Resolving aquifer behavior near a focused recharge feature based upon synoptic wellfield hydrogeochemical tracer results

Abstract

Aquitard windows are known to occur in unconsolidated aquifer systems and represent areas of focused recharge to otherwise confined underlying aquifers. The study herein focused on a known window site affecting the Memphis aquifer located in the Sheahan wellfield in Shelby County, Tennessee. Uranium- and thorium-series radioisotopes were evaluated from production wells sampled in the wellfield and compared to other geochemical characteristics of the system. From the research, a conceptual model was realized whereby higher uranium concentrations from near-surface waters flow through a redox barrier in the aquitard window and become depleted in uranium. Further, 234U/ 238U activity ratio increased downgradient of the recharge source by alpha-recoil mobilization of 234U via 234Th, and possibly leaching of uranium bound by iron hydroxides via dissolution and subsequent precipitation by reducing waters flowing to downgradient wells. The behavior of the hydrogeochemical tracers could be best described in the conceptual model by normalizing uranium to Fe 2+ throughout the system. The use of the [U]–[Fe 2+] pair provides a useful tool for understanding of the mixing behavior near focused recharge sites with redox barriers and may be useful for developing aquifer behavior models.