PROJECT SUMMARY

Figure 1. Proposed ASR systems in the Comprehensive Everglades Restoration Plan. SOURCE: CERP, 2002 (NRC 2002). [larger image]

There are many uncertainties regarding the use of large-scale aquifer storage and recovery (ASR) technology in south Florida, including suitability of proposed ASR source waters, water quality concerns and the potential for mercury bioaccumulation (NRC 2002). The type of source waters used for aquifer recharge in south Florida includes treated drinking water, untreated ground or surface water, and reclaimed water. The source water planned for the Comprehensive Everglades Restoration Plan (CERP) ASR program is untreated or partially treated groundwater or surface water (Reese 2002).

The objectives of this scope of work are to determine: (1) background levels of mercury, methylmercury (MeHG) and other chemical species in selected aquifers or subaquifers; (2) the distribution of mercury and methylmercury in the aquifer system in South Florida; (3) the conditions under which methylation of mercury may occur in aquifer materials, and (4) potential changes in water chemistry that could enhance methylmercury production (eg. sulfate and carbon additions) as a result of injecting water for storage in, or post recovery from, the Floridan aquifer system (FAS) in south Florida.

Future investigations will be conducted to determine the potential for recovered water to enhance methylmercury production in the environment.

INTRODUCTION

Alternating periods of flooding and drying, called hydroperiods, vital to the historical functioning of the Everglades ecosystem, have been severely altered by human activities. Restoring these variations in water flows and levels is an integral part of the CERP. Specifically, the timing of water held and released into the ecosystem will be modified so that it more closely matches natural patterns (CERP 2002).

ASR has been proposed as a cost-effective water-supply alternative that can help meet the needs of agricultural, municipal, and recreational users and also be used for Everglades ecosystem restoration (Reese 2002). ASR is used to store excess surface water and shallow groundwater during wet periods for recovery during longer-term dry periods (NRC 2002). In an attempt to capture, store and redistribute fresh water previously lost to tides and to regulate the quality, quantity, timing and distribution of water flows, the CERP proposes the construction of over 300 ASR wells (Figure 1) in south Florida (Reese 2002).

Figure 2. Map of locations and status of aquifer storage and recovery sites as of April 2001 (Reese 2002). [larger image]

ASR technology has been tested and implemented in some areas of south Florida. Existing and historical ASR sites in southern Florida are mostly associated with public water utility systems located along the southeast and southwest coasts (Figure 2). Under the CERP, ASR wells will be constructed in inland areas around Lake Okeechobee, in central Palm Beach County, and along the Caloosahatchee River in Hendry County (Figure 1). Recovered water is to be used for purposes that include maintaining water levels in Lake Okeechobee and wetlands areas and reduction of surface-water flows to tides (estuarine and bay areas) during storm events (Reese 2002).

The storage zone being used at most ASR sites is the FAS (Figure 3). This aquifer system is continuous throughout southern Florida, and its overlying confinement is generally good. Shallower storage zones are in the mid-Hawthorn and sandstone aquifers of the intermediate aquifer system and the surficial aquifer system (Reese 2002).

Mercury, derived from atmospheric deposition and accumulated in the aquatic ecosystem of south Florida is an environmental concern in south Florida. About 2 million acres of the south Florida Everglades ecosystem are under fish consumption advisories because of mercury contamination. High levels of mercury were found in fish from the Everglades in 1989 during monitoring by the Florida Fish and Wildlife Conservation Commission, Department of Environmental Protection, and Department of Health. Consumption of mercury-contaminated food is known to be neurotoxic to humans. These findings led the State Health Officer to issue health advisories urging fishermen not to eat some species of fish caught from the Everglades, and to limit consumption of largemouth bass and several other predatory fish species taken from other fresh or coastal waters of Florida (Florida DEP 2001). Figures 4 and 5 show mercury concentrations and wet deposition are higher in Florida relative to the rest of the United States.

Figure 3. Generalized geology and hydrogeology of Lee, Hendry, and Collier Counties (Reese 2002). [larger image]

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Figure 4. U.S. Mercury concentration trends (NADP 2001). [larger image]

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Figure 5. U.S. Mercury wet deposition trends (NADP 2001 ). [larger image]

Objectives

The objective of this scope of work is to develop a hydrogeochemical framework of mercury and methylmercury in the hydrogeologic system in which ASR will be developed in the Upper Floridan Aquifer. Task 1 of this study will be to develop a conceptual understanding of "ambient" mercury concentrations within the hydrogeologic system, in which the ASR will be developed, drawing from reports in the literature and other available data on mercury and the hydrogeologic system.

Task 2 will involve field sampling and laboratory analyses of water from municipal wells, private wells, existing wells at other ASR project sites, and pilot ASR wells that are part of this large scale project. The objective of the sampling is to provide an assessment of the occurrence and distribution of mercury and methylmercury concentrations in various hydrostratigraphic units of the FAS. This phase of the study will also include obtaining lithologic samples of the FAS from cores from preexisting ASR wells or new pilot wells.

Tasks 3, 4 and 5 will consist of laboratory experiments using FAS material and native surface and groundwater to: (1) evaluate the potential for ASR to methylate mercury within the FAS; (2) if methylation is observed, to determine the amount of time required for stored water to methylate; (3) determine the geochemical conditions under which methylation does or does not occur in ASR stored water; and (4) to assess the potential to leach intact mercury/methylmercury from aquifer materials and determine if aquifer materials are a net source/sink of mercury.

Task 6 will present results of the study in oral and written formats.

Background Information on Mercury

The processes that control mercury methylation are not completely understood, but if mercury pollution is to be managed, there needs to be a better understanding of the behavior of mercuric contaminants in the natural environment.

Alkali and metal processing, incineration of coal, and medical and other waste, and mining of gold and mercury contribute greatly to mercury concentrations in some areas, but atmospheric deposition is the dominant source of mercury over most of the landscape. Natural sources of atmospheric mercury include volcanoes, minerals containing mercury, and volatilization from the ocean. Although all rocks, sediments, water, and soils naturally contain small but varying amounts of mercury, some mineral occurrences and thermal springs, generally found in the mineralized areas of the western US, are naturally high in mercury (USGS FS 2000).

Air currents and rainfall transport and deposit mercury gases, ionic and particulate mercury, with less amounts of methylmercury, from the atmosphere to the earth's surface. Mercury is normally a problem where the rate of natural formation of methylmercury from inorganic mercury is greater than the reverse reaction. The principal key in understanding the mercury problem is unraveling the complexities of mercury methylation. Methylmercury is the form of mercury that accumulates at the top of the food web. Environments that are known to favor the production of methylmercury include certain types of wetlands, dilute low-pH lakes in Northeast and North-central United States, parts of the Florida Everglades, newly flooded reservoirs, and coastal wetlands (USGS FS 2000).

Mercury is one of the most toxic contaminants that may be present in the aquatic environment, but its ecological and toxicological effects are strongly dependent on the chemical species present. Species distribution and transformation processes in natural aquatic systems are controlled by various physical, chemical, and biological factors. Inorganic mercury forms may be transformed into organic, methylated species that are many times more toxic to aquatic organisms. Studies suggest that methylmercury is formed primarily in anoxic waters and sediments and is degraded under aerobic conditions. Field studies in estuarine sediments indicate that metal methylation occurs most rapidly under anoxic conditions in the presence of active microbial sulfate reduction (Ullrich et al. 2001). Researchers have shown that sediments can act as sinks or sources of mercury. Mason et al (1999) found that sedimentation is important in removing particulate mercury from the water column and it is likely that mercury is returned from sediments to the water column during periods of low oxygen (anoxia). A study by Covelli et al shows that a budget based on benthic flux measurements indicates that 75% of total mercury is buried in the sediment and 25% of total mercury, approximately 23% in methylated form, is annually recycled and released at the sediment-water interface.

There are two ways in which mercury can be methylated to methylmercury, by organisms (biomethylation) and chemically (abiotic methylation), which is purely chemical methylation of mercury if suitable methyl donors are present (Ullrich et al. 2001), but biotic methylation generally is more important. Demethylation, like methylation, appears to be maximal near the sediment-water interface, but is favored at higher redox potentials than methylation (Gilmour et. al 1992). The chemical form of mercury in aquatic systems is strongly influenced by redox (E_h) and pH conditions, sunlight exposure, as well as by concentrations of inorganic and organic complexing agents. The cycling and distribution of mercury between the sediment and water phases may be physically, chemically, or biologically mediated. The release of methylmercury from sediments also increases with increasing temperature and nutrient addition and decreasing pH (Ullrich et al. 2001).

Hydrogeology of the Floridan Aquifer System

Figure 6. Transmissivity determined for storage zones in the Floridan aquifer system at aquifer storage and recovery sites in southern Florida (Reese 2002). [larger image]

The three principal hydrogeologic units in south Florida are the surficial aquifer system, intermediate aquifer system, and Floridan aquifer system. Water-bearing rocks in the intermediate aquifer system grade or pinch out to the east, and in southeastern Florida the intermediate aquifer system becomes the intermediate confining unit (Figure 3). The Floridan aquifer system consists of the Upper Floridan aquifer, middle confining unit, and Lower Floridan aquifer (Reese 2002).

The Upper Floridan aquifer is 500 to 1,200 ft thick in southern Florida (Figure 3). The Upper Floridan aquifer consists of several thin water-bearing zones of high permeability interlayered with thick zones of low permeability (Reese 2002). This aquifer is well confined above by thick units in the Hawthorn Group consisting of clay, marl, silt, or clayey sand. The middle-confining unit of the Floridan aquifer system consists of micritic limestone (wackstone to mudstone), dense dolomite, and in some areas, beds of gypsum, and underlies the Upper Floridan aquifer and provides good to leaky confinement (Reese 2002).

In southwestern Florida, the Upper Floridan aquifer includes the lower part of the Hawthorn Group, Suwannee Limestone, Ocala Limestone, and in some areas, the upper part of the Avon Park Formation (Figure 3). In southeastern Florida, the Suwannee Limestone and Ocala Limestone are commonly absent. In both eastern and western areas, the top of the Upper Floridan aquifer usually is contained within a basal Hawthorn unit (Figure 3). In some areas along the east coast, the Suwannee Limestone is either interpreted as being absent or present in the lower part of this basal Hawthorn unit (Reese 2002). Significant permeable zones occur in the upper part of the Suwannee Limestone and from the lower part of the Ocala Limestone to the upper part of the Avon Park Formation (Sacks 1996). In the upper limestone sections, porosity is dominated by fractures (Sacks 1996).

The transmissivity of the Upper Floridan aquifer varies widely. The most transmissive permeable zone is found at the top of the Upper Floridan aquifer and is associated with the unconformity at the top of the rocks of Eocene age. There is a belt of high transmissivity in the upper one-third of Eocene rocks that occurs in southern Dade County and northern Key Largo and trends east-northeast (Reese 1994). Transmissivity of the Upper Floridan Aquifer ranges from 30,000 ft²/d at the Gulf Coast to 850,000 ft²/d in northeastern De Soto County (Metz 1996). In Lee and Hendry counties transmissivity values are typically 100,000 to 200,000 ft²/d (Sacks 1996).

Figure 6 shows transmissivity values for the storage zones of proposed ASR sites. Hydraulic properties determined from tests of storage zones may apply only to the storage zone or to a thicker interval if the aquifer containing the storage zone is thicker than the storage zone. In the case where the aquifer is thicker than the storage zone, the hydraulic conductivity of a storage zone will be less than that obtained by dividing the transmissivity determined from a test by the thickness of the storage zone. However, in the Upper Floridan aquifer where thick zones of relatively low permeability separate flow zones, tests of part of the aquifer are typically not influenced by the entire thickness of the aquifer. Thus, the value of transmissivity obtained is less than the total transmissivity of the aquifer (Reese 2002).

Water Chemistry

Figure 7. Ambient water salinity of storage zones in the Floridan aquifer system at ASR sites in south Florida. [larger image]

Data on ambient water chemistry, collected from storage and monitoring wells at ASR sites, include water from storage zone intervals, intervals deeper and shallower than the storage zone, and intervals that include more than the selected storage zone. Upper FAS ASR facilities in southwestern Florida were usually sampled from shallower permeable zones of the intermediate aquifer system. The inventoried data describe water salinity, specific conductance, dissolved chloride concentration, dissolved solids concentration, temperature, and dissolved sulfate concentration.

Specific conductance range from 300µS/cm to 50,000µS/cm, dissolved solids concentrations range from 300mg/L to 31,000 mg/L, temperatures range from 21°C to 35°C, and dissolved sulfate concentrations range from 21mg/L to 1,300mg/L. Reese (2002) outlines specific details on sample locations, intervals and corresponding concentrations.

The chloride concentrations of ambient water, in ASR storage zones, in the Floridan aquifer system (Figure 7) range from 500 mg/L at the Lee County WTP site to 11,000 mg/L at the Englewood South Regional WWTP site. At most sites, the chloride concentration ranged from about 1,000 to 3,000 mg/L, and the average concentration was about 2,300 mg/L. Storage zones containing water with 3,000 mg/L or greater were considered to have high chloride concentration. The highest value found in the east coast area was 3,600 mg/L at the City of Sunrise Springtree WTP site. The highest chloride concentration found in the upper part of the Upper Floridan aquifer in southern Florida based on three previous studies was 8,000 mg/L in northeastern Palm Beach County; the lowest concentration found was 400 mg/L in Lee County (Reese 2002). At these chloride levels, the mercury-chloride ligand pair (HgCl₂⁰, HgCl₃^-1, or HgCl₄^-2) likely dominates the speciation of mercury in the aquifers of south Florida. The presence of a zero charged ligand pair (e.g., HgCl₂⁰) is thought to be one of the controlling factors regulating mercury bioavailability for methylation (Benoit et al., 1999), but the chloride levels that this species would predominate is also affected by other water quality parameters (pH, E_h, dissolved organic carbon, sulfide, etc.).

PROJECT PLAN

Methods

Task 1 - Conceptual Mercury Model

A conceptual model of mercury within the FAS in south Florida will be developed by assembling information from reports in the literature and other available data. The conceptual model will include background chemistry of aquifer materials, groundwater, and surface water and will attempt to put mercury, ASR, and the hydrogeology of south Florida in a consistent framework.

Task 2A and 2B - Field Collection of Water Samples and Laboratory Analyses

Water samples (a total of 75) will be collected twice during this project (Task 2A in July 2003 and Task 2B in December 2003). Sampling wells will be selected to obtain a representative distribution of sampling points. Permission to sample the wells will be obtained from representatives of municipalities, private owners, existing ASR well project sites, and pilot ASR sites. ASR wells 20 (LO), 27 (SL), and those found in Palm Beach and Broward Counties will be targeted (Figure 2), as well as (USGS monitoring wells located in Glades, Hendry, Palm Beach, and Broward Counties (USGS 2003). Although the wells listed above are targeted for sampling, final determination of the sampling network will depend upon obtaining permission to sample the wells.

While in the field, water samples will be analyzed with a portable field probe for dissolved oxygen (DO), pH, redox, temperature, specific conductance, and dissolved organic carbon (DOC). Water samples will be analyzed for major cations and anions as well. Also, a series of water samples will be collected for mercury analysis using trace metal ultraclean techniques as outlined by Krabbenhoft and Babiarz (1992), to eliminate direct contact between the sample gear and field personnel. These water samples will be analyzed by the USGS Mercury Research Lab (Wisconsin District Mercury Laboratory) for mercury using method 1631, the analytical method approved by the USEPA that allows determination of mercury at a minimum level of 0.4 ppt (parts-per-trillion), and supports measurements at the ambient water quality criteria for mercury published in the National Toxics Rule (40 CFR 131.36) and in the Final Water Quality Guidance for the Great Lakes System (60 FR 15366). Methylmercury will be determined using the distillation and ethylation procedure first described by Horvat et al. (1993), and is used the standard operating procedure by the USGS Mercury Lab.

The only known true groundwater samples that have been analyzed for total mercury and methylmercury in south Florida were those reported by Harvey et al. (2002). Samples acquired for that data set were taken from piezometers screened in the surficial aquifer, above the Hawthorn and Floridian Aquifers. Nevertheless, in that sample set of 50 groundwater samples, about 10 percent of the samples contained high levels of MeHg (1-8 ng/L), and thus underscoring the reason for possible concern for large-scale implementation of ASR. However, not enough information was available to infer whether the MeHg present in these samples was produced with in the shallow aquifers, or if the MeHg was possible contained in surface waters that recharged the aquifer.

Task 2C - Obtaining Aquifer Cores Samples

Selection and acquisition of core materials will be coordinated with the USACE technical lead overseeing the drilling and other associated project tasks. To be most effective, core materials needed for this component of the project would be derived from portions of the FAS that will be used in the operational phase of the ASR project. Samples that represent a range of aquifer material types (texture, mineralogy and chemistry) would provide the best testing materials for this work. Although intact cores from pilot wells would be ideal for these tests, cuttings would also be usable for these evaluations. Core samples must be received by August 2003 for the tests for up to 6 months before taking final samples.

Task 3 - Water-Rock Incubation Studies

Although incubation tests used to assess methylmercury formation rates have been used by mercury researchers for over a decade (Krabbenhoft et al., 1998), there are no known published reports on studies using carbonate aquifer materials, or studies that target the evaluation of methylation potential and methylation rates at time scales of weeks to months. As such, we will adopt the best approach for conducting the methylation studies after we complete the synoptic field sampling and have had opportunity to evaluate what types of materials may be available for testing, what procedure would be most appropriate and feasible, and over what time scales. In essence, however, one of the following methods will be used: column studies using intact Floridian Aquifer cores; column studies of unconsolidated Floridian Aquifer materials; or, batch studies using unconsolidated Floridian Aquifer materials. With any of these procedures, we will employ geochemical controls (redox, chloride, sulfate, DOC, and mercury isotopes) to test whether these constituents affect aqueous MeHg formation rates. Representative water types and sources will also be used in these studies, such that the mixing of native surface water and groundwater will be evaluated under varying redox regimes. A series of six (6) to ten (10) column studies will be set up in the laboratory using Floridan Aquifer materials and native surface water and groundwater. The experiments conducted for this portion of the study will be conducted for time periods up to six months.

Task 4 - Leaching of Aquifer Materials

Leaching experiments will be performed to determine whether aquifer materials are acting as net sources or sinks of mercury within the aquifer. If mercury is found to be associated with specific minerals and/or geologic units, efforts will be made to perform experiments using samples characteristic of each type of mercury occurrence. The design of the leaching experiments and the selection of leaching solutions will focus on specific processes likely to occur. Details of the experiments will be developed after analyzing the information gained from the groundwater chemistry and mineralogy data to be assembled and collected in phases 1 and 2 of this project. The experiments will be designed to simulate conditions in the field as much as possible. If the results of the groundwater chemistry sampling indicate that variations in pH and /or redox conditions are important controls on mercury mobility, the pH and dissolved oxygen content of the leaching solutions will be varied accordingly. Although microbial processes affect mercury mobility, they will not be addressed in this project. To insure the processes observed in the leaching experiments are solely abiotic, the experiments will be performed under sterile conditions.

Our leaching experiments will be conducted in a fashion similar to that of Fadini and Jardim (2001), who outlined a method for leaching mercury from soils. Different samples of aquifer materials are suspended in MilliQ^® water to simulate a possible phase transfer between soil and water due to weathering processes (although we will likely use native surface and ground water). Aliquots of soil 0.4 g dry base are then suspended in 100 ml of MilliQ^® water in Erlenmeyer flasks with Teflon^® stoppers and placed in an orbital shaker at 120 strokes min^-1 for 2 hours. After 7 days of quiescent equilibrium, 30 ml aliquots are removed from each flask and centrifuged. Total mercury concentration in the supernatant solution is determined in the sub-samples by detection using USEPA method 1631, and methylmercury by the procedure of Horvat et al. (1993) at the USGS Mercury Research Lab. In addtion, our leaching experiments will employ the use of stable mercury isotopes (e.g., Me¹⁹⁹Hg or ²⁰²Hg) to specifically determine whether Hg or MeHg contained in surface water may adsorb to the aquifer material, methylate during contact with the aquifer materials, chemically reduce to gaseous elemental Hg⁰, or possibly undergo demethylation. The use of stable isotopes facilitates the discrimination of Hg and MeHg in injected surface water from Hg and MeHg possibly contained in carbonate aquifer or native groundwater. The mercury stable isotope analyses for total and methylmercury will be performed at the USGS Mercury Research Lab.

Task 5 - Geochemical Modeling

The geochemical environment that ASR waters will be expected to change radically from surface water conditions to groundwater conditions, and then back to surface water upon recovery. As indicated above, groundwater in the Floridian Aquifer is relatively high in chloride, which should have a strong effect on mercury speciation. However, other competing ligands may also be present (e.g., DOC, sulfide, hydroxide) at sufficiently high levels to "out compete" for the mercury, and/or possibly promote or retard mercury bioavailability for methylation. Recent research (e.g., Benoit et al., 1999) has suggested that the formation of zero charged complexes (HgCl₂, HgS, Hg(OH)₂) in the aqueous environment are the bioavailable species of mercury to methylating microbes, as opposed to charged species (e.g., HgCl₃^-, HgS₂⁼). For this project, we will incorporate the analytical results of the primary ligands for mercury (Cl, S, OH, DOC) and perform chemical speciation calculations using standard geochemical models (e.g., MINEQL) to estimate the geochemical speciation of mercury in aqueous samples for source waters, "injected" groundwater, and recovered water.

Task 6 - Report Preparation and Oral Presentation of Results

Throughout the project, the USGS will provide quarterly status reports to the USACE project manager and technical manager. The quarterly status reports are a brief summary of the tasks completed during the quarter and status of the ongoing effort. These reports should describe any issues or concerns that may impact completion of tasks. Generally, the quarterly report is no more than one page. The final report, a citable USGS Water-Resources Investigations Report, will be a web-distributed report and will document the project results and methods used. The final report will provide guidance on how ASR can best be implemented to minimize the potential for MeHG formation.

In September 2004, most of the analyses and interpretations will be completed. The project staff will present the results to date to the ASR Regional Study project delivery team (PDT) in September or October. Following that presentation, the final report will be completed, reviewed, and approved for USGS publication by March 2005.

TIMETABLE

PROJECT RELEVANCE

Previous mercury studies have concentrated on mercury and methylmercury in the atmosphere, aquatic environments, and top few centimeters of sediments. There have been no studies that have studied mercury and methylmercury in deep subsurface environments, such as in an aquifer. Similarly, there have been few studies to evaluate aquifer storage and recovery. This project will help to determine the conditions under which aquifer materials in south Florida, in which ASR will take place, may play a role in the methylation of mercury and thereby, will assist in determining the feasibility of ASR in south Florida. As human activity impacts our natural ecosystems, it is important to consider potential effects of the technologies used in attempts to restore these ecosystems. This study is important in determining if ASR is an appropriate technology for south Florida, specifically for the Florida Everglades ecosystem.

REFERENCES CITED

Benoit, J.M., C.C. Gilmour, R. P. Mason, and A. Heyes, 1999, Sulfide Controls on Mercury Speciation and Bioavailability to Methylating Bacteria in Sediment and Pore Waters, Environ. Sci. Technol., 33, pp. 951-957.

Comprehensive Everglades Restoration Plan (CERP). 2002. US Army Corps of Engineers and S. Florida Water Management District. http://www.evergladesplan.org.about/rest_plan.cfm.

Covellia, S., J. Faganelib, M. Horvat and A. Brambatia. 1999. Porewater Distribution and Benthic Flux Measurements of Mercury and Methylmercury in the Gulf of Trieste (Northern Adriatic Sea). Estuarine, Coastal and Shelf Science. Volume 48, Issue 4. 415-428.

Florida Department of Environmental Protection, Tetra Tech, Inc. (Lafayette, California and Gainesville, Florida), and University of Michigan Air Quality Laboratory (Ann Arbor, Michigan). 2001. Integrating Atmospheric Mercury Deposition with Aquatic Cycling in the Florida Everglades: A Pilot Study for conducting a Total Maximum Daily Load Analysis for an Atmospherically Derived Pollutant Integrated Summary, Final Report (Prepared for: United States Environmental Protection Agency).

Fadini, P.S. and W.F. Jardim. 2001. Is the Negro River Basin Amazon impacted by naturally occurring mercury. The Science of the Total Environment. Volume 275. 71-82.

Gilmour, C.D., E.A. Henry, and R. Mitchell. 1992. Sulfate Simulation of Mercury Methylation in Freshwater Sediments. Environmental Science and Technology. Volume 26, No. 11. 2281-2287.

Harvey, J.D., Krupa, S.L., Gefvert, C., Mooney, R.H., Choi, J., King. S.A., Giddings, J.L., 2002. Interactions between surface water and ground water and effects on mercury transport in the north-central Everglades, Water Resources Investigation Report 02-4050, 2002.

Horvat, M., Bloom, N.S. and Liang, L. 1993, Comparison of distillation with other current isolation methods for the determination of MeHg compounds in low level environmental samples. Part I. Sediment. Anal. Chim. Acta 282: 135-152.

Krabbenhoft, D.P., and C.L. Babiarz. The Role of Groundwater Transport in Aquatic Mercury Cycling. Water Resources Research. Volume 28, No. 12. 3119-3128.

Krabbenhoft, D.P., C.C. Gilmour, J.M. Beniot, C.L. Babiarz, A.W. Andren, and J.P. Hurley, 1998, Methylmercury dynamics in littoral sediments of a temperate seepage lake, Canadian Journal of Fisheries and Aquatic Sciences, 55, pp. 835-844.

Mason, R.P, N.M. Lawson, A.L. Lawrence, J.J. Leaner, J.G. Lee, G-R Sheu. 1999. Mercury in the Chesapeake Bay. Marine Chemistry. Volume 65. 77-96.

Metz, P.A., and D.L. Brendle. 1996. Potential for Water-Quality Degradation of Interconnected Aquifers in West-Central Florida. U.S. Geological Survey Water Resources Investigations Report 96-4030.

National Atmospheric Deposition Program: Mercury Deposition Network (NADP). 2001. http://nadp.sws.uiuc.edu/mdn/maps/.

Reese, Ronald S. 1994. Hydrogeology and the Distribution and Origin of Salinity in the Floridan Aquifer System, Southeastern Florida. U.S. Geological Survey Water Resources Investigations Report 94-4010. Tallahassee, Florida.

Reese, Ronald S. 2002. Inventory and Review of Aquifer Storage and Recovery in Southern Florida. U.S. Geological Survey Water Resources Investigations Report 02-4036. Tallahassee, Florida.

Regional Issues in Aquifer Storage and Recover for Everglades Restoration: A Review of the ASR Regional Study Project Management Plan of the Comprehensive Everglades Restoration Plan. 2002. National Research Council of the National Academies (NRC). Report.

Sacks, L.A., A.B. Tihansky. 1996. Geochemical and Isotopic Composition of Ground Water, with Emphasis on Sources of Sulfate, in the Upper Floridan Aquifer and Intermediate Aquifer System in Southwest Florida. U.S. Geological Survey Water Resources Investigations Report 96-4146.

Ullrich, S.M., T.W. Tanton, and S.A. Abdrashitova. 2001. Mercury in the Aquatic Environment: A Review of Factors Affecting Methylation. Critical Reviews in Environmental Science and Technology. Volume 31, No. 3. 241-293.

US Geological Survey (USGS). 2003. NWISWeb Data for Florida. Ground-Water Data for Florida. Real-Time Data for Florida. http://fl.waterdata.usgs.gov/nwis/current/?type=gw.

US Geological Survey (USGS). 2000. Mercury in the Environment. U.S. Geological Survey. Fact Sheet 146-00.