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Aquia Aquifer Dissolved Cl − and 36 Cl/Cl: Implications for Flow Velocities
Author(s) -
Purdy Caroline Bascom,
Helz George R.,
Mignerey Alice C.,
Kubik Peter W.,
Elmore David,
Sharma Pankaj,
Hemmick Thomas
Publication year - 1996
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/96wr00356
Subject(s) - aquifer , evapotranspiration , precipitation , groundwater recharge , hydrology (agriculture) , geology , groundwater , environmental science , geography , ecology , meteorology , geotechnical engineering , biology
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 36 Cl 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 36 Cl concentrations. A general similarity between the 36 Cl and Cl − profiles suggests that changes in precipitation and evapotranspiration rates, which influence both tracers similarly, are particularly important. To reconcile 14 C, 36 Cl, 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 36 Cl decay.