U.S. Department of the Interior

U.S. Geological Survey

INTERACTIONS OF MERCURY

WITH DISSOLVED ORGANIC

CARBON IN THE FLORIDA

EVERGLADES

There has been increased awareness by

both public and scientific communities

over mercury contamination of game

fish in South Florida. Effective

management strategies for dealing with

this problem will require a more

thorough understanding of the factors

and processes that result in the

generation and transport of mercury, and

control its reactivity in the Everglades.

The role of mercury complexation by

dissolved organic carbon (DOC) has

been proposed by several investigators

as a primary mechanism for the

transport of mercury in aquatic systems.

However, this relationship has been

simply proposed based on a strong

correlation between dissolved mercury

and DOC concentrations in ground, lake

and stream waters. Interactions with

DOC may be an important process for

controlling the transport and

bioavailability of mercury.

The issue of carbon reactivity in

South Florida is especially relevant due

to the high natural production of organic

carbon in the peat soils and wetlands,

the relatively high carbon content of

shallow ground water systems in the

region, the interactions of organic

matter with mercury, other trace metals,

and anthropogenic compounds,

accumulation of organic carbon in corals

and carbonate precipitates, and the

potential changes in the quality and

reactivity of dissolved organic carbon

(DOC) resulting from land use practices.

This study will attempt to meet the need

to more clearly understand the dynamics

of organic matter mercury interactions

in the Florida Everglades by focusing

research on the effect of dissolved

organic carbon (DOC) on the transport

and reactivity of Hg in the Everglades.

PROJECT GOAL

It is well recognized that the chemical

forms of mercury in the water column

and sediments are intimately related to

bioaccumulation and body burden.

Interactions of mercury and dissolved

organic matter may play an important

role in controlling the bioavailability

and reactivity of mercury. The goal of

our research is to provide information

about the interactions of mercury and

dissolved organic matter that will better

define this important, albeit, poorly

understood process. Ultimately, this

research will lead to will lead to a more

complete model of mercury behavior in

the Everglades. Our research will focus

on the effect of DOC on the transport

and reactivity of mercury in the

Everglades through a combined field and

laboratory study. The underlying

hypothesis of this research is that the

chemistry and structural characteristics

of organic matter in the Everglades have

a controlling influence on mercury

cycling processes such as methylation

and volatilization.

BACKGROUND

Organic matter in aqueous systems is

an important chemical constituent that

is important for a number of chemical

and biological processes. Organic

matter often controls geochemical

processes by acting as pH buffers, by

affecting the transport and degradation of

pollutants, and by participating in

mineral dissolution/precipitation

reactions. Dissolved and particulate

organic carbon may also play an

important role in the ecology of aquatic

systems by controlling the depth of the

photic zone in surface waters, by

influencing the availability of nutrients

and by serving as a carbon substrate for

microbially mediated reactions.

The organic carbon story begins with

microbial degradation of plant material

and organic detritus by bacteria, fungi

and other organisms. These biological

processes result in the formation of

many of the compounds that comprise

the organic matter found in soils, peat,

and aquatic systems. Organic matter

derived from different source materials

has distinctive chemical characteristics

associated with those materials. These

differences in chemical nature control

the reactivity of the organic matter.

Once in the system, organic

compounds, whether they be man-made

or naturally derived, can be truly

dissolved, associated with immobile

particles or associated with mobile

particles (DOC, colloids). Each species

Map of South Florida showing the study

area (darkened)

is interrelated to other chemical

components in the system, such as

mercury.

Most of the dissolved organic matter

in the Everglades originates from the

degradation and leaching of organic

detritus resulting from the algae,

bacteria and macrophytes living within

the wetland environment. In addition,

organic matter is also transported to the

Water Conservation Areas of the

Everglades in the canals that drain the

Everglades Agricultural Area. The

ecological and chemical significance of

the organic matter at a given place is

determined by its chemical nature - not

all organic compounds react in the same

way or to the same degree.

WHAT ARE THE KEY ISSUES

FOR THE EVERGLADES?

DOC-Mercury Interactions

The role of mercury complexation by

DOC has been proposed by several

investigators as a primary mechanism

for the transport of mercury in aquatic

systems. However, this relationship

has been simply proposed based on a

strong correlation between dissolved

mercury and DOC concentrations in

ground, lake and stream waters. The

vast majority of mercury in aquatic

systems resides in organic-rich soils and

sediments. Interactions with DOC,

however, may be an important process

for the release of mercury into
porewaters with subsequent transport

into surface waters. Effective

management strategies for dealing with

this problem will require a more

thorough understanding of the factors

and processes that result in the

generation and transport of Hg, and

control its reactivity in the Everglades.

Our studies will focus on the effect of

dissolved organic carbon (DOC) on the

transport and reactivity of Hg in the

Everglades.

Effects of land use practices

Land and water use practices can result

in changes in the quality, quantity, and

reactivity of DOC. Agricultural

practices, for instance, result in the

removal of native sources of DOC and

the addition of anthropogenic

compounds in the form of herbicides,

pesticides and surfactants. The DOC

resulting from these practices is,

therefore, different in nature from the

original DOC of the region. In

addition, the hydrologic condition of the

wetlands themselves will also impact

both the quantity and reactivity of the

DOC moving through the system. For

instance, water table changes can

markedly change DOC by introducing

oxygen into the unsaturated zone.

USGS research is attempting to

determine the effect of land and water

use practices on the quantity and nature

of the bulk DOC in the South Florida

system The first objective is to

demonstrate that these practices have a

significant effect on the nature of the

DOC thereby affecting the chemical

properties of the DOC. The second

objective is to demonstrate that DOC

interactions within a given environment

determine the behavior of mercury in the

system.

PLAN OF STUDY:

This project will use a combined

field/laboratory approach to assess the

significance and strength of DOC-Hg

interactions in the Everglades. Both the

inorganic composition and organic

matter associated with surface water,

pore waters, and ground water from the

Everglades will be characterized.

Temporal variations in mercury and

DOC concentrations will be measured at

appropriate sampling locations to

provide a measure of the mercury and

DOC loadings in the system. These

measurements will be coupled with

determinations of the nature of the DOC

inputs under different hydrologic

conditions (low and high flow

conditions). Major fractions of the

DOC, such as aquatic humic

substances, will be isolated from the

water samples and characterized by

determining elemental composition,

molecular weight, functional group

content and structural characteristics.

The isolates and whole water samples

will be used to study interactions of

DOC with mercury in laboratory

experiments using both radio-labeled

(203Hg) and non-labeled mercury under

a range of pH and concentration

conditions. In addition, these

measurements will also be made with

previously isolated samples that vary

significantly in structural composition.

The goal of these measurements will be

to provide distribution coefficients for

relevant mercury species with aquatic

humic substances and DOC, in general.

These coefficients will then be correlated

with the structural properties of the

organic matter to provide inexpensive

analytical parameters to be used to

estimate the strength of DOC-mercury

interactions in the Everglades.

Finally, using the binding constants

determined in the laboratory studies, a

computer program will be used to model

the geochemical behavior of mercury in

the Everglades. This model will allow

us to synthesize the overall effects of

the DOC-Hg associations and how they

change as the qualitative and quantitative

character of the DOC varies in our study

systems.

Anticipated Schedule:

March 1995: Begin seasonal field

sampling and sample analyses at

select sites within the study area.

January 1996: Begin compiling water

quality data as a USGS Open File

report for ongoing field work.

July 1996: Continue field sampling.

Begin characterizing organic

matter isolates and measuring

DOC-Hg binding constants.

January 1997: Continue field sampling.

Compile report on characteristics

of DOC in the Everglades.

Continue determination of Hg

binding constants.

January 1998: Compile report on Hg

binding constants.

Planned Products:

- USGS Open File report series of water

quality data.

- Journal articles detailing the spatial,

seasonal and hydrologic

variability in DOC.

- PhD thesis and journal articles

describing DOC-Hg binding

constants

COLLABORATORS:

Data collection for this project

requires the collaboration of scientists

from the USGS (Reston, Madison and

Menlo Park), from the Wisconsin

Department of Natural Resources, the

Benedict Estuarine Laboratory, Academy

of Natural Sciences of Philadelphia, and

the South Florida Water Management

District. Binding constants will be

determined in collaboration with

scientists from the USGS (Madison) and

the University of Colorado, Boulder,

Colorado. Chemical modeling will be

done in collaboration with the US EPA,

Athens, Georgia.

BENEFITS:

-Detailed understanding of the

influence of hydrologic and seasonal

factors on the variation in the nature and

amount of DOC and mercury in the

Florida Everglades.

-Distribution coefficients for mercury

with dissolved organic carbon and

aquatic humic substances that are

currently unavailable.

-The identification of easily

determined analytical parameters that can

be used to predict potential organic

matter-mercury interactions.

-Incorporation of organic matter

properties and distribution coefficient

data for mercury into a speciation model

to more adequately determine speciation

of mercury in aquatic systems, and to

provide predictive capabilities for the

behavior of mercury in other aquatic

systems.

FOR MORE INFORMATION:

George Aiken or Mike Reddy

U.S. Geological Survey

3215 Marine Street

Boulder, Colorado 80303