Dating of shallow groundwater: Comparison of the transient tracers ³H/³He, chlorofluorocarbons, and ⁸⁵Kr

TLDR

The study directly compares apparent ages derived from 3H/3He, chlorofluorocarbons, and 85Kr in shallow groundwater. Apparent ages were measured in wells completed in unconfined surficial aquifers of the Delmarva Peninsula’s late Cenozoic Atlantic Coastal Plain sediments. Tracer ages agree within 2 years for recharge dates 1965–1990, with discrepancies attributable to hydrological mixing; recharge‑rate estimates from age gradients match previous values, and high recharge rates produce nearly complete dissolved‑gas confinement, indicating negligible mixing, dispersion, gas loss, and sorption‑desorption.

Abstract

This paper describes a direct comparison of apparent ages derived from 3 H/ 3 He, chlorofluorocarbons (CCl 3 F and CCl 2 F 2 ), and 85 Kr measurements in shallow groundwater. Wells chosen for this study are completed in the unconfined surficial aquifers in late Cenozoic Atlantic Coastal Plain sediments of the Delmarva Peninsula, on the east coast of the United States. Most of the apparent tracer ages agree within 2 years of each other for recharge dates between 1965 and 1990. Discrepancies in apparent tracer ages usually can be explained by hydrological processes such as mixing in a discharge area. Recharge rate calculations based on apparent tracer age gradients at multilevel well locations agree with previous recharge estimates. High recharge rates on the Delmarva Peninsula result in nearly complete dissolved‐gas confinement in the groundwater. The remarkable agreement between the different tracer ages indicates negligible mixing of waters of different ages, insignificant dispersion, minimal gas loss to the atmosphere, and insignificant sorption‐desorption processes at this location.