Integrating environmental tracers and groundwater flow modeling to investigate groundwater sustainability, Gibsons, BC

Environmental tracers can provide information on groundwater age, recharge conditions and flow processes. This information is useful for evaluating groundwater sustainability and vulnerability by identifying groundwater provenance and information for water budgets. Gibsons, British Columbia is a growing coastal community relying on groundwater to supply drinking water to two thirds of its 4,300 residents. The Town of Gibsons is proud of its untreated groundwater resource and proactive about keeping it protected and sustainable for future generations. Samples of noble gases, tritium, and stable isotopes of oxygen and hydrogen were collected from the aquifer. Tracer results improved the site conceptual model by identifying a previously unknown contribution of mountain block recharge (MBR) and by providing recharge elevation estimates using noble gas thermometry. The updated conceptual model including the mountain block was integrated into a regional three-dimensional numerical groundwater flow model calibrated to both hydraulic heads and to recharge elevation, a non-traditional approach to model calibration. This is the first study to use recharge elevation as a calibration target, which proved to be imperative for constraining bedrock geometry and minimizing model non-uniqueness. Tracer and modeling results indicate that groundwater in the Gibsons aquifer contains a mixture of approximately 45% MBR and 55% bench recharge. The MBR component is pre-modern (> 50 years) groundwater that recharged at elevation and cold temperatures (~5°C) and has evolved hydrogeochemistry and high concentrations of excess air (EA; >0.005 ccSTP/g) and ⁴Heterr (>10-⁹ ccSTP/g). Bench recharge is modern (< 10 years) groundwater recharged at low elevations and warm temperatures (~9°C), and has non-evolved hydrogeochemistry and low concentrations of EA (0.001-0.003 ccSTP/g) and ⁴Heterr (