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Groundwater use from the Atlantic Coastal Plain aquifers in South Carolina has increased during the past 70 years as the population has increased along with demands for municipal, industrial, and agricultural water needs. While South Carolina works to increase development of water supplies in response to the rapid population growth, the State is facing a number of unanswered questions regarding availability of groundwater supplies and the best methods to manage these important supplies.
The objective of the proposed investigation is to update the groundwater flow model of the South Carolina Coastal Plain presented in Campbell and Coes, 2010.
The Atlantic Coastal Plain aquifers and confining units of South Carolina are composed of crystalline carbonate rocks, sand, clay, silt, and gravel and contain large volumes of high-quality groundwater (fig. 1). The aquifers have a long history of use dating back to the earliest days of European settlement in the late 1600s. Although extensive areas of some of the aquifers have or currently (2015) are experiencing groundwater level declines from large-scale, concentrated pumping centers, large areas of the South Carolina (SC) Atlantic Coastal Plain contain substantial quantities of high-quality groundwater that currently (2015) are unused.
An in-depth assessment of groundwater availability of the Atlantic Coastal Plain aquifers of North and South Carolina was completed in 2008 by the U.S. Geological Survey (USGS) (Campbell and Coes, 2010), the South Carolina Department of Natural Resources (SCDNR), and the North Carolina Division of Water Resources (DWR). This assessment includes (1) a determination of the status (2004) of the Atlantic Coastal Plain groundwater resources; (2) an explanation for how these resources have changed over time; and (3) development of tools to assess the system’s response to stresses from potential future climate variability.
The primary products of this effort were (1) comprehensive hydrologic datasets such as groundwater levels, groundwater use, and aquifer properties; (2) a revised hydrogeologic framework; (3) simulated water budgets of the overall study area along with several subareas; and (4) construction and calibration of a numerical modeling tool that is used to forecast the potential effects of climate change on groundwater levels. However, there has been a significant amount of new data collected since 2004 such as groundwater levels from an expanded monitoring network, water-use data, and new hydrogeologic framework interpretations.
A three-dimensional finite-difference numerical code (MODFLOW-2000) was used to simulate groundwater flow within the aquifers and confining units of the Coastal Plain of North and SC and parts of Georgia and Virginia. The approximately 70,500-square-mile (mi2) study area was represented in the model by a grid of 130 rows and 275 columns made up of 4-square-mile cells. The hydrogeologic system of alternating layers of permeable sand or crystalline carbonate rocks separated by confining units of silt, clay, or low-permeability crystalline carbonate rocks was represented by 16 deformed grid model layers. The flow simulation began with a steady-state stress period representing predevelopment flow conditions prior to 1900 and concluded with transient stress periods representing subsequent pumping and variable recharge through 2004. The model was calibrated to conditions representing the three flow systems of pre-1900, 1980-82, and 2004.
Groundwater use data has been collected in SC since 1983 (S.C. Department of Health and Environmental Control, 2006) (fig. 2A, 2B). Groundwater use in the SC Coastal Plain counties has increased from about 97 million gallons per day (Mgal/d) in 1982 to a high in 2002 of 223 Mgal/d. In 2013, the latest year that data is available, groundwater use was about 197 Mgal/d.
The Campbell and Coes (2010) model was calibrated to 2004 conditions, which are now over 10 years old and during that time there have been many changes in groundwater withdrawal patterns and use in the SC Coastal Plain. An example of these changes is the difference in the groundwater levels from 2004 to 2011, in the Middendorf aquifer, one of the primary water-supply units in the SC Coastal Plain. The best illustration of these changing groundwater level patterns compares groundwater level maps of the primary water-supply aquifers in the SC Coastal Plain that are regularly produced by SCDNR. The potentiometric surface (groundwater level surface) maps (Hockensmith, 2008a and Hockensmith and others, 2013a) show changing patterns of groundwater use in several of the primary aquifers. An example is in the Hemingway, SC (Williamsburg County) area where a small depression in the groundwater levels was observed in 2004. By 2011, this depression had grown to a much larger extent to the west and northwest. Additionally, the groundwater levels and water use had changed from 2004 to 2011 within the Middendorf aquifer in the Charleston and Berkeley County area. In the Black Creek aquifer, the depression in groundwater levels in Georgetown County deepened from 2004 to 2012 (Hockensmith, 2008b and Hockensmith and others, 2013b). However, small depressions in the Black Creek aquifer groundwater levels in Florence and Sumter Counties that were mapped in 2004 had recovered in 2012. These changing patterns of groundwater use and groundwater levels present management challenges to water users and managers. One way to assist water managers and users is to have a comprehensive tool, such as an up-to-date groundwater flow model that can be used to simulate the changing groundwater conditions and to provide insight into various potential management strategies.
The SCDNR, S.C. Department of Health and Environmental Control (SCDHEC) and USGS maintain equipment in wells around the SC Coastal Plain that continuously record groundwater levels on an hourly basis in the primary water-supply aquifers. Data collected at these wells from 2004 to 2015 show changing groundwater-use patterns across the SC Coastal Plain (Harder and others, 2012; US Geological Survey Groundwater Watch, 2015). Some examples of the changes are in Aiken County, where groundwater levels in the Black Creek and Middendorf aquifers have declined 3-10 feet (ft) since the mid 1990’s. A Black Creek aquifer well in Marion County has shown a steady decline of about about 40 ft from 1985 to 2015.
Overall, groundwater use in the SC Coastal Plain from 2004-2013 has increased slightly from about 202 million gallons per day (Mgal/d) in 2004 to an average of 209 Mgal/d from 2005 to 2013 (SCDHEC, written commun, 2015) (fig. 2A). There has been a significant increase in irrigated agriculture in SC, with much of this new water demand met by groundwater use (fig. 2B). In SC this trend is likely to continue. Potential adverse effects of the continued increase in groundwater withdrawals include groundwater level declines and reduced baseflow to streams and other surface-water bodies.
The population of SC has increased substantially from 2000 to 2014 growing from about 4.0 to about 4.6 million residents (U.S. Census Bueau, 2015). The 13-percent increase in population is not evenly distributed across the state but tends to be concentrated in the larger cities and coastal areas. Some of the water needs for this growing population are being met by groundwater use which tends to be concentrated public supply use in a few muncipalities in the SC Coastal Plain (fig. 3).
The SC Agriculture Commission is actively recruiting industrial-scale farms to locate in the Coastal Plain counties of SC (SC Commissioner of Agriculture, oral commun., March 2015) (fig. 4). These new farms will most likely use groundwater for their primary source of irrigation water and are likely to be clustered in areas with suitable soils, transportation, and labor. In addition, the Palmetto Agribusiness Council has a plan to increase the economic impact of agriculture, forestry, and allied industries from $34 billion to $50 billion by 2020 (Palmetto Agribusiness Council, 2015). This new agricultural activity will demand large quantities of water that will most likely be supplied primarily by groundwater. It is not known if these proposed increases in groundwater use for agricultural uses will have negative effects on groundwater levels, surface-water flows and/or other groundwater users.
Precipitation patterns are changing across the SC Coastal Plain with implications for groundwater recharge rates. Reduced precipitation rates, especially in the cooler months (November – March) could lead to lower groundwater recharge rates and groundwater levels. A study by Mizzell and others (2014) of historic precipitation trends, indicates a general decrease in precipitation in SC from 1900 to 2010 for summer rainfalls, increasing fall precipitation, and mixed increases and decreases for winter and spring. Future precipitation patterns could follow these trends.
The South Carolina Water Plan, 2nd Edition (Badr and others, 2004) states: “A comprehensive groundwater flow model of the Coastal Plain should be developed and used to predict the effects of future pumping and to determine optimal well spacings”. The proposed update to the Campbell and Coes (2010) model will provide the model described in the SC Water Plan. An updated groundwater flow model of the South Carolina Coastal Plain will benefit the State of SC by providing a tool that can be used by water-resource managers to estimate current available water resources and to assess the effects of future water use development and climate variability on available water resources. The USGS proposes to work cooperatively with the SCDNR, SCDHEC, and SC Department of Agriculture to develop an updated groundwater flow model of the South Carolina Coastal Plain.
OBJECTIVES AND SCOPE
The objective of the proposed investigation is to update the groundwater flow model of the South Carolina Coastal Plain presented in Campbell and Coes, 2010. Updates to the model will include activating and rediscretizing the surficial aquifer model layer, adding recent groundwater-related data (2005-2015), refining the model grid in some areas, incorporating a more detailed representation of the Fall Line area, re-calibration, and applying the model to a series of 6 scenarios. Numerous sites have been added to the SC groundwater level monitoring network, primarily by the SCDNR and SCDHEC, since 2004 when the Campbell and Coes (2010) model was last calibrated (Harder and others, 2014; H. Gilkerson, SCDHEC, personal commun., 2015). Water-use data have been compiled by SCDHEC for 2005-2014 and will be simulated in the proposed groundwater model update. A series of 6 future scenarios will be simulated with the updated model to attempt to give some insight into how potential changes in groundwater withdrawal patterns and rates and climatic changes could affect groundwater levels and stream baseflows.
The scope of this investigation will include the South Carolina Atlantic Coastal Plain aquifers and confining units. However, to simulate model boundary conditions, the model is extended into the Georgia and North Carolina Coastal Plain. A USGS Scientific Investigations report documenting the investigation will be published during the final year of the study.
The objectives listed above will be met during this 3-year investigation by implementation of the following 6 tasks.
Task 1: Data Acquistion
Task 2: Convert Data to Various Model Inputs
Task 3: Re-Configure Groundwater Flow Model
Task 4: Re-Calibrate Model to Past and Current (Year 1) Conditions
Task 5: Scenarios
Task 6: Report Preparation
A USGS SIR will be published to document the updated groundwater flow model and summarize the results of the scenarios. The SIR will not have to be as detailed as the original model documentation report (Campbell and Coes, 2010) since it will describe an update to the original model.
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