AQUIA AQUIFER DISSOLVED CL- AND CL-36 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. T hose changes have been influenced by the rise and fall of sea level, w hich has altered the distance of the recharge region from the coastlin e by similar to 200 km. The Cl-36 concentration along the flow path is not as smooth as the Cl- profile. Historic variations in cosmogenic p roduction, atmospheric transport, precipitation, and evapotranspiratio n all might have influenced Cl-36 concentrations. A general similarity between the Cl-36 and Cl- profiles suggests that changes in precipita tion and evapotranspiration rates, which influence both tracers simila rly, are particularly important. To reconcile C-14, Cl-36, and hydrolo gic 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 th an modern, prepumping rates: At greater depths, flow rates were much S lower and less variable; water in this region may be old enough to rec ord some Cl-36 decay.