Typical Sandhills Terrain

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Development and Application of a Groundwater Flow and Management Model of the 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 June 30, 2010

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 2007, the population of the 806 square mile county grew over 7 percent from 42,768 to 43,191 people (U.S. Census Bureau, 2000). Associated with this rate of population growth is an increased demand for drinking water resources both 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.

PROBLEM

Proposed additional groundwater withdrawals, along with the current withdrawals may overstress the groundwater resources in the Chesterfield County area and lower groundwater levels and possibly reduce groundwater baseflows to streams and rivers in the immediate area. Downgradient groundwater users also may be impacted. Chesterfield County is home to a major state forest (Sandhills State Forest) and an extensive Federal wildlife refuge (Carolina Sandhills National Wildlife Refuge) and any reductions in stream baseflows could have consequences for the plants and wildlife managed in these areas. The Carolina Sandhills National Wildlife Refuge and the Sandhills State Forest, located in central and southern Chesterfield County comprises over 95,000 total acres of critical longleaf pine habitat and contain a large population of endangered red-cockaded woodpeckers. Reductions in groundwater levels or the groundwater baseflows to the area's streams could have a negative effect on these valuable natural resources.

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 and to minimize the potential for excessive groundwater level declines and adverse impacts on surface-water resources. 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 Coastal Plain aquifers 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 2008. 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.

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.

Task 2: Installation of groundwater level monitoring wells

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: Publications:

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

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.