Inference of young groundwater ages and modern groundwater proportions using chlorofluorocarbon and tritium/helium-3 tracers from West Hawai‘i Island

Abstract

The geologically complex western aquifers of Hawai‘i Island serve as the primary reservoir of fresh potable water for residents, yet the area’s hydrogeologic characteristics are still not well understood. Groundwater age is a fundamental parameter used to better understand groundwater flow and storage, yet few studies have attempted to estimate apparent groundwater ages in Hawai‘i. This study implements a multi-tracer approach using chlorofluorocarbons, sulfur hexafluoride, and tritium/helium-3 to determine the apparent ages of young water masses from the West Hawai‘i aquifers. Chlorofluorocarbon and tritium/helium-3 tracer measurements suggest that the aquifers contain young groundwater with a median age of 53 years, but that a significant volume of older groundwater exists, which was recharged prior to the release of these anthropogenic tracers. Mixing models suggest that the groundwater consists of mixtures with an average 28% modern water (<70 years), with the remainder being recharged prior to 1953. Further, helium measurements demonstrate that the area’s groundwater is influenced by hydrothermal and magmatic processes from Hualālai volcano, which bias estimated groundwater ages if not properly considered. It is therefore beneficial to utilize a multi-tracer approach in an oceanic volcanic island setting, where groundwater ages are impacted by various external processes. The analysis of young apparent ages and proportions can aid in coastal aquifer characterization and subsequent groundwater quality assessment and is crucial for sustainable groundwater management.