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.
ABSTRACT, The Aquia (Paleocene) and Magothy (Late Cretaceous) Formations of the Atlantic Coastal Plain represent two well-characterized (hydrodynamically and geochemically) aquifers in southern Maryland.14C measurements of the dissolved organic (DOC) and inorganic carbon (DIC) of Aquia and Magothy groundwaters have been made using accelerator mass spectrometry (AMS). Both DI14C and D014C concentrations in the initial flow path are unexpectedly low. As the water progresses farther from the recharge area, the D114C percent modern carbon (pMC) is consistently lower than the D014C pMC; this difference stays constant for all samples. The 14C-derived ages for an Aquia water sample downgradient at Site 4 are 17 ka and 12 ka for D114C and D014C, respectively. Radiocarbon ages have been compared to ages determined by two other independent dating methods: computer-simulated hydrodynamic modeling and age estimates based on changes in C1-,180 and 2H distributions, which are interpreted to be influenced by sea level and climate.
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