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