of processes can occur based on the forms of mercury that exist. Hg0 and Hg2+ are the most common forms of mercury that exist in the atmosphere. The removal process of mercury from the atmosphere plays a major role in the contamination of waterways due to the ability of atmospheric mercury to be transported long distances. Deposition can take place in wet or dry processes. The interaction of mercury with other atmospheric gases, such as aerosols and ozone, assist in the deposition process. High levels of mercury within the atmosphere results in both wet and dry deposition, lower Hg levels rely on the oxidation 7 Figure 2. Eh ? pH diagram drawn at 25?C for Hg-Sulfur system showing pertinent mercury species pH at aHg = 0.001, aSO4 = 0.1. Blue areas indicate conditions where mercury is soluble while green areas indicate conditions in which mercury exists in a solid state. The diagram was constructed using the Geochemist?s Workbench (Bethke, 1996). 8 Figure 3. Eh ? pH diagram drawn at 25?C for Hg-Sulfur-Chlorine system showing pertinent mercury species pH at aHg = 0.001, aSO4 = 0.1, aCl- = 0.1. The addition of chlorine increases the solubility of mercury. This diagram was constructed using the Geochemist?s Workbench (Bethke, 1996). 9 of Hg0 to allow mercury to bond with other atmospheric gases resulting in deposition. An additional process that may result in dry deposition is the formation of mercuric oxide (HgO), a solid particulate, which may return to the surface under dry conditions (Schroeder and Munthe 1997). The amount of mercury contamination resulting from atmospheric deposition is monitored by the National Atmospheric Deposition Program (NADP). A special series of sites have been devoted to collecting wet atmospheric mercury deposition data as part of the Mercury Deposition Network (MDN). Throughout the United States there are 88 MDN mercury monitoring sites that monitor Hg concentrations (?g/L) and total Hg wet deposition (?g/m2). These sites collect precipitation, which is then sent to analytical laboratories for total concentration analyses. Mercury Methylation The methylation of mercury to create methylmercury (CH3Hg) is a process that takes place in aqueous environments under favorable geochemical conditions. Recent research suggests that methylation can occur in both the water column and in sediments (Sunderland et al., 2004; 2006; Warner et al., 2005). In aqueous environments, methylation ideally occurs in conditions of low pH, low dissolved organic content (DOC), low salinity, and low oxidation-reduction potential (ORP), with high temperature and sulfate content (Suzuki et al., 1992; Celo et al., 2005; and Mason et al., 2006). Many of these factors do not exist at the requisite levels for methylation in seawater. Mixing of freshwater with seawater provides more favorable geochemical conditions. 10 The aqueous geochemistry conditions involved with methylation are closely related to each other and can have a wide range of effects based on various factors. DOC, pH, and ORP help create conditions that assist in microbial activity. High DOC values have been shown to hinder the methylation process; however, DOC provides electron donors for bacteria, such as sulfate-reducing bacteria (SRB). Low ORP values are a result anaerobic conditions caused by increased microbial activity Finally, low pH allows heavy metals to be released and become available for chemical reactions (Baeyens et al.; 1998, Mehrotra, et al., 2003) . High temperatures appear to assist in the methylation process as evidenced by increased methylation during the summer months, when waters are warmer. Generally, seawater in the region is warmer than freshwater and may provide higher temperatures for methylation. However, the increase of salinity due to the addition of seawater hinders methylation process due to increased sulfides that may take in Hg to form insoluble HgS (Compeau and Bartha, 1986). It is likely that maximum methylation may occur in the freshwater-seawater mixing zone. Climate also plays a vital role in mercury methylation in estuarine environments, where methylation commonly takes place. Factors such as rainfall and wind can hinder or assist in the methylation process. Substantial rainfall events can affect the water chemistry of aqueous environments and may add nutrients and additional heavy metals from erosion or surrounding terrain. Wind can also play a part in methylation. Little to no wind can result in thermal and haline stratification of water bodies, effecting methylation. On the other hand, strong sustained winds can cause mixing of shallow 11 waters which may bring low oxygen waters to the surface and affect water column biota (Mason et al., 1999). Biomagnification and Bioaccumulation The health concerns involved with mercury are mainly the result of the ingested of mercury contaminated organisms. The methylation process converts mercury to CH3Hg, a form that can be consumed by various organisms and enter the food chain. Plankton and other bacteria consume methylmercury becoming contaminated. As predators feed on mercury contaminated lower organisms, concentrations of mercury within the food chain increase (Lawson and Mason, 1998). This results in the top predators, such as eagles and humans, ingesting the highest levels on mercury creating increased health concerns. Weeks Bay Geology Weeks Bay is located in Baldwin County, Alabama on the Gulf Coast coastal plain. Gulf Coast deposits typically consist of mostly sands and gravel deposits with some interbedded silts and clays. The coastal plain sediments can be divided into three major deposits that are considered as local aquifers. Sediments of the Miocene age are composed of white- to light-grey, fine to very coarse sands with some interbedded sandy, silty clay. Pleistocene deposits are similar to those of the Miocene but have greater abundance of interbedded sandy, silty clays. These deposits are overlain by sediments of Holocene age and consist of, white to pale-orange, fine-to coarse grained sands, with some silt, clay, and shell hash (Chandler et al., 1996). 12 Hydrologic Effects on Mercury Cycling Groundwater discharge and surface evaporation can also affect the chemistry and salinity of surface water, which in turn control the biotransformation of Hg in aqueous environments. Groundwater discharge can be analyzed by measuring hydraulic heads of clustered wells at different depths. The evaporation and geochemical evolution of surface water and groundwater can be traced by stable isotope (18O and 2H) analyses. 13 METHODOLOGY Field, laboratory, and computer analyses were conducted to study the source of Hg and how near-surface processes such as water mixing and evaporation could potentially affect the biogeochemical cycle of Hg in Weeks Bay. Weekly concentrations of total Hg in precipitation were collected from the Mercury Deposition Network (MDN) for a period of one year to gain information on seasonal trends in Hg deposition from atmospheric sources. To study the hydrology effects of water mixing and evaporation within the bay, water samples were collected from nearby groundwater wells, surrounding rivers, and from the bay for major ion, trace element, and stable isotope analyses. The data were then compiled and analyzed using a variety of computer and mapping programs to understand better water mixing within the bay and how it affects mercury methylation. Four trips were taken to the Weeks Bay area to collect data and water samples for chemical analyses. These trips were taken over the course of a year so that seasonal variations of hydrology and water chemistry within the bay could be determined. Mercury Deposition Analysis Mercury deposition data were compiled from two MDN sites, AL02 and AL24 (http://nadp.sws.uiuc.edu/mdn/) in and around the Weeks Bay watershed (Fig. 4). 14 Figure 4. Map of locations of MDN sampling sites in the southeastern region of the United States. Data from sites AL02 and AL24 were used for analysis of Hg deposition near Weeks Bay. 15 Seasonal variations in mercury deposition were studied over a period of one year from January, 2003, to March, 2004 (Lindberg and Vermette, 1995). In addition to MDN site data, USGS precipitation data were also collected from two rain gauges on the Fish River and Magnolia River (USGS Waterdata). These two data sets were graphed together to establish possible correlations that may exist between precipitation and mercury deposition. Surface Water and Groundwater Sampling Four research trips were taken to the Weeks Bay watershed from July, 2005, to May, 2006. Water-chemistry data were collected from the bay and from nearby USGS monitoring wells WW13, WW14, WW15, and WW16 (Fig. 5). These USGS monitoring wells were drilled in clusters of two in a small area along the west bank of Weeks Bay. The clustered wells were completed at different depths, which enable the examination of changes in hydraulic head with depth. The water-table elevations of the wells were measured to determine vertical hydraulic gradients. An upward gradient would indicate that groundwater flow has an upward component as it discharges into the bay. Water- chemistry data were collected from the USGS wells using the multi-parameter TROLL 9000 (manufactured by In-Situ, Inc.). The wells were bailed by removing three well volumes before sampling to insure that a representative groundwater sample was collected (EPA, 1995). To study water table and tidal fluctuations a multi-parameter TROLL 8000 was left in WW16 for a 24-hour period to record water-pressure changes. Using a boat provided by the Weeks Bay National Estuary Research Reserve 16 Figure 5. Digital elevation map of Weeks Bay with sampling locations and USGS monitoring wells. The blue circles represent areas where field chemistry data was collected while the black triangles represent areas where water samples were collected for major ion, trace element and isotope analysis. 17 (WBNERR), water-chemistry data were collected from 32 locations throughout the bay (Fig 5). Measurements were repeated at the same locations for each field trip to find seasonal variations. A Garmin GPSMAP CS60 was used to ensure sampling points were consistent during repeated trips. Water chemistry data, including temperature, pH, specific conductance, dissolved oxygen (DO), oxidation reduction potential (ORP), and turbidity were collected using the multi-parameter TROLL 9000 at each location at different water column depths to study their spatial changes. The data were recorded at half-meter depth intervals until bay bottom sediments were reached. In addition to water- chemistry data, ten water samples were collected from the bay for laboratory chemical and isotope analysis (Fig. 5). Using a sampler, 150 ml bottles were lowered and filled near river and bay bottom. Laboratory Geochemical Analyses Samples collected from groundwater, bay water, and river water were sent for geochemical and stable isotope analyses to study the source and evolution of various waters in the Weeks Bay watershed. Oxygen, hydrogen and carbon isotopic ratios were measured using the Finnigan Mat delta PLUS XP Mass Spectrometer at Florida State University. The analysis of DOC was conducted in the Civil Engineering Water Quality Laboratory at Auburn University. A raw and an acidified sample from each water well and surface water locations were sent to ACTLABS for major ion and trace element analysis using Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and Optical Emission Spectrometer (ICP-OES). Anion concentrations were measured in the ACTLABS using Dionex 2000 Ion Chromatograph (IC). 18 Computer Modeling and Visualization The collected and analyzed data were then graphed and plotted to provide a visual representation of mixing processes occurring within Weeks Bay. ArcGIS, Excel, Surfer, and Tecplot were all used to accomplish this. An ArcGIS base map was created using a digital elevation model of the USGS Magnolia Spring Quadrangle. Surface-water and groundwater sampling locations were plotted using the GPS measurements from the field. Transects of Weeks Bay water chemistry were created at various depths using Excel to plot the parameters collected by the In-Situ probes. The oxygen and hydrogen isotopic ratios of water relative to the Local Mean Water Line (LMWL) and seawater were analyzed using Tecplot to determine the effects of mixing and evaporation on water chemistry and salinity. The graphs created from Techplot were used for comparing and correlating water chemistry parameters spatial variations in order to find relationships. Spatial variations of water-chemistry parameters within the bay and river were determined using Surfer. Color and contour maps for water-chemistry field parameters were generated at different depths. These maps could be compared to find trends, thermal and salinity stratification, and mixing zones within the bay and river. Hg speciation at various Eh-pH conditions was analyzed using Geochemist?s Workbench (Bethke, 1996). 19 MERCURY DEPOSTION AND WATER CHEMISTRY IN THE WEEKS BAY WATERSHED The following sections present data related to major ion chemistry, trace elements content, and isotopic composition of waters collected within the Weeks Bay watershed. The results obtained in this study were comprised of data from the MDN as well as field sampling and laboratory analyses. The data can be used to understand the complex interrelationships of bay, ground, and surface waters taking place within Weeks Bay and the effects of these interactions on water chemistry and mercury methylation. Mercury Deposition Table 1 shows data collected by the NADP MDN at the Alabama site the AL02 near Mobile Bay. This table lists the weekly deposition (ng/L) and concentration values of total mercury (ng/m2) in precipitation collected from January, 2003, to March, 2004. Mercury deposition and concentration values were also analyzed from NADP?s MDN site AL24 (Table 2) over a twelve month period from January, 2003, to January, 2004, with the same parameters recorded at site AL02. Both NADP MDN sites show seasonal trends of mercury deposition throughout the analyzed collection period with highest deposition occurring during June and July months and lowest mercury deposition occurring from November to January. 20 Table 1. Data from NADP MDN site AL02 showing Hg concentration and deposition from January 2003 to March 2004. Site ID Date On Date Off Total Hg Concentration (ng/L) Total Hg Wet Deposition (ng/m?) AL02 12/30/2002 14:24 1/7/2003 19:51 3.74 285.14 AL02 1/7/2003 19:57 1/14/2003 12:49 -- 0 AL02 1/14/2003 12:53 1/21/2003 14:13 -- 0 AL02 1/21/2003 14:18 1/28/2003 14:36 -- -- AL02 1/28/2003 14:45 2/4/2003 14:13 10.23 70.21 AL02 2/4/2003 14:20 2/11/2003 15:03 9.35 142.49 AL02 2/11/2003 15:06 2/18/2003 16:01 12.09 322.62 AL02 2/18/2003 16:08 2/25/2003 14:48 9.04 597.13 AL02 2/25/2003 15:06 3/4/2003 14:10 5.4 382.81 AL02 3/4/2003 14:17 3/11/2003 15:27 14.36 857.38 AL02 3/11/2003 15:32 3/17/2003 15:45 16.6 928.05 AL02 3/17/2003 15:57 3/25/2003 17:40 -- -- AL02 3/25/2003 17:43 4/1/2003 15:57 16.74 93.58 AL02 4/1/2003 16:09 4/8/2003 14:55 11.16 1063.08 AL02 4/8/2003 15:00 4/15/2003 13:10 8.54 238.74 AL02 4/15/2003 13:20 4/22/2003 13:02 11.9 86.74 AL02 4/22/2003 13:11 4/29/2003 15:05 14.39 347.42 AL02 4/29/2003 15:11 5/6/2003 13:05 23.59 59.94 AL02 5/6/2003 13:10 5/13/2003 13:15 14.38 54.78 AL02 5/13/2003 13:17 5/20/2003 15:05 10.37 2308.71 AL02 5/20/2003 15:12 5/27/2003 15:00 6.46 1050.3 AL02 5/27/2003 15:10 6/3/2003 14:03 14.36 430.42 AL02 6/3/2003 14:05 6/10/2003 16:06 3.84 859.21 AL02 6/10/2003 16:12 6/17/2003 15:22 11.68 816.12 AL02 6/17/2003 15:27 6/24/2003 18:58 9.17 1010.86 AL02 6/24/2003 19:05 7/1/2003 18:39 13.53 3667.41 AL02 7/1/2003 18:45 7/8/2003 18:03 17.17 1252.02 AL02 7/8/2003 18:08 7/15/2003 16:35 -- -- AL02 7/15/2003 16:38 7/22/2003 15:29 16.35 1939.64 AL02 7/22/2003 15:40 7/29/2003 16:00 11.97 1368.97 AL02 7/29/2003 16:05 8/5/2003 12:20 18.87 906.01 21 Table 1. Continued. Site ID Date On Date Off Total Hg Concentration (ng/L) Total Hg Wet Deposition (ng/m?) AL02 8/5/2003 12:30 8/12/2003 13:55 20.61 492.29 AL02 8/12/2003 14:01 8/19/2003 16:09 11.5 306.75 AL02 8/19/2003 16:13 8/26/2003 16:07 23.41 624.53 AL02 8/26/2003 16:10 9/2/2003 16:34 123.39 689.5 AL02 9/2/2003 16:40 9/9/2003 13:06 20.45 587.09 AL02 9/9/2003 13:12 9/16/2003 18:33 32.91 585.3 AL02 9/16/2003 18:39 9/23/2003 15:43 19.87 514.79 AL02 9/23/2003 15:48 9/30/2003 17:09 -- -- AL02 9/30/2003 17:19 10/7/2003 16:59 -- 0 AL02 10/7/2003 17:04 10/14/2003 18:47 5.04 202.34 AL02 10/14/2003 18:51 10/21/2003 13:00 -- 0 AL02 10/21/2003 13:05 10/28/2003 13:54 8.86 562.86 AL02 10/28/2003 14:00 11/4/2003 13:45 -- 0 AL02 11/4/2003 13:55 11/10/2003 21:35 6.29 47.99 AL02 11/10/2003 21:42 11/18/2003 16:10 11.12 5.65 AL02 11/18/2003 16:10 11/25/2003 16:00 7.33 212.3 AL02 11/25/2003 16:05 12/2/2003 17:05 7.38 628.04 AL02 12/2/2003 17:10 12/9/2003 12:50 5.76 102.53 AL02 12/9/2003 12:57 12/16/2003 13:10 5.6 273.49 AL02 12/16/2003 13:16 12/23/2003 19:35 -- 0 AL02 12/23/2003 19:39 12/30/2003 18:25 5.14 359.3 AL02 12/30/2003 18:27 1/6/2004 13:05 9.45 19.2 AL02 1/6/2004 13:12 1/13/2004 14:20 10.04 114.79 AL02 1/13/2004 14:26 1/20/2004 17:25 3.43 156.82 AL02 1/20/2004 17:34 1/27/2004 12:47 8.03 397.97 AL02 1/27/2004 12:55 2/3/2004 13:05 8.87 83.38 AL02 2/3/2004 13:15 2/10/2004 12:30 8.71 189.56 AL02 2/10/2004 12:40 2/17/2004 13:15 8.44 845.73 AL02 2/17/2004 13:25 2/23/2004 18:30 16.61 251.58 AL02 2/23/2004 18:32 3/2/2004 12:50 4.51 309.97 AL02 3/2/2004 12:59 3/9/2004 13:58 14.64 100.42 AL02 3/9/2004 14:07 3/16/2004 12:20 9.33 142.26 AL02 3/16/2004 12:28 3/23/2004 18:03 -- 0 AL02 3/23/2004 18:03 3/30/2004 13:29 -- -- 22 Table 2. Data from NADP MDN site AL24 showing Hg concentration and deposition from January 2003 to January 2004. Site ID Date On Date Off Total Hg Concentration (ng/L) Total Hg Wet Deposition (ng/m?) AL24 12/30/2002 15:08 1/7/2003 14:40 5.77 341.34 AL24 1/7/2003 14:45 1/14/2003 17:33 -- 0 AL24 1/14/2003 17:38 1/21/2003 16:00 -- 0 AL24 1/21/2003 16:03 1/28/2003 16:42 -- 0 AL24 1/28/2003 16:44 2/4/2003 14:51 3.98 32.36 AL24 2/4/2003 14:53 2/11/2003 16:08 7.78 128.46 AL24 2/11/2003 16:10 2/18/2003 16:50 8.14 140.64 AL24 2/18/2003 16:52 2/25/2003 17:15 5.22 493.22 AL24 2/25/2003 17:20 3/4/2003 22:55 5.58 513.43 AL24 3/4/2003 22:57 3/11/2003 15:46 15.21 216.47 AL24 3/11/2003 15:48 3/17/2003 17:13 7.37 619.96 AL24 3/17/2003 17:16 3/25/2003 16:31 -- 0 AL24 3/25/2003 16:34 4/1/2003 16:32 17.4 66.3 AL24 4/1/2003 16:35 4/8/2003 21:36 12.4 677.54 AL24 4/8/2003 21:39 4/15/2003 16:52 5.76 5.85 AL24 4/15/2003 16:55 4/22/2003 15:17 8.37 95.74 AL24 4/22/2003 15:20 4/29/2003 17:55 13.41 105.63 AL24 4/29/2003 17:58 5/6/2003 17:52 -- 0 AL24 5/6/2003 17:55 5/13/2003 17:38 19.16 272.53 AL24 5/13/2003 17:41 5/20/2003 17:15 8.49 718.77 AL24 5/20/2003 17:19 5/27/2003 15:15 4.47 203.36 AL24 5/27/2003 15:20 6/3/2003 22:31 15.69 111.6 AL24 6/3/2003 22:44 6/10/2003 22:16 7.22 716 AL24 6/10/2003 22:20 6/17/2003 15:57 23.26 407.79 AL24 6/17/2003 16:00 6/24/2003 22:40 10.48 2316.36 AL24 6/24/2003 22:42 7/1/2003 16:50 7.25 1339.37 23 Table 2. Continued. Site ID Date On Date Off Total Hg Concentration (ng/L) Total Hg Wet Deposition (ng/m?) AL24 7/1/2003 16:54 7/8/2003 19:54 9.21 346.44 AL24 7/8/2003 20:00 7/15/2003 14:50 20.94 393.58 AL24 7/15/2003 14:54 7/22/2003 19:30 15.61 444.21 AL24 7/22/2003 19:33 7/29/2003 15:45 8.49 750.62 AL24 7/29/2003 15:47 8/5/2003 19:38 15.04 676.16 AL24 8/5/2003 19:40 8/12/2003 16:12 16.04 407.59 AL24 8/12/2003 16:14 8/19/2003 16:10 6.75 257.17 AL24 8/19/2003 16:10 8/26/2003 14:18 11.91 381.39 AL24 8/26/2003 14:20 9/2/2003 19:45 17.27 381.63 AL24 9/2/2003 19:48 9/9/2003 14:31 27.78 21.16 AL24 9/9/2003 14:33 9/16/2003 15:55 89.1 113.15 AL24 9/16/2003 15:55 9/23/2003 20:12 22.58 1066.91 AL24 9/23/2003 20:15 9/30/2003 13:42 35.26 8.95 AL24 9/30/2003 13:50 10/7/2003 15:56 -- 0 AL24 10/7/2003 15:58 10/14/2003 17:28 4.32 318.8 AL24 10/14/2003 17:30 10/21/2003 14:24 -- 0 AL24 10/21/2003 14:26 10/28/2003 20:15 13.68 340.74 AL24 10/28/2003 20:20 11/4/2003 15:36 -- 0 AL24 11/4/2003 15:40 11/10/2003 20:35 -- 0 AL24 11/10/2003 22:30 11/18/2003 14:50 7.61 17.41 AL24 11/18/2003 14:52 11/25/2003 19:52 5.91 169.65 AL24 11/25/2003 19:55 12/2/2003 19:58 7.06 281.77 AL24 12/2/2003 20:00 12/9/2003 16:30 6.47 231.89 AL24 12/9/2003 16:30 12/16/2003 15:12 5.01 155.43 AL24 12/16/2003 15:15 12/23/2003 18:15 -- 0 AL24 12/23/2003 18:18 12/30/2003 19:30 -- -- AL24 12/30/2003 19:32 1/6/2004 15:10 19.36 83.63 AL24 1/6/2004 15:15 1/13/2004 20:40 8.61 124.65 AL24 1/13/2004 20:45 1/20/2004 16:35 4.11 267.37 AL24 1/20/2004 16:40 1/27/2004 16:50 -- -- AL24 1/27/2004 16:55 2/3/2004 17:00 15.59 103 24 Surface-Water Chemistry Data from sampling surface-water is shown in Table 3 and records latitude/longitude, temperature, pH, ORP, conductivity, DO, and turbidity data from the July, 2005, research trip, data from additional trips can be found in the appendix (see attached CD). Measurements were taken at different depths to delineate possible thermal and haline stratifications. River water and surface water near the river mouth are characterized by relatively low pH (5.99 to 6.54), low temperature (27.80?C to 31.65?C), and low conductivity (138 ?S/cm to 2017 ?S/cm). In contrast, surface waters near the bay mouth have relatively high pH (7.8 to 8.75), high temperature (32.0?C to 33.25?C), and high conductivity (3350 ?S/cm to 5706 ?S/cm). In general, surface water sampled near the river mouth where river water mixes with bay water has the lowest DO or ORP values The major ion, trace metals, and DOC contents of surface waters are shown in Table 4, complete ICP-MS, ICP-OES, and IC table can be found in the appendix (see attached CD). Surface waters generally have a lower metal content and high chlorine, sodium, and sulfate concentrations. Groundwater Chemistry Temperature, pH, ORP, conductivity, DO, and turbidity were measured from groundwater taken from the four USGS monitoring wells (Table 5). The pH and temperature of the water in the monitoring wells were generally lower (4.53 to 6.04 and 20.6?C to 26.1?C, respectively) than those of bay water. Conductivity was also low, in the monitoring wells. The deep well closest to the Bay, WW15, however, recorded higher conductivity measurements (4688?S/cm) with respect to other wells. Major ion 25 Table 3. Surface water chemistry from locations within Weeks Bay and Fish River taken at surface level and one-meter depth during July, 2005. See Fig. 5 for sample locations. Sample ID Location GPS ID Temperature (?C) pH ORP (mV) Conductivity (uS/cm) DO (ug/L) Turbidity (NTU) WB1-S 30.413306/87.825500 009 30.93 6.47 540 909 5950 22.4 WB1-1 30.413306/87.825500 009 30.16 6.3 632 950 7125 30.2 WB2-S 30.409472/87.826583 010 31.65 6.41 609 942.6 5020 22.5 WB2-1 30.409472/87.826583 010 27.80 5.99 646 219 2930 100 WB9-S 30.407222/87.827361 017 32.17 6.38 502 204 6233 22 WB9-0.75 30.407222/87.827361 017 28.71 6.06 541 217.1 3970 28.6 WB7-S 30.402861/87828694 015 31.50 6.28 488 300 4455 19.8 WB7-0.75 30.402861/87828694 015 31.24 7.19 455 2800 6031 NA WB8-S 30.400806/87.829639 016 32.16 6.6 518 1311 5498 19.7 WB8-0.8 30.400806/87.829639 016 31.82 8.45 344 4235 6220 NA WB6-S 30.398083/87.830611 014 33.02 8.43 384 2715 6360 23.3 WB6-1 30.398083/87.830611 014 31.89 8.29 361 5147 5879 NA WB5-S 30.390417/87.833083 013 33.56 8.77 493 5174 8100 17.6 WB5-1 30.390417/87.833083 013 32.13 8.58 515 5630 7080 27.8 WB4-S 30.382889/87.834806 012 32.89 8.6 467 5522 7394 21.6 WB4-1 30.382889/87.834806 012 32.36 8.5 518 5706 6790 31.1 WB3-S 30.376861/87.835806 011 32.82 8.49 533 5700 6700 19.9 WB3-1 30.376861/87.835806 011 32.76 8.45 558 5444 6900 26 WB10-S 30.389639/87.816167 028 32.00 7 457 3000 5038 29 WB10-0.75 30.389639/87.816167 028 31.70 7.19 455 3254 5116 77.5 WB11-S 30.392194/87.820056 029 32.66 8.09 432 3975 6330 28.2 WB11-1 30.392194/87.820056 029 32.19 7.8 437 4296 5537 NA WB12-S 30.393194/87.824694 030 32.60 8.62 381 4002 7360 24.1 WB12-1 30.393194/87.824694 030 32.25 8.28 414 4616 5357 34.2 WB13-S 30.393889/87.829083 031 33.17 8.66 365 5136 6110 19.8 WB13-1 30.393889/87.829083 031 32.43 8.36 370 5442 5388 29.6 WB14-S 30.395250/87.836778 032 32.66 8.3 356 2340 6658 18 WB14-1 30.395250/87.836778 032 32.40 8.23 404 4860 5224 41.7 WB15-S 30.396850/87.841750 033 32.69 8.06 361 1865 7150 31 WB15-1 30.396850/87.841750 033 32.18 7.8 422 3985 5256 41.6 WB16-S 30.390917/87.840861 034 33.06 8.41 344 2880 6800 19.9 WB16-1 30.390917/87.840861 034 32.11 8.18 397 3981 6057 30.6 WB17-S 30.386694/87.840139 035 33.22 8.75 327 3350 7356 23.3 WB17-1 30.386694/87.840139 035 32.10 8.28 400 4980 5241 38.5 WB18-S 30.383028/87.838944 036 33.22 8.79 324 3255 7746 21 WB18-1 30.383028/87.838944 036 32.08 8.25 387 5056 5572 44 WB19-S 30.378444/87.837056 037 33.25 8.8 330 3436 7815 20.9 WB19-1 30.378444/87.837056 037 32.38 8.4 394 5636 5942 30.1 WB20-S 30.401222/87.837389 038 33.63 8.19 290 2108 7255 24.4 WB20-1 30.401222/87.837389 038 32.46 7.92 346 3676 5543 47.6 WB21-S 30.406722/87.834222 039 32.72 6.69 386 560 5369 26.2 WB21-1 30.406722/87.834222 039 30.50 6.64 402 1180 4193 42.5 WB22-S 30.410306/87.829889 040 31.99 7.03 409 1119 7150 29.9 WB22-1 30.410306/87.829889 040 28.61 6.23 473 330 3020 34.2 Fish1-S 30.445000/87.804333 018 31.40 6.8 403 66.01 8851 9.9 Fish1-1 30.445000/87.804333 018 28.56 6.3 485 60.29 7719 8.9 Fish2-S 30.442722/87.802806 019 31.54 6.63 482 65.31 8805 9.5 Fish2-1 30.442722/87.802806 019 28.50 6.26 526 61.82 7900 8.3 Fish3-S 30.443611/87.807389 020 30.20 6.57 514 63.97 9043 10.7 Fish3-1 30.443611/87.807389 020 29.06 6.29 553 63.04 7762 10.5 Fish4-S 30.440833/87.811306 021 31.98 6.9 455 71.4 9210 13.1 Fish4-1 30.440833/87.811306 021 30.47 6.74 503 66.06 9500 12.4 Fish5-S 30.436000/87.812611 022 31.83 6.98 447 69.26 9500 12.9 Fish5-1 30.436000/87.812611 022 28.81 6.27 530 64.15 7795 10.4 Fish6-S 30.435639/87.818917 023 31.65 6.82 454 76.5 9540 15 Fish6-1 30.435639/87.818917 023 30.11 6.65 489 72.27 9440 14.1 Fish7-S 30.431306/87.823722 024 32.17 6.83 443 98.3 9140 13.9 Fish7-1 30.431306/87.823722 024 31.44 6.6 477 102.5 9000 13.5 Fish8-S 30.427500/87.828556 025 31.97 6.61 444 105.2 8600 14.6 Fish8-1 30.427500/87.828556 025 30.18 6.36 521 88.22 8058 14.5 Fish9-S 30.424111/87.824778 026 32.50 6.7 441 123.3 8659 15.6 Fish9-1 30.424111/87.824778 026 31.71 6.6 496 123.3 8600 15.7 Fish10-S 30.418944/87.822806 027 32.03 6.54 551 201.7 7800 15.6 Fish10-1 30.418944/87.822806 027 30.58 6.27 568 138 7460 15.3 26 Table 4. Selected major ions and trace metals concentrations of sampled surface waters. Only one surface water site measured detectable mercury levels. Hg levels of sampled waters below detection limits. Sample Na Mg K Ca Ni Br Rb Sr Ba Cl Br F SO4-2 HCO3- ID ppm ppm ppm ppm ppb ppm ppb ppm ppb ppm ppm ppm ppm ppm 009 3890 508 142 170 80 22.9 40 2.74 50 7020 55 < 4 925 347 011A 5650 741 221 240 140 33.6 58.2 3.99 40 10400 74 < 5 1360 150 011B 6140 806 232 250 200 36.5 60.4 4.1 40 11200 79 < 5 1460 304 014 -- 684 196 210 220 30 51.2 3.53 50 9180 65 < 4 1190 773 018 3420 472 125 150 80 19.7 34.5 2.42 50 6130 47 < 3 809 827 023 3650 479 132 150 80 21.1 38.2 2.54 50 6710 53 < 3 881 110 027 -- 322 93 100 60 14.5 25.1 1.73 50 4500 37 < 2 595 891 028 3220 429 124 140 110 19.6 33.3 2.25 50 5960 45 < 3 788 57.2 033 4210 556 157 180 180 25.1 42.6 2.9 50 7720 58 < 4 1010 185 038 4570 604 168 190 200 26.5 45.1 3.1 40 8390 60 < 4 1100 147 041 4860 637 174 190 260 28.1 47.2 3.37 50 8750 60 < 4 1140 409 Seawater 10760 1290 399 411 0.05 67 120 8 10 19350 67 1300 2710 145 27 Table 5. Groundwater chemistry recorded at USGS monitoring wells WW13, WW14, WW15, and WW16. Well ID Temperature pH ORP Conductivity DO Turbidity (?C) (mV) (??S/cm) (?g/L) (NTU) WW-13 21.9 4.53 365 67.74 7.176 2460.8 WW-14 21.8 4.63 375 63.69 4.884 932.9 WW-15 20.6 4.87 260 50.44 8.411 34 WW-16 26.1 6.04 -9 4688 7.687 779.9 28 and trace metal analysis from the groundwater samples (Table 6) show higher metal and lower sulfate numbers than those of surface water samples. Mercury levels were above detection limits in three of the monitoring wells, WW13, WW14, and WW16. Oxygen, Hydrogen, and Carbon Isotopic Signatures Collected surface water and groundwater samples were sent for stable isotope analysis at Florida State University. ?18? and ?D values (Table 7) are used to determine the source of water and its geochemical evolution in the Weeks Bay watershed. All waters are depleted in 18O and 2H relative to seawater, indicating the strong influence from isotopically-light meteoric water. Groundwater has the lowest ?18O and ?D values. In contrast, Weeks Bay water has the highest ?18? and ?D values. River waters are in the intermediate range between groundwater and bay water. Carbon within the environment can be a result of several sources, which include; organic material, atmospheric CO2, CO2 gas from biologic activity, and the dissolution of carbonate material. Stable carbon isotopic signatures are based on 12C and 13C values, the resulting ratios can help determine carbon sources (Fetter, 2001). Bay and river waters from the Weeks Bay watershed have negative DOC ?13C ranging from -23.6 0/00 to -26.0 0/ 00. The monitoring wells reported no ? 13C values except for WW15 which showed values similar to those found within the bay and rivers. 29 Table 6. Selected major ions and trace metal concentrations of groundwater from USGS clustered monitoring wells. Two wells, one in each well cluster was found to contain detectable mercury. Well ID Na Mg Al Si K Fe Br Sr Ba Hg Cl F SO4-2 HCO3- ppm ppm ppb ppm ppm ppm ppb ppb ppb ppb ppm ppm ppm WW13 -- 2.46 20 3 0.5 -- -- 23.1 71 8 340 < 10 -- 364000 WW14 5180 1.06 -- 3 0.6 -- 30 10.7 36 12 220 < 7 -- 418000 WW15 -- 131 2900 -- 17 30 8500 452 220 -- 2610 < 2 100 589000 WW16 -- 0.79 -- 4 -- -- -- 9.3 21 6 -- < 8 -- 41400 30 Table 7. Oxygen, hydrogen, and carbon isotope composition of groundwater, bay water, and river water. Sample Name Water Source ?18O, 0/00 (SMOW) ?D, 0/00 (SMOW) ?13C (PDB) DOC (ppm) 9 Transition Zone -2.1 -14.9 -25.8 2.1 011A Bay -1.2 -7.5 -25.2 1.8 011B Bay -1 -9.6 -23.6 1.7 14 Bay -1.5 -10.1 -25.1 1.8 18 River -2.3 -11.9 n/a 1.7 23 River -2.2 -15.5 -25.4 1.9 27 River -2.7 -16.5 -26.0 2 28 Bay -2.3 -15.6 -25.7 2.2 33 Bay -1.8 -9.4 -25.0 1.7 38 Bay -1.6 -8.1 -25.0 1.9 41 Bay -1.6 -6.3 -25.1 1.4 41 Bay -1.5 -4.3 -25.3 n/a WW13 Groundwater -4.4 -23.1 n/a 1.4 WW14 Groundwater -4.1 -26.6 n/a 1.9 WW15 Groundwater -2.7 -17.3 -25.5 3.3 WW16 Groundwater -4.4 -26.9 n/a 1.7 Sea Water 0 0 0 0.5 ? 1.5 31 DISCUSSION OF CHEMICAL AND HYDROLOGICAL DATA FROM THE WEEKS BAY WATERSHED The chemistry of Weeks Bay plays a part in the contamination of mercury. In estuary environments these conditions are constantly changing and can affect the severity of contamination. New data from the field study can be added to the results of previous investigators to reveal how the hydrodynamics of Weeks Bay affects water chemistry and mercury methylation. With field data, computer models were created for visual interpretations of water movement and mixing. Chemical and isotope analysis of collected water samples assisted in discovering how water flow and evaporation affected the water chemistry in the watershed. These data sets were analyzed and compared to get a better understanding of the hydrodynamics of Weeks Bay. These different sets of data will be compared to find possible correlations and how they may effect the methylation of mercury in Weeks Bay. Mercury Deposition and Precipitation Mercury deposition data from the MDN compared with USGS precipitation data from the same time period show possible correlations between mercury deposition and precipitation for the Weeks Bay watershed (Fig. 6 and 7). USGS precipitation data was collected from the Fish River and the Magnolia River stream gauges, both of which feed into Weeks Bay. Figure 6 shows that total mercury wet deposition increases with the amount of weekly atmospheric precipitation observed near the Fish River. A similar 32 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 3/2 5/0 3 4/1 5/0 3 5/6 /03 5/2 7/0 3 6/1 7/0 3 7/8 /03 7/2 9/0 3 8/1 9/0 3 9/9 /03 9/3 0/0 3 10/ 21/ 03 11/ 10/ 03 12/ 2/0 3 12/ 23/ 03 1/1 3/0 4 2/3 /04 2/2 3/0 4 3/1 6/0 4 Date Pr ec ip ita tio n (in ch es ) 0 500 1000 1500 2000 2500 3000 3500 4000 Me rc ur y D ep os iti on (n g/ m2 ) Precipitation (inches) HgDep Well Al02 (ng/m2) Figure 6. Plot of total mercury wet deposition at site AL02 and weekly atmospheric precipitation observed at the Fish River stream gauge. This correlation suggests that the most likely source for mercury in the Weeks Bay is from atmospheric deposition. 33 0 1 2 3 4 5 6 7 3/25/03 4/29/03 6/3/03 7/8/03 8/12/03 9/16/03 10/21/03 11/25/03 12/30/03 2/3/04 3/9/04 Date Pr ec ip ita tio n (in ch es ) 0 500 1000 1500 2000 2500 3000 3500 4000 Hg D ep os iti on (n g/ m2 ) Precipitation (inches) HgDep Well Al02 (ng/m2) Figure 7. Plot of total mercury wet deposition at site AL02 and weekly atmospheric precipitation observed at the Magnolia River stream gauge that shows similar correlation. Like the Fish River data, these data suggest that the most likely source for mercury in Weeks Bay is from atmospheric deposition. 34 correlation of mercury wet deposition increase with increased atmospheric precipitation is observed near the Magnolia River (Fig. 7). The correlation of mercury wet deposition and precipitation confirms that the likely source of mercury in Weeks Bay is the result of atmospheric mercury deposition. Surface-Water Chemistry and Seawater Intrusion Chemistry data of surface water demonstrates a mixing zone in the Weeks Bay, as a result of seawater intrusion along the bottom of the bay. Grouping of groundwater, river water, and bay water can be seen in the plots relating different water parameters (Fig. 8-11). Waters have distinct characteristics, such as pH, conductivity, DO, and temperature signatures. Groundwater has the lowest temperatures, pH, and conductivity, which would favor Hg methylation (Ullrich et al., 2001; Mason et al., 2006). Surface water near the bay mouth has the highest pH, temperature, and conductivity values. These parameters may help explain the spatial distribution of Hg methylation and its relation to the mixing of seawater, river water, and groundwater in the bay. A transition zone located at the mouth of the Fish River entering Weeks Bay is indicated by a comparison of data from the groupings of bay and river waters. The pH and conductivity measurements taken along a north-south transect from the mouth of the Fish River to the mouth of Weeks Bay show a trend of saline, high pH seawater entering the bay (Figs. 12 and 13). Water collected from 1m depth has higher pH and conductivity with respect to those collected from the surface. This trend suggests that the seawater intrudes along the bottom portion of the water column in the bay before mixing with acidic freshwater from the Fish River. The trend can also be seen in contour plots of 35 pH Co nd uc tiv ity (u S/ cm ) 5 6 7 8 0 1000 2000 3000 4000 5000 6000 TransitionZone RiverWater BayWater Groundwater Figure 8. Plot of pH vs. conductivity showing a general relationship in water chemistry parameters that demonstrate the mixing of seawater, river water, and groundwater in Weeks Bay. 36 Conductivity(uS/cm) DO (u g/L ) 0 2000 4000 6000 3000 4000 5000 6000 7000 8000 9000 TransitionZone RiverWater BayWater Groundwater Figure 9. Plot of conductivity vs. DO showing a relationship in water chemistry parameters to locations within Weeks Bay. 37 Figure 10. Plot of temperature vs. pH demonstrating a relationship in the mixing of seawater, river water, and groundwater within Weeks Bay. 38 Temperature (?C) Co nd uc tiv ity (u S/ cm ) 20 22 24 26 28 30 32 34 0 1000 2000 3000 4000 5000 6000 TransitionZone RiverWater BayWater Groundwater Figure 11. Plot of temperature vs. conductivity that demonstrates a relationship between water chemistry and location within Weeks Bay. 39 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 0 400 650 1150 1450 1750 2650 3500 4200 Distance (m) pH Surface pH 0.5 m pH 1 m pH Figure 12. Plot of pH values at three different depths along a north-south transect from the mouth of the Fish River to the mouth of the Weeks Bay. A high pH front is created by the intrusion of seawater into Weeks Bay. Mouth of the Weeks Bay Mouth of the Fish River 40 0.00 1000.00 2000.00 3000.00 4000.00 5000.00 6000.00 7000.00 0 400 650 1150 1450 1750 2650 3500 4200 Distance (m) Co nd uc tiv ity (m S/c m) Surface Conductivity (?S/cm) 0.5 m Conductivity (?S/cm) 1 m Conductivity (?S/cm) Figure 13. Plot of conductivity values at three different depths along a north-south transect from the mouth of the Fish River to the mouth of the Weeks Bay. A high salinity front, indicated by increasing conductivity, is created by the intrusion of seawater into Weeks Bay. Mouth of the Fish River Mouth of Weeks Bay 41 samples taken at the surface and at 1-meter depths through out the bay (Figs. 14-16). Steep contour gradients of conductivity, temperature, and pH in the upper bay indicate the saline wedge formed by mixing of saltwater and freshwater. Waters at 1 meter depth are more saline than those at the surface, indicating that denser seawater intrudes farther into Weeks Bay at depths below relatively fresh surface water. These plots illustrate that warm dense seawater penetrates beneath cold fresh waters from the rivers in same cases up the mouth of the Fish River, Surfer maps from additional trips can be found in the appendix (see attached CD). The saltwater wedge within Weeks Bay, clearly demonstrates thermal and saline stratification. Interestingly, studies in different watershed basins suggestion that highest mercury methylation mainly occurs near the saline wedge, where acidic water and low-salinity water are both present by mixing (Ullrich et al., 2001; Celo et al., 2005). Contour maps of DO and ORP for Weeks Bay indicate areas of low oxygen and reducing conditions. The reduced oxygen levels in these areas may indicate microbial activity, which is an important factor in the methylation of mercury. Areas with the lowest DO exist at the mouth of the Fish River near the upper bay. The ORP contour maps show areas of Weeks Bay that exhibit spatial variations in oxidized or reduced conditions (Fig. 18). Some bacteria, such as SRB, prefer anaerobic waters with low ORP values that may contribute to the methylation of mercury (King et al., 2002). Interestingly, the lowest ORP values are located in the upper-central portion of the bay, near the interface of fresh and brackish waters. Additional trips exhibited lower ORP values indicating anaerobic conditions during certain times of the year (see CD). These 42 -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 82 82.5 83 83.5 84 84.5 85 85.5 86 86.5 87 87.5 88 88.5 89 89.5 90 90.5 91 91.5 92 Te mp era tur e ( F) La titu de Longitude 33.33 32.77 32.22 31.66 31.11 30.55 30.00 29.44 28.88 28.33 27.77 Te mp era tur e ( ?C ) Te mp era tur e ( F) La titu de Te mp era tur e ( ?C ) -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 82 82.5 83 83.5 84 84.5 85 85.5 86 86.5 87 87.5 88 88.5 89 89.5 90 90.5 91 91.5 92 Te mp era tur e ( F) La titu de Longitude 33.33 32.77 32.22 31.66 31.11 30.55 30.00 29.44 28.88 28.33 27.77 Te mp era tur e ( ?C ) Te mp era tur e ( F) La titu de Te mp era tur e ( ?C ) Figure 14. Contour maps of surface (top) and 1-meter (bottom) temperature levels in Weeks Bay and Fish River. Warmer waters at 1-meter depth are shown farther into the bay than those at the surface. 43 -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8 8.2 8.4 8.6 pH La titu de Longitude pH La titu de pH La titu de -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8 8.2 8.4 8.6 pH La titu de Longitude pH La titu de p H La titu de Figure 15. Contour maps of surface (top) and 1-meter (bottom) pH levels in Weeks Bay and Fish River. Like the temperature readings, higher pH water can be found closer to the mouth of the Fish River at 1-meter depth than at the surface. 44 -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 Co nd uc tiv ity (u S/ cm ) La titu de Longitude Co nd uc tiv ity (? S/ cm ) Co nd uc tiv ity (u S/ cm ) La titu de Co nd uc tiv ity (u S/ cm ) La titu de Co nd uc tiv ity (? S/ cm ) -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 Co nd uc tiv ity (u S/ cm ) La titu de Longitude Co nd uc tiv ity (? S/ cm ) Co nd uc tiv ity (u S/ cm ) La titu de Co nd uc tiv ity (u S/ cm ) La titu de Co nd uc tiv ity (? S/ cm ) Figure 16. Contour map of surface (top) and 1-meter (bottom) conductivity levels in Weeks Bay. Similar to temperature and pH, higher conductivity readings can be found farther up the bay at 1-meter depth than those at the surface. 45 -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 Di ss olv ed O xy ge n ( ug /L) La titu de Longitude Di ss olv ed O xy ge n ( ?g /L) Di ss olv ed O xy ge n ( ug /L) La titu de Di ss olv ed O xy ge n ( ug /L) La titu de Di ss olv ed O xy ge n ( ?g /L) -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 Di ss olv ed O xy ge n ( ug /L) La titu de Longitude Di ss olv ed O xy ge n ( ?g /L) Di ss olv ed O xy ge n ( ug /L) La titu de Di ss olv ed O xy ge n ( ug /L) La titu de Di ss olv ed O xy ge n ( ?g /L) Figure 17. Contour map of surface (top) and 1-meter (bottom) DO levels. The lowest DO readings are located at the mouth of Fish River. 46 -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 275 300 325 350 375 400 425 450 475 500 525 550 575 600 625 OR P (m V) La titu de Longitude OR P (m V) La titu de Figure 18. Contour map of surface (top) and 1 meter (bottom) ORP contour maps. Lowest ORP zones are found on the western side of the bay and the highest at the mouth of the Fish River. -87.87 -87.86 -87.85 -87.84 -87.83 -87.82 -87.81 -87.8 30.37 30.38 30.39 30.4 30.41 30.42 30.43 30.44 275 295 315 335 355 375 395 415 435 455 475 495 515 535 555 575 595 615 635 OR P (m V) La titu de Longitude OR P (m V) La titu de 47 same conditions can also increase the affinity of mercury to sulfides removing mercury from the system and reducing methylation (Kannan, 1998). Along with field parameters chemical analysis of collected water samples were completed for major ions and trace elements. Data from major ion analyses are consistent with the results of the measured parameters. The surface water in the bay is predominated by an average Na/Cl molar ratio of about 0.86, similar to that of seawater. The surface water has lower SO4/Cl ratios (average 0.048) with respect to that of seawater (~0.052), indicating that SO4-2 may be removed by mineral precipitation (such as gypsum, CaSO4) or bacterial sulfate reduction: SO4-2 + 2CH2O +3H+ ? 2CO2 + H2S + 2H2O Where CH2O represents organic matter. Sulfate reduction is an important factor in Hg cycling as SRB take up Hg in its inorganic from and convert it to methylmercury through metabolic processes (King et al., 2002). This process may play an important role in the methylation of mercury in Weeks Bay. The surface water in the bay also has elevated metal content of Sr, Ni, and Rb, similar to that of seawater (Table 5). Cl/Br ratios of surface water, ranging from ~306 to ~318, are slightly higher than that of seawater (~288). Figure 19 shows the distribution of sampled waters using a piper diagram. The surface waters contain high amounts of Na and Cl, similar to values found in seawater. The groundwater analyzed contains high amounts of Na and HCO3-, indicating sodium bicarbonate type of groundwater. The Na-HCO3 type high alkalinity of the groundwater 48 C A T I O N S A N I O N S%meq/l Na+K HCO +CO3 3 Cl Mg SO4 Ca Calcium (Ca) Chloride (Cl) Su lfat e (S O4 ) + Ch lor ide (C l) Ca lciu m (Ca ) + M agn esi um (M g) Ca rbo nat e ( CO 3) + B ica rbo nat e ( HC O3 ) So diu m (N a) + Po tas siu m (K ) Su lfa te ( SO 4)Magn esi um (M g) 80 60 40 20 20 40 60 80 80 60 40 20 20 40 60 80 20 40 60 80 80 60 40 20 20 40 60 80 20 40 60 80 80 60 40 20 80 60 40 20 009 011A 011B 014 018 023 027 028 033 038 041 WW13 WW14 WW15 WW16 Seawater Figure 19. Piper diagram showing surface and groundwater compositions compared to that of seawater. Surface waters resemble those of seawater (i.e., Na-Cl types), whereas groundwater is sodium bicarbonate rich. 49 is most likely a result of the combination of dissolution of calcite and ion exchange (Marimuthu, 2005, Penny et al., 2005). Chemical analyses of major ions also provide further information on the physical mixing and accompanying biogeochemical reactions. A graphic technique is used to evaluate mixing behavior (Figs. 20-23). In this method, chloride, a conservative (non- reacting) species, is plotted on the x-axis. The species of interest, which may or may not be conservative, is plotted on the y axis. The mixing behavior can be determined based on the proximity of data points to the straight line drawn between the seawater and freshwater end-members. Data points lying on or close to the conservative mixing line indicate that dissolved species exhibit conservative behavior. Non-conservative behavior is indicated if the data points deviate significantly from the conservative mixing line. Enrichment or depletion of species in solution may be caused by biogeochemical processes such as mineral dissolution/precipitation, ion-exchange, or microbial processes. In all of the graphs, a linear trend reveals the conservative mixing between seawater and freshwater. River waters plotted near those of freshwater and bay waters, as influenced most by saltwater intrusion, has the highest major ions concentrations. The results of the graphical analyses indicate Na+, Ca+2, Mg+2 and Br- exhibit conservative behavior during mixing. Only sulfate exhibits non-conservative depletion (about 10%). The reason for this depletion is unclear and may be a result of bacterial sulfate reduction. Previous studies have found that some estuary environments show the highest levels of methylmercury in the upper portions near the mouths of tributaries. In the study area the upper estuaries contained low DO levels, low pH, and low salinity (Baeyens, 1998; Benoit, 1998; and Leermakers, 2001). These conditions are ideal for sulfate- 50 Chlorine (ppm) Br om ine (p pm ) 5000 10000 15000 2000010 20 30 40 50 60 70 TransitionZone Seawater RiverWater BayWater Chloride (ppm) Br om ide (p pm ) Br om ine (p pm ) Br om ide (p pm ) Figure 20. Plot of Cl versus Br. Linear pattern between various waters found within Weeks Bay. 51 Chlorine (ppm) Ca lci um (p pm ) 5000 10000 15000 2000050 100 150 200 250 300 350 400 450 500 TransitionZone Sea Water RiverWater BayWater Chloride (ppm) Ca lci um (p pm ) Figure 21. Plot of Cl versus Ca graph showing a similar linear pattern between waters. 52 Chlorine (ppm) Ma gn es ium (p pm ) 5000 10000 15000 20000200 400 600 800 1000 1200 1400 TransitionZone Seawater RiverWater BayWater Chloride (pp ) Ma gn es ium (p pm ) Figure 22. Plot of Cl versus Mg graph with a linear pattern between Weeks Bay watershed waters. 53 Figure 23. Plot of Cl versus Na graph showing mixing of waters within Weeks Bay with a linear pattern between waters. Chlorine (ppm) So diu m (p pm ) 5000 10000 15000 200000 2000 4000 6000 8000 10000 12000 TransitionZone Seawater RiverWater BayWater Chlorid ( ) So diu m (p pm ) 54 reducing bacteria to exist which have been shown to play a part in the methylation of mercury (Benoit 2001, King 2002). These same conditions may exist within Weeks Bay at the mouth of the Fish River. According to Drever (1997), DOC contents for rainwater range from 0.5 mg/L to 1.5 mg/L, seawater is about 0.5 mg/L, and river and lake waters range are 2 mg/L to 10mg/L. The DOC data collected from the surface waters range 1.4 mg/L to 2.2 mg/L. These results may indicate that the surface water of the bay and rivers are the result mostly rainwater of meteoric origin with some samples falling in the rivers and lakes range. However, these results may be skewed due to the mixing of lower DOC seawater and groundwater diluting the amount of DOC in the bay. The narrow range of DOC values indicate that DOC level is not an important factor in controlling the spatial distribution of Hg methylation in the Bay. Groundwater Movement Head levels of four clustered USGS monitoring wells on the western shore of Weeks Bay indicate that groundwater are discharging upward into Weeks Bay (Fig. 24). Head levels at WW13 and WW14, the farthest from the bay, are equal, suggesting horizontal groundwater movement. WW15 and WW16 are located closer to the bay and show a different groundwater flow pattern. WW16, slightly further inland from WW15, has head levels higher than those of WW13 and WW14, while the deeper well WW15, the closest to the bay, is an artesian well and has higher hydraulic head than the shallow WW16 in a nearby location. This suggests that groundwater is moving upward into Weeks Bay. 55 Figure 24. Diagnostic illustration of groundwater movement near the Week Bays. Deeper well WW 15 is an artesian well and has a higher hydraulic head than the adjacent shallow well WW16, indicating an upward movement of groundwater into Weeks Bay. Distance clustered wells WW 13 and 14 have the same hydraulic head, indicating horizontal flow predominates in the aquifer away from the bay. Weeks Bay NW SE WW14 Depth 25 ft WW13 Depth 65 ft WW16 Depth 8.5 Ft WW15 Depth 82.5 Ft Groundwater Flow Water Table Levels 56 Groundwater Influences Data retrieved from USGS monitoring wells are different than that derived from surface waters. Three of these wells (WW13, WW14, and WW16) were found to contain mercury levels (6 ? 8 ppb) higher than detection limits. The DOC in these wells ranges from 1.4 to 1.9 mg/L, with one well, WW15, reaching a value 3.3 mg/L. The DOC values of WW15 more closely resemble those of river and lake values, this may be the result of the surface water infiltration into the groundwater. This possibility is consistent with the higher conductivity found in WW15, which is similar to the conductivity of Weeks Bay (Table 5). Stable isotope analyses provides more details about the mixing of waters and demonstrates the role that groundwater plays in influencing the chemistry of Weeks Bay surface water. Comparing oxygen and hydrogen isotopes of sampled sites along with seawater signature as well as the local meteoric water line sheds more lights on the nature of mixing and evaporation (Fig. 25). Again we can see the grouping of bay water, river water, and groundwater with the transition zone at the mouth of the Fish River between those of river water and bay water groupings. Groundwater isotope data values plot on or near the local meteoric water line (LMWL) (Cook, 1997; Penny et al., 2003), suggesting that groundwater has not undergone great evaporation or water rock interaction since its recharge. Water rock interaction is unlikely due to the increase in hydrogen to which water rock interactions would not contribute. In contrast, the Fish River waters are enriched in 18O and 2H and plot farther off the LMWL, indicating greater evaporation than groundwater. Bay water has the highest 18O and 2H values, 57 -8 -6 -4 -2 0-40 -30 -20 -10 0 10 ? O (per mil, SMOW)18 ????D (pe r m il, SM OW ) seawater groundwater river water bay water mixing evaporation LM WL seawater ????D (pe r m il, SM OW ) ????D (pe r m il, SM OW ) ????D (pe r m il, SM OW ) Figure 25. Plot of evaporation trajectory of local meteoric water line (LMWL) (Holser, 1979; Cook, 1997) and seawater mixing trend, shown using deuterium (?D) and oxygen (?18O) isotope ratios of groundwater and surface water in Weeks Bay. As evaporation occurs in river waters, the waters become enriched in their isotopic signatures as evaporation preferentially removes lighter 16O and 1H. The data show that Fish River and Weeks Bay waters, which are farther off the LMWL, undergo greater evaporation than groundwater. The Weeks Bay water represents a mixture of two ?end-member? waters: one of seawater and one of river water or groundwater of meteoric origin impacted by variations in evaporation rates. 58 indicating stronger influence from mixing with isotopically heavier seawater. The stable isotope signatures indicate that the Weeks Bay water represents a mixture of two ?end- member? waters: one of seawater and one of river water or groundwater of meteoric origin impacted by various degrees of evaporation. 59 CONCLUSIONS The most likely source for mercury contamination found in the Weeks Bay watershed is from atmospheric deposition. Examining the precipitation data from the USGS and mercury deposition data from the MDN show that an increase in precipitation results in an increase mercury deposition in the Weeks Bay watershed. The complex mixing that is taking place within the Weeks Bay watershed impacts the water chemistry and methylation of mercury. The addition of seawater, freshwater, and groundwater contribute to the conditions (e.g., warm, acidic, low-salinity) necessary for methylation. The oxygen and hydrogen isotope data suggest that the chemistry and quality of surface waters of river and bay are affected by evaporation, meteoric recharge, groundwater discharge, and mixing with seawater. The stable isotope signatures of groundwater fall close to the LMWL, indicating minimum evaporation prior to surface infiltration. River water and bay water show enrichment of 18O and 2H relative to groundwater, indicating that they undergo greater evaporation or mixing with isotopically heavier seawater. Similar isotopic signatures and evaporation patterns are found in a coastal salt march in Australia (Marimuthu, 2005). Geochemical analysis suggests that, most major ions (Na+, Ca2+, Mg2+, Br-, etc.), with the exception of SO4-2, exhibit conservative behavior during water mixing. In addition to physical mixing and evaporation, biochemical processes such as bacterial sulfate reduction may be at work in the watershed as indicated by non-conservative depletion of SO4-2. Bacterial 60 sulfate reduction is known as the critical first strep for Hg methylation and bioaccumulation. The intrusion of seawater into Weeks Bay creates a front of high salinity, high pH water that penetrates below low pH, low-conductivity freshwater. At this wedge interface, a depth difference of only a few centimeters causes rapid changes in water chemistry. Such salinity and thermal stratifications are pronounced in the bay at various seasons. The seasonal changes in salinity and thermal stratifications may control the location of mercury methylation and require further study. In addition, the discharge of acidic, low-salinity groundwater into the bay and river may contribute vital conditions that promote the methylation of mercury. The most rapid changes in water chemistry occur at the mouth of the Fish River leading into Weeks Bay. At this point, the mixing of several waters of different chemical characteristics creates a favorable environment (i.e., the presence of warm, acidic, and low-salinity waters) where methylation may occur and cause the spread of mercury contamination throughout the watershed via bioaccumulation. Low DO and ORP values observed in this mixing zone suggest active microbial processes that are an important factor in initializing Hg methylation. Seasonal variations also allow for increased and decreased methylation rates in the watershed. Similar variations also are seen in other mercury contaminated estuary environments (Benoit, 1998; Mason, 1999; Leermakers, 2001; Conaway, 2003). Summer conditions are characterized by relatively high temperature, lower pH, low conductivity, low DO and high ORP. Summer months are also the wet test season for the region as indicated by the data of mercury deposition and precipitation. The bay chemistries during winter and spring times were characterized by low temperatures, high pH, low 61 conductivity, high DO, and low ORP. The spring and winter data appears to have more uniform conditions throughout the bay, indicating greater mixing and less stratification. This could be a result of increased winds and less stream discharge as a result of reduced precipitation. Comparing observed conditions to those of similar estuaries in North America, generalizations can be made of when and where mercury methylation may occur within Weeks Bay. In the Chesapeake Bay, Scheldt Estuary, and San Francisco Bay mercury concentrations are highest during high water flow seasons. During these seasons all of these estuaries exhibited high temperatures, low pH, low conductivity, and low DO (Benoit, 1998; Mason, 1999; Leermakers, 2001; Conaway, 2003). These conditions are similar to those found in Weeks Bay and indicate that the greatest methylation is occurring during summer months. 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