CL-36: a tracer for ground water in the Aquia formation of southern Maryland

Chlorine - 36 measurements using accelerator mass spectrometry can be routinely made on 1-5 mg environmental samples with 36ClCl isotopic ratios down to 10-16. Chlorine is an ideal tracer for hydrological systems due to its geochemistry. The Aquia Formation (Eocene-Paleocene), predominantly made of sands but characterized by 20-40% glauconite, represents one of the many productive, heavily pumped and well characterized aquifers of the Southeastern Coastal Plain. The 36Cl (t12 = 3.01 x 105y) values, measured in ground water taken at regular intervals along the flow path, show an unexpected buildup and then decrease with age. Cyclic fluctuations of chloride concentrations correlate with a 36ClCl pattern characteristic of sea-level shifts, and thus provide time information for the ground water within the aquifer. However, additional effects are observed which may be the result of selective ion filtration causing a buildup of 36Cl concentrations over time. An unusually high activity (∼ 15X modern water) observed in an outcrop sample is interpreted as a result of the bomb pulse and has important implications for recharge rates