Case Study of the Sheahan Wellfield Using H3 ∕ He3 Field Data to Determine Localized Leakage Areas

Abstract

Source water protection zones are not easily determined for semiconfined aquifers with highly localized areas of leakage from overlying shallow aquifers. This research is a companion case study to a theoretical study detailing the inverse modeling of aquifer mixing zones using age-distribution models. Where the first study detailed the capabilities of inverse age-distribution modeling given various sets of environmental tracer data and prior information, this study demonstrates the successful application of the technique to a real-world problem with a robust conceptual model verified in the current literature. The case study presented here considers a hydrogeologic setting in the northern Mississippi Embayment where highly localized leakage features exist between a shallow alluvial aquifer and the Memphis aquifer. Geochemical analyses, environmental, and radiochemical tracers have been used to develop and verify a conceptual model of the flow system at the Sheahan Wellfield in Memphis, Tenn. This study used inverse age-distribution modeling of tritium and helium-3 at multiple wellheads to determine the most highly probable location of a near field leakage source that is impacting wellheads. The method was used to identify the most likely location of the leakage site, and to identify an area at most risk for wellhead management considerations.