Aquia aquifer dissolved Cl^- and ³⁶Cl/Cl: Implications for flow velocities

The Aquia 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 historic changes in the deposition of Cl^- in this region. Those changes have been influenced by the rise and fall of sea level, which has altered the distance of the recharge region from the coastline by ~200 km. The ³⁶Cl concentration along the flow path is not as smooth as the Cl^- profile. Historic 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 (<60 m) 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.