C.B. Purdy, G.R. Helz, A.C. Mignerey, P.W. Kubik, D. Elmore, P. Sharma, T. Hemmick

Water Resources Research 32 (1996) 1163-1171

The Aqua aquifer (southern Maryland) contains a remarkably smooth Cl^- profile (0.46-3.23 ppm) along its flow path. This is interpreted as a record of histoic changes in the deposition of Cl^- in this region. Those changes have been influenced by the coastline by ~200 km. The ³⁶Cl concentration along the flow path is not as smooth as the Cl^- profile. Histoic variations in cosmogenic production, atmospheric transport, precipitation, and evapotranspiration all might have influenced ³⁶Cl concentrations. A general similarity between the ³⁶Cl and Cl^- profiles suggests that changes in precipitation and evapotranspiration rates, which influence both tracers similarly, are particularly important. To reconcile ¹⁴C, ³⁶Cl, and hydrologic data, we propose a two-tier model for flow in the Aquia. Shallower portions of the aquifer (<60m) were subjected to hydraulic gradients and flow rates approximately 5 times larger during the Pleistocene than modern, prepumping rates. At greater depths, flow rates were much slower and less variable; water in this region may be old enough to record some ³⁶Cl decay.