Assessment of Groundwater Availability in Aiken County, South Carolina

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The objective of this project is to develop a groundwater-flow model that can be used by Aiken County water managers to manage current and projected reported and unreported demands on groundwater resource and to ensure the highest quality of groundwater.

Project Chief: Bruce Campbell
Cooperator: Aiken County, Gilbert-Summit Rural Water District, City of Aiken, Breezy Hill Water and Sewer and Montmorenci-Couchton Water District
Period of Project: October 1, 2014 to December 31, 2018

PROBLEM STATEMENT

Study area - Assessment of Groundwater Availability in Aiken County, South Carolina
Map of the study area.

Aiken County is located in South Carolina midway between the mountains and the coast and is bordered by the Savannah River on the west, Edgefield and Saluda Counties on the north, Barnwell and Orangeburg Counties on the southeast and by Lexington County on the east (map). The 2010 census counted 160,099 persons living in the County, an increase of about 18,000 from 2000. Aiken County is the fourth largest South Carolina County by land area, about 1,073 square miles.
Most of the potable water produced in Aiken County, with the exception of North Augusta, is supplied by groundwater produced from the various Atlantic Coastal Plain (ACP) aquifers underlying the County. The ACP aquifers underlying Aiken County are very productive and generally contain high-quality groundwater that requires little treatment prior to use.

Reported water use data for 2010 reveals that about 15 million gallons per day (MGD) of groundwater was pumped by users in Aiken County. For example, pumpage for the following water suppliers for 2010 includes the Montmorenci-Couchton Water District (0.3 MGD), New Ellenton Commissioners of Public Works (0.75 MGD), Valley Public Service Authority (1.0 MGD), Breezy Hill Water and Sewer (1.1 MGD), College Acres Public Water District (0.15 MGD), and the City of Aiken (7 MGD). Other groundwater users include Jackson, Beech Island, Monetta, Salley, Wagner, Talatha, Windsor, the Savannah River Site, and various commercial irrigation users not required to report their use. In all likelihood, there is additional (but not reported) seasonal irrigation pumping in Aiken County, as in most Atlantic Coastal Plain counties in SC. The impact on groundwater resources from this unreported irrigation pumping is not known, but the distribution of large areas of agricultural land use in Aiken County suggests that it may be substantial and has the potential to increase in the future.

Map of aquifers, Assessment of Groundwater Availability in Aiken County, South Carolina
FIgure 2- Most groundwater pumped in Aiken County comes from a series of unconsolidated Tertiary and late Cretaceous–age sand aquifers, with intervening clay confining (or semi-confining) units.

Most groundwater pumped in Aiken County comes from a series of unconsolidated Tertiary and late Cretaceous–age sand aquifers, with intervening clay confining (or semi-confining) units (fig. 2; Gellici, 2007). The primary aquifers used to produce groundwater in Aiken County are the Crouch Branch and McQueen Branch aquifers (formerly known as the Middendorf aquifer; Campbell and Coes, 2010). The Crouch Branch aquifer occurs from approximately 200 to 300 feet (ft) below land surface (BLS). The McQueen Branch aquifer occurs from approximately 325 to 450 ft BLS. Most of the wells in Aiken County are screened across both aquifers.

An analysis of 100 years of precipitation data (1913 – 2013) from 66 climate stations across South Carolina by Mizzell and others (2014) indicated a generally decreasing trend. However, the climate station at the City of Aiken was the only one of the 66 stations analyzed that had an increasing trend in precipitation of over 1 inch over all four seasons. The groundwater level declines could, therefore, be related more to the increasing rates of groundwater withdrawals than changes in precipitation rates due to changes in climate.

The reliance on groundwater by Aiken County has increased steadily since the 1950’s, but it is unknown if this withdrawal of groundwater will affect the current or future availability or quality of groundwater in Aiken County. An assessment of the factors that affect groundwater availability and quality across Aiken County is hindered by the lack of a comprehensive network of wells to monitor short-and long-term changes in groundwater levels or baseline water quality. Monitoring wells are located at Aiken State Park, to the west of New Ellenton and north of Jackson. Groundwater levels in these wells are monitored by the South Carolina Department of Natural Resources (SCDNR) and have shown declines of 6 to 7 feet over the past 20 years (figs. 3a and 3b; Joe Gellici, SCDNR, written commun., 2012). Figure 3b shows groundwater levels collected in well AIK-0847 at Aiken State Park in the Black Creek (Crouch Branch aquifer) aquifer. Groundwater levels in this well have declined from approximately 23 ft BLS in 1993 to about 30 ft BLS in 2012―a total decline of 7 ft. Groundwater levels in well AIK-0824 (fig. 4) located west of New Ellenton, SC have declined from 178 ft BLS in 1993 to about 184 ft BLS in 2012―a total decline of 6 ft. Because rainfall levels between 1901 and 2010 showed an increasing precipitation trend for Aiken County (Mizell and others, 2014), it is likely that these lowered groundwater levels are due to increased pumpage rather than changes in precipitation due to climate change.

Recent potentiometric maps (Hockensmith, 2012a, b) indicate that groundwater levels in the deeper McQueen Branch aquifer are lower than the groundwater levels in the shallower Crouch Branch aquifer. This indicates a potential for the downward vertical flow of water from land surface to the McQueen Branch aquifer. The time required for water to move from land surface to the aquifer, and then enter Aiken County public-supply wells is currently unknown. A longer amount of time that groundwater would remain in the aquifer prior to being pumped (“older” groundwater age) would enable any entrained contamination to be attenuated. Conversely, a shorter residence time (“younger” groundwater age) may not be sufficient to protect the water-quality produced from the “capture zone” that supplies groundwater to each wellhead. Multiple potential sources of contamination occur at land surface in Aiken County that include contaminants such as volatile organic compounds (VOCs), petroleum hydrocarbons, and metals, but their location in reference to current or planned Aiken County potable supply wells is unknown (http://www.scdhec.gov/environment/water/gwci.htm).

 

Figure 3A. Groundwater levels in well AIK-0847
Figure 3A. Groundwater levels in well AIK-0847 screened in the Crouch Branch aquifer (previously the Black Creek aquifer) located at Aiken State Park from 1993–2012. See inset location map on figure 2 for well location. (Written communication, Joe Gellici, 2012). and Figure 3B) Groundwater levels in well AIK-0824 screened in the Crouch Branch aquifer (previously the Black Creek aquifer) located west of New Ellenton, SC from 1993–2012.  See inset location map on figure 2 for well location. (Written communication, Joe Gellici, 2012).

 

This situation presents the Aiken County water managers with the challenge to efficiently meet current and predicted demands for groundwater without causing water-quantity or water-quality problems. These problems could result from interference between well capture zones causing groundwater levels to decline at a more rapid rate or in combination with overall increasing pumping rates in the Aiken County area.

 

OBJECTIVES

The objective of the proposed assessment of the groundwater availability for Aiken County is simply put: To develop a groundwater-flow model that can be used by Aiken County water managers to manage current and projected reported and unreported demands on groundwater resource and to ensure the highest quality of groundwater.

This objective can be discretized into 4 tasks, listed here in order of implementation: (1) Develop a state-of-the science groundwater-flow and management model (2) Calculate the water budget for the Aiken County area (3) Document the general water-quality characteristics for public-supply wells across Aiken County. (4) Assess the occurrence of chemical contamination in selected public-supply wells.

APPROACH

The objectives listed above will be met by implementation of the following 4 tasks.

Cretaceous-age sediments near Aiken, SC
Cretaceous-age sediments near Aiken, SC  (Public domain.)

Task 1: Develop State-of-the-Science Groundwater-Flow and Management Model

A -- Compile existing hydrostratigraphic data: This task will include compiling all data necessary to update the regional hydrogeologic framework at a finer scale.  Existing geologic reports and maps, subsurface data and hydrogeologic information will be assembled to refine the current hydrostratigraphy. Project personnel will work closely with state or local agency scientists who have extensive knowledge of the proposed Aiken County study area.

B -- Compile and interpret existing aquifer-test data:
Aquifer-test data are available from several sources in the region. Data from the USGS, state agencies, and the project partners will be collected and interpreted to access areas with little or no data.

C -- Compile existing groundwater-level data: There are several sources of groundwater-level data in the study area. Existing reports, the USGS database, state partners, and data from local entities will be the primary sources of historic groundwater-level data collected in the area.

D -- Refine the existing regional hydrogeologic framework to incorporate local-scale data: The current hydrogeologic framework for the proposed study area will be refined, where needed, using currently available stratigraphic and hydrogeologic data. The refined framework would use existing wells that have the required data available (geophysical logs, cores, and drill cuttings descriptions).

E -- Groundwater model development: The existing groundwater-flow model of the North Carolina–South Carolina Atlantic Coastal Plain (Campbell and Coes, 2010) will be updated using MODFLOW-2005 (Harbaugh, 2005) by decreasing the grid spacing in the Aiken County area from the original model grid-spacing of 2 x 2 miles to approximately 300 x 300 ft. A commercial graphical user interface will be used to enhance pre- and post-processing tasks, as well as to allow for visualization, ease of use, and updating. This modeling effort will include all of the Atlantic Coastal Plain aquifers in the study area. Boundary conditions will be defined to best simulate natural groundwater flow and surface-water –groundwater interaction conditions within the aquifers.

F -- Model calibration to predevelopment and 2015 conditions: Test initial model parameters and model sensitivity, and perform a steady-state calibration to ground-water measurements and any appropriate surface-water flow observations. In subsequent applications, the steady-state model will be used to refine a transient-model calibration. The transient-model will simulate groundwater conditions over time (1900–2015) that generally result from groundwater withdrawals. Calibration criteria will be determined based on the degree of accuracy of the measured data.

Task 2: Calculate the Water Budget for the Aiken County Area In general, a water budget is an assessment of water inputs versus water outputs. Such an assessment typically includes measurement of inputs such as precipitation, artificial recharge, and increased infiltration due to stormwater management improvements, and measurements of outputs, such as pumpage, discharge to streams, evaporation, and transpiration. There are water-use data available for the Aiken County area that have previously been gathered. This modeling effort will compile recent (2005-present) monthly water-use data from the project partners and state agencies to account for any seasonal variations.

Task 3: Document the general water-quality characteristics for public-supply wells across Aiken County
Groundwater from public-supply wells across Aiken County will be sampled at the well head for measurements of temperature, dissolved oxygen, dissolved iron, specific conductance, turbidity, and pH. These measurements will be made using a digital multi-parameter sonde. The concentrations of iron will be mapped to document the “high iron belt” that exists for groundwater in some parts of the County.

Task 4:  Assess the occurrence of chemical contamination in selected public-supply wells.

A --Assess the occurrence of chemical contamination in wells owned by Montmorenci-Couchton.  Groundwater samples from impacted wells will be sampled for volatile organic compounds (VOCs) to help determine the source of the contaminants. Groundwater samples will also be collected for age-dating analyses to assist in the calibration of the groundwater flow model.

B -- Assess the occurrence of potential contamination in wells owned by Breezy Hill. Groundwater samples from selected wells will be sampled for VOCs and age dates to assist in the calibration of the groundwater flow model.

C -- Assess the occurrence of high radium concentrations in wells owned by Gilbert-Summit. Groundwater samples from selected wells will be sampled for total radium concentrations and age dates to assist understanding the source of the radium and to help calibrate the groundwater flow model.

D -- Assess the occurrence of contamination in the selected public-supply wells mentioned in A-C. The recharge area, or capture zone, that supplies groundwater to these public-supply wells will be determined. The size of the simulated capture zones will be calibrated to the age dates by using the particle-tracking module in the groundwater-flow model MODFLOW.

BENEFITS

Benefits of this project to partners include:
• Availability of a comprehensive groundwater-resource data set for Aiken County.
• Scientifically-based groundwater flow model developed and documented by the USGS.
• Assessment of the water quality of a subset of public-supply wells in Aiken County and of potential sources of groundwater contamination.

PUBLICATIONS

The planned products for this project:
• All model datasets and model computer files (provided upon the completion of the project).
• Project findings will be published in a USGS Scientific Investigations Report.

REFERENCES

  • Campbell, B.G., and Coes, A.L., eds., 2010, Groundwater availability in the Atlantic Coastal Plain of North and South Carolina: U.S. Geological Survey Professional Paper 1773, 241 p., 7 pls.
  • Fry, J., Xian, G., Jin, S., Dewitz, J., Homer, C., Yang, L., Barnes, C., Herold, N., and Wickham, J., 2011. Completion of the 2006 National Land Cover Database for the Conterminous United States, PE&RS, Vol. 77(9):858-864.
  • Gellici, J.A., 2007, Hydrostratigraphy of the AIK-2448 and AIK-2449 core holes in the Breezy Hill area of Aiken County, South Carolina: South Carolina Department of Natural Resources, Land, Water and Conservation Division Water Resources Report 43, 29p.
  • Harbaugh, A.W., 2005, MODFLOW-2005, The U.S. Geological Survey modular ground-water model—the ground-water flow process: U.S. Geological Survey Techniques and Methods 6-A16, variously paged.
  • Hockensmith, B.L., 2012a, Potentiometric surface of the Middendorf aquifer in South Carolina, November 2009: South Carolina Department of Natural Resources Report 51, 10p. 1 pl.
  • Hockensmith, B.L., 2012b, Potentiometric surface of the Black Creek aquifer in South Carolina, November 2009: South Carolina Department of Natural Resources Report 52, 10p. 1 pl.
  • Mizzell, H., Malsick, M., and Abramyan,  I., 2014, South Carolina’s climate report card: understanding South Carolina’s climate trends and variability: Journal of South Carolina Water Resources, Vol. 1, no. 1, p. 4-9.
  • U.S. Census Bureau, 2012, Population Estimates: http://www.census.gov/popest/metro/metro.html accessed on December 15, 2012