Simulation of Groundwater Flow in the Charleston Aquifer near Mount Pleasant, SC – an Update

Science Center Objects

The objective of the investigation is to use an existing, updated groundwater-flow model of the South Carolina Coastal Plain aquifers and confining units in the Mount Pleasant area created by Petkewich and Campbell to simulate current groundwater conditions and to include current planning goals developed by the Mount Pleasant Waterworks. The scope of the groundwater simulations will include the Coastal Plain aquifers underlying the Mount Pleasant area. 

Problem Statement:

Map of Mt. Pleasant Waterworks wells in the Charleston aquifer
Figure 1. Map of Mt. Pleasant Waterworks wells in the Charleston aquifer

 Mount Pleasant, South Carolina (SC) is located in the metropolitan Charleston, SC area (fig. 1), and has grown between 1960 and 2015 from a small town of about 7,000 persons to a city of about 78,000. This growth has increased demand on groundwater pumped from the Middendorf aquifer (currently called the Charleston aquifer, Campbell and Coes (2010)), about 1,800 to 2,000 feet (ft) below land surface, in the Mount Pleasant area. Mount Pleasant Waterworks (MPW), the town’s independent public works agency, has produced potable water from the Charleston aquifer since 1969 and operates 6 wells that produce groundwater from the Charleston aquifer. This groundwater has high dissolved solids and is treated at 4 reverse osmosis (RO) plants. The total capacity of groundwater that can be pumped and treated is about 7 million gallons per day (Mgal/D) (J. Ouellet, MPW, 2012, written commun.). In fiscal year 2012, the MPW distributed an average of 8 MGAL/D, where groundwater provided about 32 percent (2.2 Mgal/D) and treated surface water from the Charleston Water System provided the balance (5.8 Mgal/D). 

Groundwater has been pumped from the Charleston aquifer by the City of Charleston, MPW and other users since development started in the late 1800s. There are presently a number of users of Charleston aquifer groundwater including MPW, Kiawah Island, private industry, Sullivan’s Island, and Isle of Palms. During this time, pumpage primarily by MPW and Kiawah Island to meet irrigation needs, past use by the Town of Summerville, and private industry has resulted in a deep, regional cone of depression in the potentiometric surface of the Charleston (fig. 2; Wachob, 2015).

 

Potentiometric map of Charleston, South Carolina
Figure 2. Potentiometric map of Charleston, South Carolina

Groundwater levels have declined from about 126 feet (ft) above land surface in downtown Charleston prior to pumpage (Aucott and Speiran, 1985) to current (2015) groundwater levels of approximately 40 ft below land surface as measured in observation well CHN-14 and about 60 ft below land surface at BRK-431 both of which are open to the Charleston aquifer near Mount Pleasant (figs. 2 [inset] and 3).

A consequence of these regional declines in groundwater levels has been that Charleston, Berkeley, and Dorchester Counties have been designated by the South Carolina Department of Health and Environmental Control (SCDHEC) as a Capacity Use Area (CUA) (South Carolina Department of Health and Environmental Control, 2001). In CUAs, groundwater withdrawals in excess of 3 million gallons per month must be permitted by and reported to the SCDHEC. A concern faced by current groundwater users in a CUA is that future regulations could be imposed to limit further increases in groundwater pumped from the Charleston aquifer.

This scenario of regional cone of depression, competing demands for groundwater within a CUA, and current and potential future increased demands on groundwater creates a concern for the MPW regarding the sustainability of groundwater pumped from the Charleston aquifer. As of 2015, pumping levels in the 6 MPW wells have been as low as approximately 400 ft below land surface and the decreasing groundwater levels result in pumps being lowered and higher costs for electricity incurred to lift water from these progressively deeper depths.

Graph of groundwater level at observation well CHN-14
Figure 3a. Graph of groundwater level at observation well CHN-14
Graph of groundwater level at observation well BRK-431
Figure 3b. Graph of groundwater level at observation well BRK-431

 

 

 

 

 

 

 

 

 

 

 

 

Objective and Scope: 

The objective of the proposed investigation is to use an existing, updated groundwater-flow model of the South Carolina Coastal Plain aquifers and confining units in the Mount Pleasant area created by Petkewich and Campbell (2007) to simulate current groundwater conditions and to include current planning goals developed by MPW (C. Duffie, MPW, 2015, personal commun.). The scope of the groundwater simulations will include the Coastal Plain aquifers underlying the Mount Pleasant area. The simulation period of the model will be from about 2015 to 2045 in the future determined by discussions with the MPW. The results of these investigations will be documented in a peer-reviewed USGS Scientific Investigations Report.

 

Approach:

Task 1: Inclusion of Recent Groundwater Data into Model

Task 2: Update Model and Run New Simulations

     Scenarios:

      a. Maximize current reverse-osmosis plant capacity: Simulate current wellfield maximum pumping of 8.5 Mgal/d to 2045 with no variation in annual withdrawal rates;

      b. Maximize current well capacity: Increase withdrawal rates from 9.0 Mgal/d in 2015 to 10.5 Mgal/d for the period of 2020-2045. The increase from 9.0 to 10.5 Mgal/d will be applied in 20-percent increments each year from 2016 to 2020;

      c. Minimize Charleston Water System (Surface-Water Source) Purchases: Add 2 new Charleston aquifer wells in 2020 capable of pumping 1,100 gallons per minute. The increase in withdrawals from 9.0 to 10.5 Mgal/d will be applied in 20-percent increments each year from 2016 to 2020. And then from 10.5 to 12.5 Mgal/d from 2020 – 2025; also at 20-percent increments.

      d. Minimize Reverse-Osmosis / Well Usage: Seasonal variations in pumping rates based on historical MPW demand will be used. Total pumping will be incrementally increased from 6 Mgal/d in 2015 to 8.5 Mgal/d in 2045.

Task 3: Publication

       A USGS Scientific Investigations Report will be published to document the revisions to the groundwater-flow model.

Products

The primary products of this investigation include:

  1. All model datasets and model computer files (provided upon the completion of the project)
  2. Project findings published in a USGS Scientific Investigations Report