United States Environmental Protection Agency	Office of Research and Development

National Exposure Research Laboratory
Research Abstract

Government Performance Results Act (GPRA) Goal 4
Annual Performance Measure 235

Significant Research Findings:

Watershed Mercury Simulation Software for TMDL Assessments

Scientific	Mercury (Hg) has been declared a primary pollutant by the U.S. Environmental

Problem and	Protection Agency (USEPA), the United Nations Environmental Science

Policy Issues	Committee (UNESCO), the United Nations Environmental Council of Europe

(UNECE), and the trilateral Council of North American Environment Ministers.
Human and wildlife exposure to mercury is primarily due to the consumption of
contaminated fish. Out of 189 compounds identified as hazardous air pollutants in
the 1990 Clean Air Act, mercury was singled out for separate study to examine
anthropogenic (human-caused) emission and to define thresholds at which
mercury affects human health and the environment.

Most mercury in the atmosphere is the gaseous element, Hg", which can be
oxidized to divalent mercury, Hg(II) and then deposited to receptor sites. Hg(II)
depositing from the atmosphere onto terrestrial components of the watershed may
accumulate in soils, where a fraction is reduced and re-emitted back to the
atmosphere as gaseous Hg". A small but significant, fraction of the deposited
mercury is delivered to the surface water network via runoff and erosion. Total
Hg(II) loading to surface water bodies includes, therefore, direct atmospheric
deposition and indirect atmospheric deposition; that is, fluxes depositing first onto
the land and then washing into an adjacent water body. Within some soils,
wetlands, and water bodies, Hg(II) can be methylated to organic forms, primarily
monomethylmercury, CH3Hg+ (here designated MeHg), which is the most toxic of
the mercury species and strongly bioaccumulates.

States or EPA Regions are responsible for setting Total Maximum Daily Loads
(TMDL) for water bodies adversely affected by high mercury levels. TMDL
analyses must determine the present indirect contributions of atmospheric mercury
deposition to the watershed, in order to predict water body mercury levels under
regulatory control strategies. Watershed mercury loading estimates must account
for atmospheric deposition directly to the aquatic systems and elements of the
terrestrial watershed, transport and transformation through biota and soils in the
watershed, delivery to the tributary network, and transport and transformation
through the network and the main water body. A complete description of these
fluxes in a watershed must be inferred from a judicious combination of
measurements and calculations, because mercury cycling in watersheds is complex
and incompletely understood. To support defensible mercury TMDLs, a
convenient watershed characterization and modeling framework is needed to help

users integrate watershed transport and mercury cycling processes described in the
literature with site-specific watershed and water body data. This product uses
Geographic Information System (GIS) technology to link terrestrial and aquatic
simulation models along with atmospheric mercury deposition data on a watershed
scale to quantify the direct and indirect atmospheric mercury sources to the
terrestrial and aquatic components of a selected watershed, providing the states
and regions a model capable of supporting a TMDL assessment of methylmercury
levels in fish resulting from atmospheric deposition, point sources, and internal
watershed processes.

Research	The objective of this research was to produce and demonstrate practical, second-

Approach	generation mercury watershed simulation technology that can be applied at basin

scales to investigate proposed remediation and load reduction options. The
approach was to assimilate present scientific knowledge of hydrology, sediment
transport, and mercury cycling into descriptive modules linked by recently
developed GIS watershed software and supporting databases. This technology
takes advantage of the recently-released ArcGIS 9 to implement watershed
calculations on a grid rather than a sub-watershed basis. The software was
produced by combining new hydrology, sediment, and mercury fate modules with
components of the GIS- based Watershed Characterization System (WCS)
(Greenfield et al., 2002) which calculates soil mercury concentrations and loadings
from pervious and impervious surfaces in individual sub-basins. The software has
been peer reviewed and tested on two watersheds in Georgia, where it was used to
calculate mercury TMDLs.

Results and	A distributed grid-based watershed mercury loading model was developed to

Impact	characterize spatial and temporal dynamics of mercury from both point and non-

point sources. The model simulates flow, sediment transport, and mercury
dynamics on a daily time-step across a diverse landscape. The model is composed
of six major components: (1) an ArcGIS interface for processing spatial input data;
(2) a basic hydrological module; (3) a sediment transport module; (4) a mercury
transport and transformation module; (5) a spreadsheet-based model post-
processor; and (6) links to other models such as WASP and WhAEM 2000
developed by EPA. The model fully uses the grid processing capacity of the latest
ArcGIS technology. The water balance, sediment generation and transport, and
mercury dynamics are calculated for every grid within a watershed. Water and
pollutants are routed daily throughout the watershed based on a unique and
flexible algorithm that characterizes a watershed into many runoff travel-time
zones. The mercury transport and transformation module simulates the following
key processes: (1) mercury input from atmospheric deposition; (2) mercury
assimilation and accumulation in forest canopy and release from forest litter; (3)
mercury input from bedrock weathering; (4) mercury transformation in soils; (5)
mercury transformation in lakes and wetlands including reduction and net
methylation; (6) mercury transport through sediment and runoff; and (7) mercury
transport in stream channels. By using the grid-based technology, flow and
mercury dynamics can be examined at any of several points in the watershed. The
model is capable of supporting large-scale watershed modeling with high-
resolution raster datasets. The model is programmed in Visual Basic and requires
two ArcGIS (version 9.0) components—ArcView 9 and the Spatial Analyst

Research	This software was produced under contract with Tetra Tech, Inc. Along with the

Collaboration and software, a document is available describing the conceptual design and modeling
Research	procedures.


Publications from this study include:

Tetra Tech, Inc., "Development of Second Generation Mercury Watershed Simulation Technology -
Technical Document," U.S. EPA, National Exposure Research Laboratory, Athens, GA, 2004.

Ting Dai, Ambrose, R. B., Alvil, K., Wool, T.A., Manguerra, H., Chokshi, M., Yang, H., and
Kraemer, S. "Characterizing Spatial and Temporal Dynamics: Development of a Grid-Based
Watershed Mercury Loading Model," to be presented at the American Society of Civil Engineers,
Environmental and Water Resources Institute 2005 Watershed Management Conference,
Williamsburg, VA, July, 2005.

Future Research EPA and other governmental and private organizations are supporting research to
better understand mercury transport and transformation processes in watersheds
and water bodies. METAALICUS is a particularly notable collaborative effort
investigating the cycling of historical versus newly deposited mercury in an
experimental lake in Ontario, Canada. We plan to incorporate new research
findings from this and other ongoing research into future versions of the mercury
TMDL software.

Questions and inquiries can be directed to:

Robert B. Ambrose, Jr., P.E.

U.S. EPA, Office of Research and Development
National Exposure Research Laboratory
960 College Station Road, Athens, GA 30605-2700
Phone: 706-355-8334
E-mail: ambrose.robert@epa.gov

Contacts for



Federal funding for this research was administered under EPA contract number