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Development and Application of a Groundwater Flow and Management Model and Assessment of Groundwater Contamination, Chesterfield County Region, South Carolina

Project Number: 2519-D8201
Project Chief: Bruce Campbell
Cooperator: South Carolina Department of Natural Resources
Period of Project: October 1, 2007 to December 31, 2013

SUMMARY

Chesterfield County is in the northern part of South Carolina and adjacent to the North Carolina border. Chesterfield County also lies on the Fall Zone, the geologic boundary between the Coastal Plain and Piedmont physiographic provinces (see map). Between 2000 and 2010, the population of the 806 square mile county grew over 9 percent from 42,768 to 46,734 people (U.S. Census Bureau, 2012). Associated with this rate of population growth is an increased demand for drinking water resources in the Chesterfield County area. In 2004, the total water-use reported for Chesterfield County by South Carolina Department of Health and Environmental Control was about 2 million gallons per day.

Water-quality issues also present a concern in the Chesterfield County area. Recently, concentrations of ethylene dibromide (EDB), dibromochloropropane (DBCP), and naturally occurring radium (Ra, as the isotopes ²²⁶Ra and ²²⁸Ra) in concentrations above the U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Limit (MCL) have been detected in several public supply wells and numerous domestic wells in the McBee, SC area, in central Chesterfield County. These wells are screened to various depths in the sole-source aquifer beneath this part of Chesterfield County.

PROBLEM

The detection of EDB is of concern because it is a known carcinogen with a low MCL of 0.015 parts per billion. The exact source and release history of EDP and DBCP is unknown, but believed to be related to the legacy use of EDB and DBCP as soil fumigants and EDB as a gasoline additive. The source of radium is most likely linked to the natural radioactive decay of minerals that contain uranium (which produces ²²⁶Ra and then radon gas (²²²Rn)) and thorium (which produces ²²⁸Ra) in the rocks and sediments of the aquifers.

Levels of EDB and DBCP that exceed the MCL’s have been detected in both public supply and domestic wells in the McBee area, and consequently the local water utility has been forced to install and maintain cost–intensive granular-activated carbon filters (GAC) for several production wells. Also, numerous individuals have had to discontinue use of domestic wells and purchase drinking water from the local water utility. The duration that these remedial actions and domestic-well restrictions must be in place, however, is uncertain, because the source or sources of these contaminants remains unknown.

OBJECTIVES AND SCOPE

The objective of the proposed investigation is to develop and apply a groundwater flow and groundwater management model that can be used to better manage the groundwater resources in the Chesterfield County area. Objectives of these better management practices would be to help assure sustainability of the groundwater resources in the area. A groundwater-flow model will be developed for the Coastal Plain aquifers in the Chesterfield County area. The updated model will enable Chesterfield County water-resource managers to:

• Determine flow rates within aquifers and drawdown effects from current pumping centers.
• Test possible scenarios for future groundwater withdrawals from the Coastal Plain aquifers and the effects of these withdrawals on existing wells and on stream baseflows in the area.
• Optimize future groundwater withdrawals to meet demands and not exceed constraints such as impacts on surface-water baseflows and groundwater level declines.

The scope of this modeling effort will include the Atlantic Coastal Plain aquifers in Chesterfield County and all or parts of neighboring counties in both North and South Carolina (referred to as the Chesterfield County region herein). The simulation period of the model will be from about 1900 (predevelopment) to 2012. Scenarios of possible future groundwater management strategies along with the impacts of the proposed withdrawals will be evaluated. The results of the investigation will be documented in a USGS Scientific Investigations Report anticipated to be released to the public in late 2013.

A better understanding of the relation between groundwater contamination of certain drinking-water wells by EDB, DBCP, and radium, and their potential sources and transport will provide guidance on the future fate of these contaminants.

• Examine scenarios to help determine source areas of EDB and DBCP detected in both public supply and domestic wells, and to examine if and when the contaminants may be flushed from the system.
• Examine the factors that have led to the recent occurrence of radium detections in some supply wells.

APPROACH

The approach for each task of the proposed study is presented below.

Task 1: Assemble pertinent hydrogeologic and geologic data

USGS Hydrologist collecting data from a groundwater production well in the Chesterfield County study area (USGS Photo)

The purpose of this task is to gather the data necessary to update the regional USGS hydrogeologic framework to a more detailed scale. Existing geologic reports and maps, subsurface data and hydrogeologic information will be assembled to refine the current hydrostratigraphy. There are water-use data available for the Chesterfield County region that has previously been gathered; however, the regional USGS model only uses yearly withdrawals. This new modeling effort will compile recent (1997-present) monthly water-use data from the project partners and state agencies to account for seasonal variations. Aquifer-test data is available from several sources in the region. Data from the USGS, state agencies, and the project partners will be collected and interpreted to fill in areas with little or no data. There are several sources of groundwater level data in the region. Existing reports, the USGS database, the SCDNR database and the North Carolina Division of Water Resources (NCDWR) database along with data from local entities will be the primary sources of historic groundwater-level data collected in the area. A synoptic water-level data collection will occur in the fall of 2007 and early spring 2008 to create updated potentiometric surface and water-level maps of the Chesterfield County region. Wells measured will include the project partner's production and observation wells and other available wells in the study area.

Groundwater contaminant data for EDB and DBCP detected in wells and surface-water (springs) in the immediate McBee, SC area, will be compiled. Also potential source areas for these contaminants, such as the multiple former underground storage tanks (UST) and historical agricultural usage, will be delineated. Groundwater contaminant data for radium detected in wells in the immediate McBee, SC area,will be compiled. The use of radon gas as a surrogate for radium detection will be tested.

Task 2: Installation of groundwater level monitoring wells

CTF-221 and 222 groundwater monitoring station.

Several sites will be selected that will allow for the collection of regional groundwater levels in both the surficial and Middendorf aquifers. Two wells will be installed at each site, one screened in the surficial aquifer and one in the Middendorf aquifer. The four wells will be fitted with near real-time continuous groundwater-level recording equipment that will enable the project partners and the public to retrieve near-real-time groundwater level data through the USGS web pages (http://waterdata.usgs.gov/sc/nwis/current/?type=gw).

Task 3: MODFLOW model development:

USGS will develop a groundwater-flow model using MODFLOW-2005 (Harbaugh and others, 2005) and will use a commercial graphical user interface to enhance pre- and post-processing tasks, as well as to allow for ease of use, model refinement, and updating. This modeling effort will include all of the Coastal Plain aquifers in the study area. Model development will include modifying the current USGS regional model to incorporate data compiled during the data collection task.

Task 4: Simulate Management Scenarios:

After calibration, the transient groundwater-flow model will be applied to simulate possible future withdrawal scenarios by using the Ground-Water Management (GWM) package for MODFLOW (Ahlfeld and others, 2005). The GWM package is a tool that allows the flow-model to be enhanced by incorporating management goals and constraints such as minimizing the loss of stream baseflows or maximizing groundwater production while minimizing the impact on existing groundwater users. Groundwater users and other stakeholders will be involved in developing management scenarios to be tested.

Task 5: Groundwater and selected surface-water contaminant data collection and interpretation:

Streamflow measurement on Juniper Creek.

The source or contributing area of groundwater derived from selected public supply and domestic wells will be determined from advective transport or particle-tracking techniques. The redox geochemistry of groundwater and select surface-water locations will be assessed during contaminant sampling. Delineation of the prevailing redox status is important in understanding the long-term fate of EDB and DBCP, because these highly oxidized compounds tend to resist degradation when dissolved oxygen is present. Detection of potential breakdown products of EDB and DBCP will help understand the fate and potential for long-term contamination in the aquifer. Surface-water samples will be collected in representative seeps, streams, and tributaries.

Selected wells will be sampled for chlorofluorocarbons (CFCs). The detection of chlorofluorocarbons in groundwater indicates recharge by water that has been in contact with the atmosphere since the 1940’s, and absolute-CFC concentrations can indicate the age of groundwater. Detection of CFCs in ground water beneath McBee and surrounding areas will help determine the age of the contamination of EDB and DBCP, possible sources, and even groundwater flow rates. This data can be used to help predict when the contaminants will be purged from the groundwater system. Some surface-water samples will be collected and analyzed for CFCs.

Groundwater and surface-water samples will be collected for radium isotopes and radon gas.

Task 6: Publications:

A USGS Scientific Investigations Report will be published to document the model and to summarize the results of the simulations.

PUBLICATIONS

Landmeyer, J.E., and Campbell, B.G., 2010, Contamination of Groundwater by the Fumigants Ethylene Dibromide (EDB) and Dibromochloropropane (DBCP) near McBee, South Carolina: Proceedings of the 2010 South Carolina Water Resources Conference, held October 13-14, 2010, at the Columbia Metropolitan Convention Center

Campbell, B.G., and Landmeyer, J.E., 2010, Development of a Conceptual Model of Groundwater Flow, Chesterfield County, South Carolina: Proceedings of the 2010 South Carolina Water Resources Conference, held October 13-14, 2010, at the Columbia Metropolitan Convention Center

REFERENCES

Ahlfeld, D.P., Barlow, P.M., and Mulligan, A.E., 2005, GWM-A ground-water management process for the U.S. Geological Survey modular ground-water model (MODFLOW-2000): U.S. Geological Survey Open-File Report 2005-1072, 124 p.

South Carolina District., 2004, Sorth Carolina District Science Plan Science Goals for 2004-2009: U.S. Geological Survey Administrative Report, 52p.

Harbaugh, A.W., 2005, MODFLOW-2005, The U.S. Geological Survey modular groundwater model-the ground-water flow process: U.S. Geological Survey Techniques and Methods 6-A16, variously paged.

Newcome, R., Jr., 2004, Ground-water resources of Chesterfield County, South Carolina: South Carolina Department of Natural Resources, Land, Water and Conservation Division, Water Resources Report 36, 16p.

U.S. Census Bureau, 2007, State and county quick facts http://quickfacts.census.gov/qfd/states/45000.html

U.S. Geological Survey, 1999, Strategic directions for the Water Resources Division, 1998-2008: U.S. Geological Survey Open-File Report, 99-249, 19p.

U.S. Geological Survey, 2000, U.S. Geological Survey Strategic Plan 2000-2005: Reston, Va., 20p.