2011 Ground Water Summit and 2011 Ground Water Protection Council Spring Meeting

Forecasting Salt-Water Intrustion In the Eastern Shore of Virginia Using a Three-Dimensional Numerical Model

Monday, May 2, 2011: 10:45 a.m.
Constellation C (Hyatt Regency Baltimore on the Inner Harbor)
Ward E. Sanford*, U.S. Geological Survey;
Jason P. Pope, U. S. Geological Survey;

A three-dimensional groundwater model of the Eastern Shore of Virginia (the southernmost Delmarva Peninsula) was constructed using the USGS code SEAWAT to help forecast salt water intrusion.  The model simulates advective-dispersive mixing in the transition zone and is currently being used by state and county water resource planners to help manage the fresh water resource.  The model represents explicitly the water-table aquifer, three underlying confined aquifers and four confining layers by discretization of the system into 42 model layers.  The areal resolution of the model is between 1000 and 5000 ft.  Sea water is present in the system beneath the Atlantic Ocean and coastal bays, and brackish water is present beneath the Chesapeake Bay and bayside marshes.  Salt water is encroaching locally along both the ocean and bay sides of the peninsula.  The model was calibrated to match 605 water levels at 20 nests of observation wells, and to 17 dissolved chloride concentrations from wells in the salt-water transition zones.  The distribution of hydraulic conductivities (K) of the aquifers and confining layers were estimated using automated calibration, pilot points, and kriged fields of K.

One challenging aspect of the model creation was by the lack of adequate data to accurately and fully define the spatial distribution of the current or predevelopment transition zone.  The simulated steady-state equilibrium transition zone was not a good match to many current chloride observations.  An approach was developed that created an initial transition zone that respected both chloride concentrations and near-equilibrium flow and transport conditions.  Future scenarios simulated to 2050 indicate increases in chloride concentrations of up to 1,000 mg/L at a number of locally stressed sites.  Specific predictions of the exact levels and timing of chloride increases, however, remain highly uncertain because of the paucity of data at the local sites of intrusion.