Groundwater Modeling in the South Carolina Coastal Plain Bruce Campbell, USGS – South Atlantic Water Science Center Alex Butler and Joe Gellici South Carolina Department of Natural Resources

What is a groundwater flow model?

Computer models of groundwater systems simulate the flow of groundwater, producing groundwater levels and budgets Framework on which to organize their knowledge and understanding of groundwater systems Groundwater models may be used to: predict the effects of hydrological changes -- groundwater withdrawals, irrigation developments, or climate change) on the behavior of the aquifer 2) used in various water management plans for urban and rural areas.

Representative GW Flow Model Primary inputs: - Model Grid - Hydrogeologic Framework - Aquifer Properties - Observation Data - Boundaries - Wells – Water Use Data Primary Outputs: - Groundwater Levels - Budgets Representative GW Flow Model

USGS Regional Aquifer System Analysis (RASA) 1980’s Georgia Bob Faye and Greg Mayer North Carolina Gerry Geise Ron Coble and Jodie Eimers South Carolina Walt Aucott and Gary Speiran

Mount Pleasant Waterworks – Groundwater Modeling Studies

Chesterfield County - Groundwater Modeling Studies

McBee, SC Area Alligator Rural Water - Well Field Simulated Groundwater Levels Particle Tracks Groundwater Age Dates

USGS Groundwater Resources Program – Started in 2004 GW Model of NC and SC Coastal Plain Calibrated to 2004 conditions Revised hydrogeologic framework Analysis of GW monitoring networks Climate change predictions

SC Coastal Plain Issues Increase in Atlantic Coastal Plain population Increased demand on groundwater resources Groundwater quality issues Increased agricultural withdrawals SC Agriculture Dept recruiting industrial-scale farms Impact of increased groundwater withdrawals on surface water flows

South Carolina Water Plan South Carolina Department of Natural Resources Recommendation: “A comprehensive ground-water flow model of the Coastal Plain should be developed and used to predict the effect of future pumping and to determine optimal well spacing's.”

A) South Carolina groundwater use, 1983-2013; B) Reported irrigation groundwater use, 1983 – 2013.

Irrigated Farm - Aiken County

Irrigated farmland in Barnwell County, SC

USGS Focus Area Study / SCDNR Model Update Combines funding sources: Internal USGS Water Census Program / Focus Area Study Cooperative funding from South Carolina Department of Natural Resources/USACOE 1) Water Census Program / Focus Area Study: Internal USGS funding from the SECURE Water Act Southeastern Atlantic Coastal Basins of the Carolinas 2) SCDNR/US ACOE Project Funding to expand groundwater model area to include all of the SC Coastal Plain

SCDNR / Focus Area Study Groundwater Model Area SCDNR / Focus Area Study Model Area

Project Objectives Update the groundwater flow model Activate and rediscretize the surficial aquifer model layer Adding recent groundwater-related data (2005-2015) Refine the model grid in some areas Incorporate a more detailed representation of the Fall Line area Incorporate new MODFLOW packages – Newton Formulation, Multi-Node Well Package, etc Variable-density offshore boundary Re-calibration, and apply the model to a series of 6 scenarios

Model Scenarios Six scenarios Will be determined by the State Cooperators Will involve: Additional potential future withdrawals Climate change Projected land use changes

Questions?

Aiken County Groundwater Availability Study

Preliminary Soil-Water Balance (SWB) Modeling

ET and Recharge (year 2010) Calculated by SWB

Aiken County Groundwater Resources Heavy reliance on groundwater Pumped primarily from Atlantic Coastal Plain sediments / aquifers Upper Coastal Plain – Fall Line Underlain by multiple aquifers Generally - very dilute water

Aiken County Groundwater Use - 2010 Total - about 14 Million Gallons per Day City of Aiken – about 7 MGD One of the largest users of groundwater in SC All public water utilities in county except one use primarily a groundwater source Irrigation use - increasing

Irrigation Water Use in Aiken County Recent siting of several “industrial” scale farms of 5,000+ acres Significant increase in water use – both surface and groundwater At least 2 more large scale farms are being sited Local concern over impact on water resources and existing users Have had lots of interest in the on-going project. County has requested state regulation of groundwater withdrawals

Approach Collect data and interpret Construct groundwater-flow model of Aiken County Determine the water budget Determine “Capture Zones” for existing and planned PSWs Determine age of groundwater to assess vulnerability and availability Comprehensive water-quality analyses (VOCs, radionuclides, iron, etc.)

Benefits to Aiken County Determine availability of groundwater for future use Coordinate planning among Aiken County groundwater users Provide SC Dept of Health and Environmental Control with data and information for potential Capacity Use designation A Management tool

Aiken County Groundwater Flow Model Numerical simulation of GW flow - MODFLOW Can use most of hydrological data See how data interacts Calibrate to past conditions Simulate future scenarios Can be used by all interested parties

Model Cells – 400 ft x 400 ft

Simulated Water Levels - Gordon aquifer

Simulated Water Levels - Crouch Branch aquifer

Breezy Hill Water and Sewer - Service Area

Breezy Hill Wellfield

Breezy Hill – Simulated Groundwater Levels

Model Scenarios Base Case: A “base case” simulation will be established that will comprise of pumping existing wells at their reported 2015 withdrawal rates out to year 2050. Base Case Plus 0.5-percent Annual Increase: The 2015 withdrawal rates will be increased by 0.5 percent per annum to 2050, for a cumulative increase of 8 percent. Drought: The effect of a 10-year drought will be simulated for the years 2020-2030 by reducing the annual average recharge rates by 5 to 10 percent and simulating the “base case” pumping rates. Agricultural Withdrawals Increase: Using Scenario (2), the effect of withdrawals by anticipated increases in agricultural pumping in the County will be simulated by adding 20 new irrigation wells in the study area.

Current Groundwater Modeling Projects – Focus Area Study / Model Update Aiken County