Simulation of Groundwater-Level and Salinity Changes in the Eastern Shore, Virginia

Abstract

Groundwater-level and salinity changes have been simulated with a groundwater model developed and calibrated for the Eastern Shore of Virginia. The Eastern Shore is the southern part of the Delmarva Peninsula that is occupied by Accomack and Northampton Counties in Virginia. Groundwater is the sole source of freshwater to the Eastern Shore, and demands for water have been increasing from domestic, industrial, agricultural, and public-supply sectors of the economy. Thus, it is important that the groundwater supply be protected from overextraction and seawater intrusion. The best way for water managers to use all of the information available is usually to compile this information into a numerical model that can simulate the response of the system to current and future stresses. A detailed description of the geology, hydrogeology, and historical groundwater extractions was compiled and entered into the numerical model. The hydrogeologic framework is composed of a surficial aquifer under unconfined conditions, a set of three aquifers and associated overlying confining units under confined conditions (the upper, middle, and lower Yorktown-Eastover Formation), and an underlying confining unit (the St. Marys Formation). An estimate of the location and depths of two major paleochannels was also included in the framework of the model. Total withdrawals from industrial, commercial, public-supply, and some agricultural wells were compiled from the period 1900 through 2003. Reported pumpage from these sources increased dramatically during the 1960s and 70s, up to currently about 4 million gallons per day. Domestic withdrawals were estimated on the basis of population census districts and were assigned spatially to the model on the assumption that domestic users are located close to roads. A numerical model was created using the U.S. Geological Survey (USGS) code SEAWAT to simulate both water levels and concentrations of chloride (representing salinity). The model was calibrated using 605 predevelopment and transient water-level observations that are associated predominantly with 20 observation nests of wells sited across the study area. Sampling for groundwater chemistry at these sites revealed that chloride has not increased significantly in the last 20 years. Environmental tracers in the samples also indicated that the water in the surficial aquifer is typically years to decades old, whereas water in the confined aquifers is typically centuries to millennia old. The calibration procedure yielded distributions of hydraulic conductivity and storage coefficients of the aquifers and confining units that are based on 21 pilot points, but vary smoothly across the study area. The estimated values are consistent with other measurements of these properties measured previously on cores and during hydraulic tests at various well fields. Simulations performed with the model demonstrated that the calibrated model can reproduce the observed historical water levels fairly well (R2 = 0.93). The chloride concentrations were also simulated, but a match with chloride concentrations was more difficult to achieve (R2 = 0.16) because of the lack of sufficient data and the unknown exact behavior of the entire transition zone in the millennia leading up to the present day. Future pumping scenarios were simulated through 2050, with pumping set to either 2003 rates or total permitted withdrawal rates. Water levels in 2050 are predicted to be lower than current levels by a few feet where stresses are currently heaviest but potentially by tens of feet if total permitted withdrawals are extracted at current low-stressed sites. Simulations of chloride concentrations through 2050 revealed some potential for seawater intrusion in the areas of Cape Charles, Chincoteague, east of the town of Exmore, and east of the town of Accomac, but precise estimates of concentration increases are highly uncertain. Simulation results were also used to estimate that the down