U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2007–5240

In cooperation with the U.S. Army Corps of Engineers, Jacksonville, Florida

An Assessment of the Potential Effects 

of Aquifer Storage and Recovery on 

Mercury Cycling in South Florida 

An Assessment of the Potential Effects 

of Aquifer Storage and Recovery on 

Mercury Cycling in South Florida

By David P. Krabbenhoft, George R. Aiken, and Mary P. Anderson

In cooperation with the U.S. Army Corps of Engineers, Jacksonville, Florida

Scientifi c Investigations Report 2007–5240

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Krabbenhoft, D.P., Aiken, G.R., and Anderson, M.P., 2007, An assessment of the potential effects of aquifer storage and recovery on mercury cycling in South Florida: U.S. Geological Survey Scientific Investigations Report 

2007–5240, 20 p.

Cover photographs:

Upper left: experimental setup used for the project;

Upper right: sampling an ASR well from a sampling tap;

Lower left: sampling in Everglades National Park.
iii

Contents

Abstract .................................................................................................................................................. 1

Introduction............................................................................................................................................ 1

Purpose and Scope ..................................................................................................................... 2

Aquifer Storage and Recovery ......................................................................................... 3

Hydrogeological Setting and Study Area ....................................................................... 3

Methods and Approach .............................................................................................................. 5

Field Sampling and Chemical Analysis ........................................................................... 5

Laboratory Experiments ..................................................................................................... 6

Evaluation of Field Sample Results .................................................................................................... 8

Evaluation of Experimental Samples Results ................................................................................. 10

Summary and Conclusions ................................................................................................................ 18

Acknowledgments .............................................................................................................................. 19

References ........................................................................................................................................... 19

Figures

 1. Map showing location of the study area in southeastern Florida ............................... 4

 2. Diagram showing laboratory incubation experiment apparatus ................................. 7

 3. Bar chart showing total mercury and methylmercury concentrations for 

the municipal water-supply wells sampled for this study............................................. 9

 4–7. Graphs showing:

 4. Results from experiments A1 and A2, replicate aerobic incubations 

containing surface water and crushed rock ........................................................ 14

 5. Results from experiments AN1 and AN2, replicate anaerobic 

incubations containing surface water and crushed rock .................................. 15

 6. Results from experiments AC and ANC, aerobic and anaerobic 

incubations, respectively, containing only filtered surface water ................... 16

 7. Dissolved organic carbon and specific ultraviolet absorbance 

results from the aerobic experiments A1 and A2, and anaerobic 

experiments AN1 and AN2, which both contain filtered surface water 

and crushed rock ...................................................................................................... 17

Tables

 1. Location, date, aquifer, and depth of the sampled municipal wells in 

southeastern Florida ........................................................................................................... 5

 2. Results of specific chemical analyses of municipal wells ......................................... 10

 3. Results of specific chemical analyses of major cations and anions 

from municipal wells ......................................................................................................... 11

 4. Results of specific chemical analyses of samples collected 

from laboratory incubation experiments ....................................................................... 12
iv

Conversion Factors

Multiply By To obtain

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

gallon per day (gal/d) 0.003785 cubic meter per day

liter (L) 1.057 quart

kilogram (kg) 2.2046 pound

Celsius (oC) 1.8 x (°C) + 32 Fahrenheit

Additional Abbreviations

ASR Aquifer Storage and Recovery 

 CERP Comprehensive Everglades Restoration Plan

 CVAFS Cold vaporatomic fluorescence spectroscopy

 Hg mercury

201Hg and Me199 isotope tracers

 MeHg methylmercury

 mg/L milligrams per liter

 mL milliliter

 ng/L nanograms per liter

 PETG polyethylene terephthalate G copolymer

 SUVA specific ultraviolet absorbance

 SnCl2

 stannous chloride

 TOC total organic carbon

 USGS U.S. Geological Survey
An Assessment of the Potential Effects of 

Aquifer Storage and Recovery on Mercury Cycling 

in South Florida

By David P. Krabbenhoft, George R. Aiken, and Mary P. Anderson

water samples collected from the Floridan and surficial 

aquifer systems. These results indicate that “recovered 

water” from the CERP ASR program would not represent 

a significant additional direct load of mercury and methylmercury to ASR “receiving waters.” Net production of 

methylmercury, however, can result from additions of 

sulfate or natural organic carbon. Thus, because the Upper 

Floridan aquifer generally has elevated concentrations of 

sulfate (relative to ambient Everglades conditions) and surface waters near Lake Okeechobee (the assumed target for 

ASR receiving waters) are elevated in organic carbon and 

sulfate, at least some potential for increased methylmercury production might arise from the release of recovered 

ASR water to locations in or near the Everglades.

 Introduction

The ecological restoration of the Everglades and 

the South Florida ecosystem is a challenging and significant environmental initiative. The Comprehensive 

Everglades Restoration Plan (CERP, 2002, http://www.

evergladesplan.org), which was authorized by Congress in 

the Water Resources Development Act of 2000, provides 

a framework for restoring this ecosystem, and calls for 

improvements to water quality and overall health of the 

Everglades, enhancement of water supply for municipal 

and agricultural use, and maintenance of flood protection. 

Succinctly put, this plan calls for optimizing water supply, 

water use, and water quality in a way that is harmonious 

with the needs of a restored ecosystem and a rapidly growing population in southeast Florida. The overall strategy 

of all ASR programs is to optimize water availability and 

use of water supplies. Surface water is stored during wet 

months by injecting it into subsurface aquifers (in this 

case the Upper Floridan aquifer). During dry months, the 

 Abstract 

Mercury contamination in the environment is a global 

concern, especially in areas with abundant wetlands, such 

as south Florida. As the causal factors of this concern 

improve, scientists find that many factors that do not 

necessarily affect mercury concentrations, such as flooding 

and drying cycles, or changes to carbon and sulfate loading, can profoundly affect net mercury toxicity. Especially 

important are ecological factors that alter the conversion 

of mercury to methylmercury, which is the most bioaccumulative and toxic form of mercury in the environment. Resource managers, therefore, need to be aware of 

possible deleterious affects to mercury toxicity that could 

result from land and water management decisions. Several 

aspects of the Comprehensive Everglades Restoration 

Plan (CERP), including the planned Aquifer Storage and 

Recovery (ASR) program, have the potential to affect the 

abundance of methylmercury. In response to these concerns, the U.S. Geological Survey and U.S. Army Corps 

of Engineers collaborated on a study to evaluate how the 

proposed ASR program may affect mercury cycling and 

toxicity.

This project was conducted as an initial assessment of 

the possible effects of the CERP ASR program on mercury 

in the south Florida environment. A twofold approach was 

employed: field sampling and controlled laboratory benchmark experiments. The field sampling survey collected 

ground-water samples from the Floridan and surficial 

aquifer systems for the ASR program to determine existing 

levels of mercury and methylmercury. Laboratory experiments, on the other hand, were designed to determine how 

the injected surface water would interact with the aquifer 

during storage periods. Overall, very low levels of mercury 

and methylmercury (mean values of 0.41 and 0.07 nanograms per liter, respectively) were observed in ground-
2 An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida

injected water is recovered to help meet peak water-use 

demands. If implemented as envisioned, the ASR program 

for south Florida could include more than 300 ASR wells 

with a combined injection capacity of up to 1.7 billion 

gal/d and would require more than 30 years to complete 

(National Research Council of the National Academies, 

2002). 

Concerns over possible deleterious environmental 

effects from the south Florida ASR program prompted 

a review by the National Research Council Committee on Restoration of the Greater Everglades Ecosystem 

(National Research Council of the National Academies, 

2002), which identified several key issues related to ASR 

that warranted further evaluation. One of the concerns was 

whether implementation of the ASR program might result 

in in situ mercury (Hg) methylation within the Floridan 

aquifer system, and (or) exacerbated Hg methylation in 

the Everglades from the release of recovered water to the 

ecosystem (National Research Council of the National 

Academies, 2002; Wanless, 2004). In response to these 

concerns, the U.S. Geological Survey (USGS) and U.S. 

Army Corps of Engineers collaborated on a study to evaluate how the proposed ASR program may affect Hg cycling 

and toxicity.

Mercury methylation is the process that results in the 

conversion of inorganic Hg to methylmercury (MeHg), the 

most bioaccumulative and toxic form of Hg. MeHg makes 

up nearly all of the Hg at the top of food webs where 

toxicological impacts are of concern, and factors controlling its genesis and transport are a focus of much Hg 

research. Concerns over elevated levels of MeHg in biota 

of the Everglades have been researched extensively, and 

the important causal factors are inputs of Hg from atmospheric deposition, variations in water quality (specifically, 

sulfate and dissolved organic carbon) related to runoff 

from the Everglades Agricultural Area, and hydroperiod 

maintenance (Krabbenhoft, 1996; Cleckner and others, 

1998; Gilmour and others, 1998; Hurley and others, 1998; 

Krabbenhoft and others, 1998, Ravichandran and others, 

1998; Bates and others, 2002). Although these studies have 

sufficiently linked the combined influences of land-use 

practices and atmospheric Hg inputs to the Everglades, and 

provided a basis for understanding and predicting possible 

impacts of various Everglades restoration activities on Hg 

toxicity, no specific information was available at the time 

of this study on the possible effects an ASR system in 

south Florida might have on Hg cycling and toxicity.

 Purpose and Scope

The purpose of this study was to provide a scientific basis for evaluating whether concerns stated by the 

National Research Council (National Research Council of 

the National Academies, 2002) over possible exacerbation 

of Hg contamination of fish and wildlife in the Everglades 

due to implementation of the ASR program are warranted. At the initiation of this study, no reliable information existed on Hg or MeHg concentrations in south 

Florida aquifers, and there were no known studies that 

demonstrated whether ASR operations may trigger MeHg 

production. In response to this scientific information need, 

this study was designed and executed to help inform natural resource managers and planners in south Florida on the 

potential implications of the proposed ASR program on 

Hg cycling and toxicity. 

For the past 15 to 20 years, researchers have 

revealed many of the underlying details regarding global 

Hg contamination, including sources, cycling, fate, and 

toxicological significance (Wiener and others, 2003). 

Relatively little research, however, has been conducted on 

Hg in ground-water systems, and no known studies have 

been conducted on MeHg formation in deep subsurface 

environments. Likewise, there are no known published 

studies that have evaluated possible interactions between 

operation of an ASR system and possible MeHg formation 

during storage periods, or the subsequent release of this 

potent neurotoxin to receiving waters during recovery. In 

addition, recent studies in the Everglades and elsewhere 

(Jeremiason and others, 2006; Orihel and others, 2006; 

Evers and others, 2007) have shown that additions of Hg, 

sulfate, or organic carbon to wetlands can stimulate MeHg 

production. Thus, for the south Florida ASR program, net 

gains in MeHg abundance in the Everglades could result if 

appreciable MeHg production occurs in the aquifer during 

storage periods, or if the release of recovered water results 

in net increases in MeHg production. With these concerns 

in mind, the four goals of this study were to: (1) determine 

background ground-water concentrations of Hg, MeHg, 

and other relevant ancillary chemical constituents in the 

Upper Floridan aquifer and surficial aquifers; (2) determine whether there are any spatial trends in Hg and MeHg 

concentrations in the aquifers; (3) determine whether net 

MeHg formation could occur during ASR storage periods; and (4) determine whether net changes in Hg, MeHg, 

sulfate, and organic carbon could occur during storage 
Introduction 3

periods. To achieve these goals, a multifaceted approach 

was employed that included field sampling and controlled 

laboratory experiments. Field sampling activities were 

designed to achieve the first two goals; controlled laboratory experiments were used to address the last two goals. 

 Aquifer Storage and Recovery

The following is a brief explanation of ASR; more 

detailed descriptions can be found in Pyne (1995) and 

National Research Council of the National Academies 

(2002). In the simplest terms, ASR is “the storage of water 

in a suitable aquifer through a well during times when 

water is available, and recovery of the water from the same 

well during times when it is needed” (Pyne, 1995). Water 

is typically stored in confined aquifers, such as the Upper 

Floridan aquifer, and observation wells are used to monitor 

hydrostatic pressure, water quality, and the movement of 

the stored water. The target storage volume is the volume 

of water that is anticipated to be needed for the recovery 

period, plus the volume of water in the buffer zone. The 

buffer zone separates the stored water from the native 

ground water and consists of a mixture of the two water 

types. ASR storage times are typically several months, but 

can range from a single day to several years. High recovery efficiency of the stored water is an important goal for 

successful ASR programs. In areas of high evaporation, 

such as Florida, recovery efficiency can be less than 100 

percent and still be of net benefit to overall water management. One of the greatest concerns of an ASR program 

is unintended changes to water quality in the receiving 

aquifer due to the storage water having greatly different 

geochemical composition (most notably oxidation reduction potential, nutrients, and salinity). The ASR systems 

in southern Florida will generally store relatively fresh 

surface water in an aquifer containing brackish to saline 

water. The limiting salinity level is 250 mg/L chloride, or 

slightly higher if the recovered water is mixed with potable 

water at a water-treatment plant (Reese, 2002).

 Hydrogeological Setting and Study Area

The study area for this project includes all or parts of 

Broward, Palm Beach, Martin, and St. Lucie Counties in 

southeastern Florida (fig. 1). A brief, general description 

of the hydrogeological setting is provided here; however, 

the reader is referred to Miller (1990), Reese and Memberg (2000), and Reese (2004) for detailed descriptions of 

the hydrogeology, lithology, and geochemical conditions 

in south Florida. The hydrostratigraphy of south Florida 

is made up of four units: the surficial aquifer system, 

Biscayne aquifer, intermediate aquifer, and Floridan 

aquifer system (Miller, 1990). The surficial aquifer system 

includes all unconfined aquifers that are present near the 

land surface (Miller, 1990), and is present everywhere in 

the study area except the southeastern corner, where the 

Biscayne aquifer is present as the surficial aquifer. Consisting mostly of quartz sand, silts, clay, shell beds, coquina, 

calcareous sandstone, and sandy, shelly limestone, the 

surficial aquifer system ranges in thickness from 50 to 

more than 300 ft (Reese and Memberg, 2000). The Biscayne aquifer consists of highly permeable limestone and 

less permeable sandstone and sand. Miller (1990) indicates 

that the Biscayne aquifer grades to the north and west into 

sandy deposits that are part of the surficial aquifer system. 

In this report, these two shallow aquifers—the surficial 

aquifer system and the Biscayne aquifer—are generally 

referred to as the “surficial aquifers.” 

Underlying the Biscayne and surficial aquifer system 

is the intermediate confining unit, which is underlain by 

the Floridan aquifer system. The intermediate aquifer, 

present in southwestern Florida, grades eastward in southeastern Florida to become the intermediate confining unit. 

This unit provides good confinement for the Floridan aquifer system, and ranges in thickness from 250 ft in Martin 

County to 750 ft in Palm Beach County (Reese, 2004). 

The Floridan aquifer system is comprised of the Upper 

Floridan aquifer, the middle confining unit, and the Lower 

Floridan aquifer, which is underlain by the sub-Floridan 

confining unit. The principal focus of this study is the 

Upper Floridan aquifer, which is the target injection zone 

for the CERP ASR program. In southeastern Florida, the 

Upper Floridan consists of a band of permeable carbonate 

rock 500 to 600 ft thick (Reese and Memberg, 2000). 
4 An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida

Figure 1. Location of the study area in southeastern Florida. Samples were collected at municipal water utilities (red circles) in 

Broward, Palm Beach, Martin, and St. Lucie Counties (modified from Reese, 2002).

COLLIER

LEE PALM BEACH

HENDRY

BROWARD

GLADES

HIGHLANDS

FLORIDA

MARTIN

HARDEE

DE SOTO

OKEECHOBEE ST. LUCIE

CHARLOTTE

INDIAN RIVER

Lake

Okeechobee

82° 81° 80°

27°

26°

STL-01

M-O2 M-O3

PB-04

PB-05

PB-08

PB-07

PB-06

PB-09

BR-10

BR-11

BR-12

BR-13

BR-14

Area detailed below
Introduction 5

 Field Sampling and Chemical Analysis

Water samples were collected from 24 municipal 

water utilities in Broward, Palm Beach, Martin, and St. 

Lucie Counties in southeastern Florida (fig. 1; table 1) 

to determine background concentrations of total Hg and 

MeHg, and general geochemical conditions of ground 

water within the surficial and Upper Floridan aquifers. 

Well depths ranged from 60 to 1,800 ft. The sampling sites 

were selected to provide a geographic representation of 

ground-water quality along the southeastern coast of Florida and a portion of the general area for the proposed ASR 

project. Permission to collect water samples was received 

from representatives of each municipal water facility. 

 Methods and Approach

This study used a combined approach of field sampling and laboratory experiments to achieve the four goals 

of this project. Sampling ground water from 24 municipal 

water utilities provided information relevant to the first 

two goals (ambient concentrations of Hg and MeHg). 

Laboratory experiments lasting 16 weeks were designed to 

address the last two goals (potential for MeHg formation 

during ASR operation and possible changes during storage 

periods). Combined, these efforts yielded complimentary 

data to provide an improved understanding of the potential linkages between ASR and Hg cycling and toxicity in 

south Florida.

Table 1. Location, date, aquifer, and depth of the sampled municipal wells in southeastern Florida.

[ID, identification number; NA, not available]

County Date sampled Water utility name Map ID

(see fig. 1) Aquifer Utility well ID Well depth(feet)

Broward 11/20/2003 City of N. Lauderdale BR-12 Biscayne 2 125

Broward 11/20/2003 City of Deerfield Bch BR-10 Biscayne 19 108

Broward 11/20/2003 Tamarac BR-11 Surficial 1 60

Broward 11/20/2003 City of Ft. Lauderdale BR-14 Biscayne 37 115

Broward 11/20/2003 City of Lauderhill BR-13 Surficial 5 150

Palm Beach 11/19/2003 Village of Tequesta PB-04 Floridan 2 1,800

Palm Beach 11/19/2003 Village of Tequesta PB-04 Surficial 19 70

Palm Beach 11/18/2003 City of Boca Raton PB-09 Biscayne BR1 NA 

Palm Beach 11/18/2003 Village of Palm Springs PB-07 Surficial 17 200

Palm Beach 11/18/2003 Village of Palm Springs PB-07 Surficial 12 NA 

Palm Beach 11/17/2003 Village of Wellington PB-08 Surficial 24 180

Palm Beach 11/17/2003 Village of Wellington PB-08 Surficial 22 160

Palm Beach 11/17/2003 Village of Wellington PB-08 Surficial 19 NA 

Palm Beach 11/18/2003 West Palm Beach PB-05 Surficial 2 150

Palm Beach 11/18/2003 West Palm Beach PB-05 Floridan ASR 1,400

Palm Beach 11/18/2003 Boynton Beach PB-06 Biscayne 7W 161

Palm Beach 11/18/2003 Boynton Beach PB-06 Biscayne 15E 233

Palm Beach 11/18/2003 Boynton Beach PB-06 Biscayne 6E 76

Martin 11/19/2003 City of Stuart M-02 Surficial 3 130

Martin 11/19/2003 Sailfish Point M-03 Floridan 2 1,000

St. Lucie 11/19/2003 Fort Pierce STL-01 Floridan F-8 1,250

St. Lucie 11/19/2003 Fort Pierce STL-01 Floridan F-4 1,000

St. Lucie 11/19/2003 Fort Pierce STL-01 Surficial N7 80

St. Lucie 11/19/2003 Fort Pierce STL-01 Surficial 16S 100
6 An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida

Samples were collected from wells that were in most cases 

(all but two) already producing upon arriving onsite, or 

were turned on and allowed to run for at least 15 minutes 

prior to sampling. Samples were collected from “sampling 

taps,” which are small faucets affixed to the main production line for each well, and are upstream from any onsite 

treatment systems (for example, reverse osmosis). 

Five individual water samples were collected sequentially from each well and subsequently analyzed for the 

following list of parameters: pH and conductivity; total 

Hg and MeHg; total organic carbon; major cations; and 

major anions. All samples were collected after rinsing 

the container at least three times with sample water. A 

multiparameter field probe (Hydro LabTM) with a flowthrough cell was used in the field for measurements of 

pH and conductivity. After collection, all samples were 

labeled and placed in darkened coolers until the end of the 

day when they were sorted by sample type and preserved 

appropriately. Samples intended for total Hg and MeHg 

analysis were collected using trace-metal-clean techniques 

as outlined by Olson and DeWild (1999). Briefly, total Hg 

and MeHg samples were collected in 1-L Teflon bottles by 

qualified field personnel who wore copolymer vinyl gloves 

at all times during sampling to minimize artifact contamination. Teflon bottles are advantageous for several reasons: 

(1) they are nonbreakable; (2) they are resistive to chemical breakdown under the rigorous cleaning procedures 

employed by the USGS Mercury Research Lab (4 molar 

hydrogen chloride at 65°C for 24 hours); and (3) they 

minimize Hg and MeHg loss to container walls and greatly 

reduce gaseous Hg exchange into or out of the container. 

After cleaning, the Teflon bottles are partially filled with 

about 100 mL of 1 percent hydrogen chloride, and stored 

in two layers of zip-type plastic bags until use. Major 

cation and anion samples were collected in precleaned 

250-mL plastic (polyethylene) bottles. Water samples for 

total organic carbon (TOC) determinations were collected 

in 40-mL glass vials with septum-piercing lids. 

Sample preservation and shipping procedures were 

as follows: total Hg and MeHg samples were preserved 

by adding concentrated, low-Hg hydrochloric acid to 1 

percent by volume (for example, 10 mL of concentrated 

hydrogen chloride was added to a 1-L sample), and then 

hermetically sealed in two layers of zip-type bags. Major 

cation samples were preserved with 2 mL of concentrated, 

high-purity nitric acid, whereas major anion and organic 

carbon samples were preserved in dark, cool (4°C) ice 

chests. All samples were shipped overnight to the USGS 

Mercury Research Lab in Middleton, Wisconsin, for processing and analysis. 

A brief description of the analytical procedures for 

total Hg and MeHg in water is given here, but a complete 

description can be found in Olson and DeWild (1999), 

DeWild and others (2004), and Olund and others (2004). 

Total Hg and MeHg determinations were performed at 

the USGS Mercury Research Lab, a facility specifically 

designed for and dedicated to low-level speciation analysis 

of environmental samples. Total Hg was determined by 

cold vapor atomic fluorescence spectroscopy (CVAFS), 

following oxidation with bromine monochloride at 50°C, 

reduction by stannous chloride (SnCl2), and purge-and-trap 

of the evolved gaseous Hg onto gold-coated, glass-bead 

columns. The analytical procedure for MeHg is a two-step 

process involving distillation of the sample to separate the 

MeHg from potential matrix interference effects of organic 

carbon, followed by aqueous phase ethylation and quantification using gas chromatography and CVAFS. Analyses 

for the isotope tracers utilized in the experimental part of 

this study were analytically determined using the same 

procedures, with the exception that CVAFS was not used 

for detection and quantification. For these samples, the 

experimentally added isotope tracers, 201Hg and Me199, 

were quantified using a PerkinElmer Elan 6100, inductively coupled plasma mass spectrometer, also located at 

the USGS Mercury Research Lab. Analysis of TOC was 

performed using a Shimadzu (model TOC-V CSH) analyzer that utilizes a high temperature (680°C) combustion 

and catalytic oxidation method, followed by detection and 

quantification using infrared adsorption. 

Major ion analyses were conducted at the Wisconsin State Laboratory of Hygiene in Madison, Wisconsin. 

Cations were measured using inductively coupled plasma 

mass spectrometry, with the exception of calcium and 

sodium, which were determined by inductively coupled 

plasma-atomic absorption spectrometry. Major anions 

(chloride and sulfate) were analyzed by liquid ion chromatography, and alkalinity was tested by standard potentiometric titration (American Public Health Association and 

others, 2005).

 Laboratory Experiments

The objective of this part of the study was to simulate, 

under controlled but varying laboratory conditions, possible changes to total Hg and MeHg concentrations in the 

Upper Floridan aquifer during operation of the CERP ASR 

project. In particular, laboratory studies were conducted to 

assess whether processes that affect Hg concentrations—

speciation and transport (methylation, demethylation, sorp-
Introduction 7

tion and desorption—might occur during storage periods, 

and to assist in the interpretation of field data. 

The experimental setup (fig. 2) involved a series 

of nine 2-L incubation bottles containing filtered water 

from the rim canal on the east side of Lake Okeechobee, 

pulverized rock material from the Upper Floridan aquifer, 

and a sparging gas (air or nitrogen) to control the redox 

condition inside the bottles. Water from the rim canal was 

chosen because it is one of the likely water sources for 

the ASR project. The South Florida Water Management 

District provided the rock material that was derived from a 

semicontinuous core drilled for a CERP ASR pilot project 

well southwest of Lake Okeechobee. The 12 specific 

core segments (about 2 in. wide by about 3 ft long) were 

from two depth intervals: 830–840 ft and 1,020–1,030 ft 

below land surface. Each core segment was crushed into 

irregular pieces with an average diameter of about 0.2 in., 

using a pre-cleaned stainless steel mallet and plate. All of 

the crushed rock was combined into a large plastic bin, 

homogenized, and then distributed into the incubation 

bottles. The bottles were made of polyethylene terephthalate G copolymer (PETG), because it has very low gas 

permeability, low sorption potential for Hg and MeHg, and 

it is opaque to ultraviolet light, which can cause photochemical changes to Hg speciation. Teflon tubing was used 

to connect 2-L PETG bottles to gas flowmeters and then to 

the sparging gas. Tight-fitting 1⁄4-in. holes were drilled into 

the lids of the PETG bottles so that 1⁄4-in. Teflon tubing 

(all tubing diameters listed are outside diameters) could be 

inserted into the bottles. Flowmeters were placed in line 

for each bottle to maintain a regular and equal flow rate 

(0.3 L per minute) for each experimental test. Each sparging line had an in-line gold trap to eliminate possible Hg 

contamination associated with the sparging gas.

Figure 2. Laboratory incubation experiment apparatus. Although nine 2-liter bottles were used for the experiment, two bottles 

are illustrated here.

A

A B

B

Nalgene

PETG

bottles

Flowmeter Teflon

tubing Gold trap

Teflon tubing

Teflon connector

GAS
8 An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida

Each PETG bottle, except for the controls (described 

below), initially received about 500 mL by volume of the 

crushed aquifer rock. Each bottle then was filled with 

1.5 L of the filtered canal water. The canal water had been 

spiked with about 3.3 ng/L of the isotope tracers Me199Hg 

and 201Hg 5 days before the start of the experiment to allow 

for equilibration of the isotopes with the water matrix. 

The starting concentrations were chosen to approximate 

the levels (low nanograms per liter) of ambient Hg and 

MeHg in the native canal water utilized in the experiment. 

The starting concentration of Me201Hg is greater than the 

typically observed MeHg levels in Everglades surface 

water (about 0.5 to 1 ng/L). To reliably detect the tracer 

during the experiment, however, a starting concentration 

of about 3 ng/L was deemed necessary. The purpose of 

the spikes was twofold: (1) to allow for distinguishing Hg 

species that are from the water phase from those that may 

be derived from sorption or desorption from the rock, and 

(2) to allow for direct examination of methylation and 

demethylation potential under varying redox conditions. 

In all, five anaerobic and four aerobic experiments 

were conducted. The five anaerobic experiments included 

the following: crushed rock and canal water; a replicate 

crushed rock and canal-water test; sterilized rock (heated 

to 125°C for 24 hours) and canal water; surface water only 

(control); and, crushed rock and deionized water. The four 

aerobic experiments included the following: crushed rock 

and canal water; a replicate crushed rock and canal-water 

test; and surface water only (control). Sampling was conducted at six specific times: 0, 2, 4, 8, 12, and 16 weeks, 

beginning in February 2004. For each sampling, about 

200 mL of water was slowly pumped into a pre-cleaned 

Teflon bottle using a peristaltic pump and a pre-cleaned 

Teflon line. Samples for TOC and sulfate analyses also 

were collected, but due to volume limitations these samples were only collected during the first and last sampling 

period. Hg samples were immediately preserved with 2 mL 

of concentrated low-mercury hydrogen chloride, and the 

TOC/sulfate samples were kept refrigerated until analysis. 

For each of the TOC samples acquired from the experiments, the specific ultraviolet absorbance (SUVA) also was 

measured. SUVA is a qualitative measure for the overall 

composition of TOC in a water sample (Haitzer and others, 

2002) and was determined by measuring adsorption at 

254 nanometers and normalizing these measurements to 

the concentration of TOC in the sample. High SUVA values indicate a TOC matrix with more aromatic components 

(for example, humic acids), whereas low SUVA values 

indicate a more aliphatic composition of the TOC matrix. 

A basic understanding of the nature of TOC character is 

important for evaluating the degree of interaction between 

organic carbon and Hg.

 Evaluation of Field Sample Results

Overall, concentrations of total Hg and MeHg in both 

the Upper Floridan aquifer and surficial aquifers are very 

low, with mean concentrations of about 0.41 ng/L and less 

than 0.07 ng/L, respectively (fig. 3; table 2). Low MeHg 

concentrations were somewhat surprising, given that 

sulfate reduction was strongly indicated by the ubiquitous 

presence of hydrogen sulfide odor from samples taken 

from the surficial aquifers and the Upper Floridan aquifer. In addition, observed MeHg levels showed very little 

spatial variability (horizontally and vertically) in either 

the Floridan or surficial aquifers (minimum and maximum 

values observed were 0.055 and 0.081 ng/L, respectively). 

Total Hg concentrations were more variable (minimum 

and maximum values observed were 0.10 and 4.14 ng/L, 

respectively), and the mean concentration for the Floridan 

was about one-third that of samples derived from the surficial aquifers (0.16 and 0.50 ng/L, respectively). This difference is likely due to the surficial aquifer samples having 

five times greater concentrations of TOC than the deeper 

Floridan, and the stabilizing effect of natural organic matter on Hg in water (Haitzer and others, 2002). Although 

MeHg concentrations in the Floridan and surficial aquifers 

are very low, the ratios of total Hg to MeHg are high, with 

an average ratio of 0.41 and a maximum observed ratio of 

0.91, which are high compared to the published literature 

(Wiener and others, 2003). A high total Hg to MeHg ratio 

is generally an indicator of greater methylation potential 

in surface water and sediments. The use of this indicator, 

however, is dubious at the very low total Hg and MeHg 

concentrations observed in this study. It is important to 

note that the total Hg and MeHg concentrations in ground 

water are substantially less than the concentrations that are 

generally observed in surface water in south Florida (about 

2–4 ng/L, and 0.1–0.2 ng/L, respectively). Thus, from the 

perspective of direct loading of Hg and MeHg to local 

wetlands, the discharge of recovered ASR waters from the 

Floridan and surficial aquifers is of little concern.

Chemical results from samples collected from the 

Upper Floridan aquifer show significantly higher conductivity, sodium, chloride, and sulfate concentrations 

compared to the samples from the surficial aquifers (table 

3). Greater sodium, sulfate, and chloride ion concentrations at depth are likely due to prolonged contact time 

for dissolution of aquifer minerals and mixing with relic 
Evaluation of Field Sample Results 9

Figure 3. Total mercury and methylmercury concentrations for the municipal water-supply wells sampled for this study. Data are 

arranged from north (left) to south (right) along the bottom axis.

METHYLMERCURY TO TOTAL MERCURY RATIO

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

STL-01

M-02

PB-04

PB-08

PB-08

PB-07

PB-06

PB-06

PB-06PB-09

BR-10BR-12BR-11

STL-01STL-01

M-03

PB-04PB-05

SAMPLE SITES (NORTH TO SOUTH)

TOTAL MERCURY AND METHYLMERCURY 

CONCENTRATION, IN NANOGRAMS PER LITER

Total mercury

Methylmercury

Methylmercury to total mercury ratio

4.1 4.1

0.2

0.3

0.4

0.5

0.6

0.1

0.0

0.8

0.9

1.0

0.7

Surficial and Biscayne

Aquifers

Upper Floridan 

Aquifer

seawater (Reese, 2004). Of these constituents, sulfate is the 

primary concern with regard to MeHg production, because 

results show that ground water in the Floridan is about five 

times greater in sulfate concentration than water collected 

from the surficial aquifers (214 and 43 mg/L, respectively). 

On the other hand, samples from the surficial aquifers are 

about five times greater in organic carbon concentration 

compared to samples from the Floridan (9.9 and 1.8 mg/L, 

respectively). The organic carbon concentrations in water 

from the surficial aquifers are similar to what is observed 

in surface water (wetlands, lakes, and streams) in south 

Florida. Injection of shallow ground water and (or) surface 

water will likely increase the organic carbon concentrations of the Floridan near the site of injection compared to 

ambient levels. Thus, recovered water from a functioning 

ASR program in south Florida could contain TOC and 

sulfate concentrations that could subsequently stimulate 

MeHg production if released to the local environments, 

including the Everglades.
10 An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida

 Evaluation of Experimental Samples 

Results

The last two goals of the project focused on assessing whether key chemical processes could occur during 

ASR storage periods. Results from the incubation experiments provided clear answers to these questions (figs. 4–7; 

table 4). First, rapid and substantial losses of total Hg and 

MeHg were observed in all incubation bottles containing 

the crushed aquifer rock. The inorganic isotopic tracers 

201Hg and Me199Hg mimicked the behavior of their natural 

analogs, generally showing an even greater loss rate from 

solution. It is important to note that the concentrations of 

both natural and isotopic Hg species in these experiments 

showed rapid declines, and were similar in concentration 

at the end of the 16 weeks. This strong corroboration supports the conclusions from these experiments and enables 

an understanding of the controlling factors of low total Hg 

and MeHg in ground water from the study area. No apparent gains in total Hg or MeHg in any of the experiments 

suggests that desorption (leaching from the Upper Floridan 

aquifer material) is not an important process.

Table 2. Results of specific chemical analyses from municipal wells.

[ID, identification number; μMhos/cm, microMohs per centimeter; mg/L, milligram per liter; ng/L, nanogram per liter; ND, no data, sample not collected; HgT, total mercury; MeHg, methylmercury; MeHg/HgT ratio, ratio of methylmercury to total mercury; TOC, total organic carbon]

Water utility name Aquifer Utility 

well ID pH Conductivity (μMhos/cm)

Alkalinity 

(mg/L)

HgT 

(ng/L)

MeHg 

(ng/L)

MeHg/

HgT ratio

TOC 

(mg/L)

City of N. Lauderdale Biscayne 2 7.6 651 230 4.14 0.07 0.02 11.66

City of Deerfield Bch Biscayne 19 7.55 512 197 .22 .068 .31 8.83

Tamarac Surficial 1 7.67 525 194 .14 .066 .49 7.43

City of Ft. Lauderdale Biscayne 37 7.72 633 218 .46 .055 .12 12.08

City of Lauderhill Surficial 5 7.62 485 190 ND ND ND ND

Village of Tequesta Floridan 2 7.73 8020 149 .12 .072 .59 1.37

Village of Tequesta Surficial 19 7.74 599 218 .45 .07 .16 6.02

City of Boca Raton Biscayne BR1 7.45 570 208 .23 .072 .32 12.35

Village of Palm Springs Surficial 17 7.41 595 240 .13 .07 .52 11.37

Village of Palm Springs Surficial 12 7.34 737 268 ND ND ND ND

Village of Wellington Surficial 24 7.36 799 268 .13 .069 .55 13.76

Village of Wellington Surficial 22 7.3 754 268 ND ND ND ND

Village of Wellington Surficial 19 7.4 737 253 .128 .071 .6 11.03

West Palm Beach Surficial 2 7.36 599 218 .298 .081 .28 6.88

West Palm Beach Floridan ASR 7.71 6570 133 .28 .07 .25 1.27

Boynton Beach Biscayne 7W 7.27 607 251 .29 .071 .24 14.24

Boynton Beach Biscayne 15E 7.47 528 199 .33 .066 .2 10.9

Boynton Beach Biscayne 6E 7.68 481 154 .1 .067 .68 2.37

City of Stuart Surficial 3 7.68 545 236 .11 .072 .65 5.51

Sailfish Point Floridan 2 7.84 4600 153 .08 .073 .91 1.97

Fort Pierce Floridan F-8 7.92 1510 147 .08 .07 .85 2.07

Fort Pierce Floridan F-4 7.9 1550 167 .24 .069 .29 2.56

Fort Pierce Surficial 7 7.92 756 183 ND ND ND ND

Fort Pierce Surficial 16S 7.59 646 256 .28 .076 .27 13.21
Evaluation of Experimental Samples Results 11

Rapid loss of Hg species was observed in all the 

experiments (aerobic and anaerobic) using crushed rock 

(figs. 4 and 5), whereas slow and incomplete loss was 

observed in the control bottles (those with no crushed 

rock, fig. 6). This indicates that Hg and MeHg are removed 

from aqueous solution by net sorption to mineral surfaces. 

Although this does not mean that some desorption is not 

occurring also, the uniform response of all the experiments strongly supports the conclusion that, overall, net 

sorption dominates the exchange process between Hg, 

MeHg, and the aquifer rock. Because the experimental 

design employed crushed aquifer rock that presumably has 

a greater rock-surface area to water ratio, the experimentally observed loss rates of dissolved Hg and MeHg may 

be greater than what might be observed under operating 

ASR conditions and an intact aquifer substrate. However, 

the overall agreement between Hg and MeHg concentrations in ground-water samples from the surficial aquifers 

and the Floridan, and those observed from the experiments 

indicates that these loss rates may be good proxies for loss 

rates under ASR conditions. Finally, because the mass Hg 

in ground water or injected surface water is probably so 

small (nanograms per liter of water) and the mass of rock 

is about 12 orders of magnitude greater (about 2 kg per 

Table 3. Results of specific chemical analyses of major cations and anions from municipal wells.

[ID, identification number; mg/L, milligram per liter; ND, no data]

Water utility name Aquifer Utility 

well ID

Calcium

(mg/L)

Magnesium

(mg/L)

Sodium

(mg/L)

Potassium

(mg/L)

Sulfate

(mg/L)

Chloride

(mg/L)

City of N. Lauderdale Biscayne 2 105 4.3 34.6 2 47.4 55.7

City of Deerfield Bch Biscayne 19 91.2 3.2 19.2 2 39.9 32.5

Tamarac Surficial 1 89.7 3.6 20.8 2 41.4 34.5

City of Ft. Lauderdale Biscayne 37 84.5 9.6 39.1 4 31.5 63.1

City of Lauderhill Surficial 5 87.5 2.4 18.3 1 33.2 30.5

Village of Tequesta Floridan 2 165 174 1,320 43 261.2 2,570

Village of Tequesta Surficial 19 73.4 1.7 17 2 27.7 29.1

City of Boca Raton Biscayne BR1 97 3.8 24.5 2 44.7 42.1

Village of Palm Springs Surficial 17 115 3.6 16.5 2 53.6 28.9

Village of Palm Springs Surficial 12 122 4.8 42.4 ND 45.3 72.3

Village of Wellington Surficial 24 124 5.9 46.4 4 62.4 75.3

Village of Wellington Surficial 22 125 5.8 34 3 62.1 57

Village of Wellington Surficial 19 119 5.2 32.9 5 51.7 61.6

West Palm Beach Surficial 2 93.4 3.2 30.6 ND 19 54.4

West Palm Beach Floridan ASR 124 136 1,100 41 369.4 1,960

Boynton Beach Biscayne 7W 105 2.8 23.9 ND 19.2 39.9

Boynton Beach Biscayne 15E 91.8 3.4 19.7 1 36 33.3

Boynton Beach Biscayne 6E 86.6 2.2 16.4 3 62.7 29.5

City of Stuart Surficial 3 100 2.9 15.1 1 31.1 18.1

Sailfish Point Floridan 2 99.4 106 688 26 160.9 1,340

Fort Pierce Floridan F-8 56.4 46.2 179 10 119.9 324

Fort Pierce Floridan F-4 51.9 42 203 13 158.9 302

Fort Pierce Surficial 7 80.4 10.2 65.9 5 71.4 92.2

Fort Pierce Surficial 16S 112 4.7 25.6 2 ND 36.1
12 An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida Table 4. Results of specific chemical analyses of samples collected from laboratory incubation experiments. [ID, identification number; ng/L, nanogram per liter; mg/L, milligram per liter; ND, no data, sample not collected; DOC, dissolved organic carbon; SUVA, specific ultraviolet absorbance; mg/L C, DOC expressed as milligrams per liter as carbon] Sample/experiment ID Sample date Ambient MeHg (ng/L) Me199Hg (ng/L) Me201Hg (ng/L) Ambient Hg (ng/L) 199Hg (ng/L) 201Hg (ng/L) Sulfate (mg/L) DOC (mg/L C) SUVA Filtered surface water 01/18/04 0.092 3.300 0.000 2.081 3.458 3.551 66.4 43 1.8 Filtered surface water 01/18/04 .090 3.268 .000 2.144 3.579 3.538 65.1 42.8 1.8 A1 02/04/04 .102 .018 .001 2.056 .023 .011 55.6 40 1.3 A1 02/18/04 .026 .007 .000 .877 .018 .017 ND 32 1.6 A1 03/17/04 .011 .005 .000 .881 .014 .010 ND 21.6 2.4 A1 04/15/04 .009 .014 .001 .440 .012 .009 ND 20.1 2.5 A1 05/13/04 .005 .004 .001 .222 .022 .037 32.1 19.9 2.6 A2 02/04/04 .085 .013 .001 2.368 .042 .032 58.1 39 1.3 A2 02/18/04 .024 .008 .000 1.747 .019 .020 ND 33 1.5 A2 03/17/04 .009 .012 .000 .673 .008 .022 ND 21.7 2.4 A2 04/15/04 .005 .009 .000 .330 .006 .004 ND 20.3 2.6 A2 05/13/04 .006 .016 .001 .165 .008 .009 28.2 19.8 2.6 AN1 02/04/04 .090 .018 .000 1.328 .062 .056 50.7 50 1.1 AN1 02/18/04 .028 .011 .000 .832 .006 .045 ND 41 1.3 AN1 03/17/04 .023 .011 .000 .672 .006 .065 ND 21.6 2.5 AN1 04/15/04 .014 .005 .000 .115 .008 .003 ND 19.8 2.6 AN1 05/13/04 .015 .022 .000 .160 .019 .011 19.2 20.5 2.6 AN2 02/04/04 .093 .017 .000 1.458 .009 .003 48.3 51 1 AN2 02/18/04 .046 .010 .000 .751 .007 .017 ND 53 1 AN2 03/17/04 .042 .013 .000 .556 .006 .014 ND 34 1.5 AN2 04/15/04 .024 .006 .000 .245 .001 .000 ND 25.6 2.1 AN2 05/13/04 .008 .015 .000 .134 .006 .083 15.9 25 2.2 AC 02/04/04 .102 3.067 .002 2.137 2.819 2.700 65.1 42 1.8 AC 02/18/04 .063 2.787 .000 1.809 2.556 2.493 ND 41 1.8 AC 03/17/04 .048 2.704 .000 1.690 2.255 2.551 ND 40 1.9

AC 04/15/04 .046 2.258 .000 1.725 2.375 2.331 ND 33 2.2
Evaluation of Experimental Samples Results 13

Sample/experiment ID Sample date Ambient MeHg (ng/L) Me199Hg (ng/L) Me201Hg (ng/L) Ambient Hg (ng/L) 199Hg (ng/L) 201Hg (ng/L) Sulfate (mg/L) DOC (mg/L C) SUVA

AC 05/13/04 .047 2.375 .000 1.551 2.200 2.100 63.2 36 2ANC 02/04/04 0.113 3.158 0.000 2.161 3.384 3.185 53.7 43 1.8ANC 02/18/04 .080 2.770 .000 1.759 2.725 2.715 ND 39 1.9

ANC 03/17/04 .048 2.777 -.001 1.825 2.530 2.343 ND 27.7 2.7ANC 04/15/04 .050 2.529 .000 1.804 2.460 2.427 ND 22.7 3.3ANC 05/13/04 .039 2.352 .003 1.899 2.352 2.034 24.4 23.3 3.2ANSC 02/04/04 .576 .068 .000 1.339 .064 .093 52.8 55 1.2ANSC 02/18/04 .085 .008 .000 .854 -.001 .004 ND 56 1.1ANSC 03/17/04 .042 .014 .000 .707 .004 .004 ND 42 1.5ANSC 04/15/04 .028 .012 .000 .465 -.011 .002 ND 25.4 2.4ANSC 05/13/04 .042 .004 .000 .287 .005 .017 20.9 26 2.6ASC 02/04/04 .073 .015 .000 2.010 .024 .019 55.6 36 1.8ASC 02/18/04 .026 .010 .000 1.465 .000 .020 ND 26 2.5ASC 03/17/04 .033 .005 .000 .922 .001 .000 ND 25.1 2.5ASC 04/15/04 .008 .007 .000 1.051 .002 .000 ND 22.6 2.7ASC 05/13/04 .002 .005 .001 1.031 .002 .009 27.1 22.9 2.7ANMQ 02/04/04 .103 .011 .000 .891 .001 .001 1.6 9 2.3ANMQ 02/18/04 .037 .000 .000 .371 .001 .002 ND 8.9 2.3ANMQ 03/17/04 .008 .004 .000 .057 .001 -.001 ND 8.1 2.4ANMQ 04/15/04 .005 .001 .000 .079 .000 .001 ND 8.6 2.3ANMQ 05/13/04 .007 .011 .001 .050 .015 .000 1.4 8.7 2.4

Table 4. Results of specific chemical analyses of samples collected from laboratory incubation experiments—Continued.[ID, identification number; ng/L, nanogram per liter; mg/L, milligram per liter; ND, no data, sample not collected; DOC, dissolved organic carbon; SUVA, specific ultraviolet absorbance; mg/L C, DOC expressed as milligrams per liter as carbon]
14 An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida

Figure 4. Results from experiments A1 and A2, replicate aerobic incubations containing surface water and crushed rock. Top 

panel shows the results for ambient methylmercury (MeHg) and the isotopic tracer Me199Hg; bottom panel shows results for 

ambient total mercury (Hg) and the isotopic tracer 201Hg.

0.00

0.02

0.04

0.06

0.08

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0.12

01/18/0402/04/0402/18/0403/17/04

04/15/0405/13/0401/18/04

02/04/04

02/18/04

03/17/0404/15/0405/13/04

AMBIENT METHYLMERCURY,

IN NANOGRAMS PER LITER 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

ADDED ME199HG,

IN NANOGRAMS PER LITER 

AMBIENT TOTAL MERCURY,

IN NANOGRAMS PER LITER 

ADDED ME201HG,

IN NANOGRAMS PER LITER 

Ambient methylmercury, in nanograms per liter

Isotopic tracer Me199Hg, in nanograms per liter

Ambient total mercury, in nanograms per liter

Isotopic tracer Me201Hg, in nanograms per liter

Experiment A1 Experiment A2

0.0

0.5

1.0

1.5

2.0

2.5

01/18/04

02/04/0402/18/0403/17/04

04/15/04

05/13/04

01/18/04

02/04/04

02/18/04

03/17/04

04/15/0405/13/04

0.0

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2.0

2.5

3.0

3.5

4.0

Experiment A1 Experiment A2

liter of rock), the aquifer itself could be an inexhaustible 

sink for Hg and MeHg from ground water.

The lack of any observed formation of Me201Hg 

(introduced as inorganic 201Hg) in the experimental results 

indicates that net MeHg production is minimal to nonexistent (table 4). Likewise, no detectable inorganic 199Hg 

(introduced as Me199Hg) in the analyzed samples indicates 

that net demethylation also is not active under anaerobic or 

aerobic conditions in contact with the crushed rock (table 

4). It should be noted, however, that because greater than 

99 percent of the starting mass of 201Hg and Me199Hg was 

not in solution at the end of the experiment (presumably 

lost to sorption as noted above) it is possible that methylation and demethylation were occurring, but that the net 

products of these reactions were lost so rapidly to the rock 

surfaces that the byproducts could not be observed. Given 

the very small mass of isotope utilized in the experiments 

(about 5 ng per bottle) and the large mass of rock (about 

1 kg), it was not possible to quantify the isotopic tracers at 

the conclusion of the experiment. The slow but compara-
Evaluation of Experimental Samples Results 15

0.00

0.02

0.04

0.06

0.08

0.10

01/18/04

02/04/04

02/18/04

03/17/0404/15/0405/13/04

01/18/04

02/04/04

02/18/0403/17/04

04/15/04

05/13/04

0.0

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3.5

Experiment AN1 Experiment AN2

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01/18/04

02/04/0402/18/0403/17/04

04/15/0405/13/0401/18/0402/04/0402/18/04

03/17/04

04/15/0405/13/04

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Experiment AN1 Experiment AN2

AMBIENT METHYLMERCURY,

IN NANOGRAMS PER LITER 

ADDED ME199HG,

IN NANOGRAMS PER LITER 

AMBIENT TOTAL MERCURY,

IN NANOGRAMS PER LITER 

ADDED ME201HG,

IN NANOGRAMS PER LITER 

Ambient methylmercury, in nanograms per liter

Isotopic tracer Me199Hg, in nanograms per liter

Ambient total mercury, in nanograms per liter

Isotopic tracer Me201Hg, in nanograms per liter

Figure 5. Results from experiments AN1 and AN2, replicate anaerobic incubations containing surface water and crushed rock. 

Top panel shows the results for ambient methylmercury (MeHg) and the isotopic tracer Me199Hg; bottom panel shows results for 

ambient total mercury (Hg) and the isotopic tracer 201Hg.

bly minimal loss of Hg and MeHg isotopes observed in the 

control bottles (those lacking crushed rock, fig. 6) is likely 

due to sorption to the walls of the bottle.

Sulfate reduction and organic carbon consumption were clearly observed in the incubation bottles 

under aerobic and anaerobic conditions (fig. 7; table 4). 

A concomitant increase in SUVA indicates the residual 

organic carbon pool in the bottles is evolving toward a 

more complex (aromatic) mixture, with the more aliphatic 

compounds preferentially utilized by the sulfate-reducing 

bacteria. The molar loss rate of carbon compared to sulfate 

is about 2:1, which is the theoretical value predicted if 

microbial sulfate reduction is the process leading to the 

dual consumption of sulfate and organic carbon (Stumm 

and Morgan, 1981). Sulfate reduction appears to have 

occurred in the experiment, without appreciable MeHg 

being observed, which strongly suggests that either loss 

by sorption to the rock virtually masked the formation of 

Me201Hg, or that high sulfide concentrations (the byproduct of sulfate reduction) inhibited the formation of MeHg 
16 An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida

Figure 6. Results from experiments AC and ANC, aerobic and anaerobic incubations, respectively, containing only filtered 

surface water. Top panel shows the results for ambient methylmercury (MeHg) and the isotopic tracer Me199Hg; bottom panel 

shows results for ambient total mercury (Hg) and the isotopic tracer 201Hg.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

01/18/04

02/04/04

02/18/04

03/17/0404/15/0405/13/04

01/18/04

02/04/04

02/18/0403/17/04

04/15/04

05/13/04

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Experiment AC Experiment ANC

0.0

0.5

1.0

1.5

2.0

2.5

01/18/04

02/04/04

02/18/0403/17/0404/15/0405/13/0401/18/0402/04/04

02/18/0403/17/04

04/15/0405/13/04

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Experiment AC Experiment ANC

ADDED ME199HG,

IN NANOGRAMS PER LITER 

ADDED ME201HG,

IN NANOGRAMS PER LITER 

AMBIENT METHYLMERCURY,

IN NANOGRAMS PER LITER AMBIENT TOTAL MERCURY, IN NANOGRAMS PER LITER 

Ambient methylmercury, in nanograms per liter

Isotopic tracer Me199Hg, in nanograms per liter

Ambient total mercury, in nanograms per liter

Isotopic tracer Me201Hg, in nanograms per liter

(Benoit and others, 1999). The fact that sulfate reduction 

appears to have occurred in the anaerobic control experiment, which did not contain the rock material, indicates 

that the sulfide inhibition process effect is the more likely 

explanation. Over the course of the 16-week experiment, 

sulfate and organic carbon concentrations appeared to stabilize at relatively elevated levels, but it was not possible 

to conclude if sulfate reduction had actually ceased or had 

simply reached a low, steady-state rate. The fact that ample 

sulfate and organic carbon remained in solution at the 

conclusion of the experiment indicates that the availability 

of labile carbon could have been limiting sulfate-reduction 

activity. 
Evaluation of Experimental Samples Results 17

Figure 7. Dissolved organic carbon and specific ultraviolet absorbance (SUVA) results from the aerobic experiments A1 and A2 

(top panel), and anaerobic experiments AN1 and AN2 (bottom panel), which both contain filtered surface water and crushed rock.

0

10

20

30

40

50

01/18/04

02/04/04

02/18/0403/17/04

04/15/04

05/13/04

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03/17/04

04/15/04

05/13/04

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Experiment A1 Experiment A2

0

10

20

30

40

50

60

01/18/0402/04/0402/18/04

03/17/04

04/15/04

05/13/0401/18/04

02/04/04

02/18/04

03/17/04

04/15/04

05/13/04

DISSOLVED ORGANIC CARBON,

 IN MILLIGRAMS PER LITER

SPECIFIC ULTRAVIOLET 

ABSORBANCE

SPECIFIC ULTRAVIOLET 

ABSORBANCE

DISSOLVED ORGANIC CARBON,

 IN MILLIGRAMS PER LITER

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Experiment AN1 Experiment AN2

Dissolved organic carbon, in milligrams per liter

Specific ultraviolet absorbance

Dissolved organic carbon, in milligrams per liter

Specific ultraviolet absorbance
18 An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida

 Summary and Conclusions

The Everglades ecosystem restoration program is a 

complex and multifaceted effort that is challenging natural 

resource managers in south Florida to meet restoration 

program goals and to prepare for anticipated increases in 

water-supply needs. Planning and establishing a largescale ASR program is one way planners are attempting to 

improve water use and availability in the future. Although 

large-scale ecosystem restoration and ASR programs are 

being conducted elsewhere, the situation in south Florida 

is unique, because there are concerns that both might affect 

Hg contamination levels of aquatic food webs. In response 

to these concerns, the U.S. Geological Survey and U.S. 

Army Corps of Engineers collaborated on a study to 

evaluate how the proposed ASR program could affect Hg 

cycling and toxicity. The project had four overall goals that 

were addressed through a combined field and laboratory 

study approach. The four project goals were to: (1) determine background concentrations of Hg, MeHg, and other 

relevant ancillary chemical constituents in the surficial and 

Upper Floridan aquifers; (2) determine whether there are 

any spatial trends in Hg and MeHg concentrations in the 

aquifers; (3) determine whether net MeHg formation could 

occur during ASR storage periods; and (4) determine 

whether net changes in Hg, MeHg, sulfate, and organic 

carbon could occur during storage periods. 

Overall, ground-water samples collected from 

water-utility wells that were screened in the target aquifers (surficial and Upper Floridan aquifers) for the ASR 

program revealed very low concentrations of Hg and 

MeHg concentrations (mean values of 0.41 and 0.07 ng/L, 

respectively). There were no apparent spatial (horizontal 

or vertical) trends in the MeHg results; however, the total 

Hg concentrations in the surficial aquifers were about three 

times greater than the mean for the deeper Floridan (0.50 

and 0.16 ng/L, respectively). The greater total Hg concentrations probably were due to the stabilizing influence 

(reduced sorption loss) of TOC, which had concentrations 

about five times greater in samples taken from the surficial 

aquifers than the Floridan. Based on these results, it does 

not appear that recovered water from the ASR program 

would represent a significant additional direct load of 

Hg and MeHg to receiving waters in south Florida. Net 

production of MeHg, however, can result from additions of 

sulfate and (or) natural organic carbon. The overall mean 

sulfate concentration observed in this study was 

80 mg/L, with much greater concentrations in samples 

from the Floridan (214 mg/L) than the surficial aquifers 

(43 mg/L). The opposite trend was observed for TOC, 

where concentrations were about five times greater in the 

surficial aquifers compared to the Floridan (9.8 and 

1.8 mg/L, respectively). Given that the presumed CERP 

ASR program storage water (surface water near Lake 

Okeechobee) is elevated in organic carbon and sulfate, and 

much of the Upper Floridan aquifer is elevated in sulfate, 

at least some potential for increased MeHg production may 

arise from the release of recovered ASR water to wetland 

environments in south Florida.

Results from the laboratory incubation experiments 

support the conclusion that low concentrations of Hg and 

MeHg are due to losses from sorption to the aquifer rock. 

Rapid losses to sorption, which were observed for naturally occurring Hg and MeHg, contained surface water 

used in the experiment as well as Me199Hg and the 201Hg 

isotope tracers that were added at the beginning of the 

experiment. These results indicate that most of the Hg and 

MeHg, which may be contained in injected surface water 

during ASR operation, will likely be present in the recovered water. The lack of any observed transformation of 

inorganic isotopes (201Hg to Me201Hg or 199Hg to Me199Hg) 

indicates that there is minimal potential for net methylation or demethylation under the conditions represented by 

the laboratory experiments. It is possible, however, that 

methylation and demethylation were occurring, but that the 

net products of these reactions were lost so rapidly to the 

rock surfaces that the byproducts could not be observed. 

Regardless, the results support the conclusion that net 

gains in MeHg during ASR storage are unlikely. 

Sulfate reduction and organic carbon consumption 

were clearly observed during the incubation experiment. 

The observed carbon-to-sulfate molar loss rate of about 2:1 

is the same as the predicted theoretical value if microbial 

sulfate reduction is the process leading to the dual consumption of sulfate and organic carbon. At the conclusion 

of the 16-week experiment, sulfate and organic carbon 

concentrations appeared to stabilize, but it was not possible to conclude if sulfate reduction had actually ceased 

or simply reached a low, steady-state rate. It is important 

to note that the apparent steady-state sulfate and organic 

carbon concentrations of about 20 to 30 mg/L at the end 

of the 16-week experiment are still elevated over what was 

observed in surface water from most unimpacted areas of 

the Everglades. Although organic carbon concentrations in 

the surface waters of the Everglades are not at concentrations considered limiting to microbial processes, sulfate is 

present at low concentrations (less than 1 mg/L) in many 

pristine areas. In general, the concentration of sulfate 

observed in samples from Floridan and surficial aquifer 
References 19

wells (19 to 369 mg/L) are substantially greater than most 

Everglades surface-water concentrations. Given the known 

links between sulfate loading and MeHg production, it is 

reasonable to conclude that sulfate added from the release 

of recovered ASR water could contribute to additional 

MeHg formation in receiving waters of the Everglades.

 Acknowledgments

The authors are very grateful to Raycine M. Hodo 

from the University of Wisconsin for her significant help 

in many aspects of this project and the personal interest 

she took in executing critical aspects of the work. Mark 

Olson, John DeWild, and Shane Olund of the USGS Mercury Research Lab provided significant assistance to the 

experimental and laboratory analysis portions of this study. 

Mark Shafer from the U.S. Army Corps of Engineers and 

Robert Verrastro from the South Florida Water Management District provided valuable review comments for this 

report.

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Prepared by the Wisconsin Water Science Center:

 U.S. Geological Survey

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This publication is available online at URL:

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Krabbenhoft and others—An Assessment of the Potential Effects of Aquifer Storage and Recovery on Mercury Cycling in South Florida—Scientific Investigations Report 2007–5240

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