GROUND-WATER CHEMISTRY AND MODELING COME TOGETHER AT VIRGINIA'S EASTERN SHORE

Ground water accounts for nearly 100 percent of the water use on Virginia’s Eastern Shore, a peninsula between the Atlantic Ocean and the Chesapeake Bay, where surface water is virtually nonexistent. Industrial, public supply, domestic, and agricultural uses are stressing the aquifer system there and increasing the likelihood of salt-water intrusion. To help the state better manage this resource, the USGS is constructing a 3D model of the aquifer system on the peninsula using the variable-density transport model SEAWAT. In order to simulate a 3D salt-water transition zone, the model requires a fine grid resolution and a total of over 1.6 million nodes. Calibration of the model is being carried out using the parameter estimation program UCODE along with a variety of environmental tracers.

The environmental tracers being used to help calibrate the model include chloride, tritium, chlorofluorocarbons (CFCs), SF₆, ¹⁴C, and natural dissolved gases (helium, nitrogen and argon). Over 50 wells were sampled for these constituents in the fall of 2003. Chloride is being used to track the transition zone between fresh and salt water. Tritium, CFCs, and SF₆ are being used to date the young ground water in the water-table aquifer. ¹⁴C ages are being used to track ground water being recharged downward into the confined aquifer. Helium is being used to track ground water upwelling from beneath the aquifer system. Nitrogen and Argon are being used to estimate recharge temperatures.

The tracers are especially useful at revealing ages and flow patterns within the confined aquifer. Helium values increase toward the bottom and saline margins of the flow system. The values are consistent with helium concentrations in deeper units and slow upward advection through the underlying sediments. ¹⁴C values are affected by dissolution of carbonate material in the aquifer, but the resulting ages have been adjusted using ¹³C values and modeling of the total chemical evolution along flowpaths. The adjusted ages correlate well with recharge temperatures; the correlation reveals a temperature decrease during the last ice age to < 4 °C near 30ka. Combining a variety of tracer data with water level data has resulted in a more diverse set of observations, which will result in a more reliable model calibration, and in turn help the state better manage the resource.