Water resource implications of ¹⁸O and ²H distributions in a basalt aquifer system

Ongoing decline of water levels in the confined basalt aquifers of the Pullman-Moscow Basin of Washington and Idaho has prompted study of the timing, amount and distribution of recharge to the system. Previous radiocarbon ages indicate residence times on the order of 10³ years and greater and suggest a low rate of recharge to the lower basalt aquifer since the end of Pleistocene time. By contrast, more recent hydrodynamic flow modeling studies invoke a larger Holocene recharge rate through the unconfined loess unit to the upper and lower basalt aquifers, which implies relatively short residence times (10² years). Stable isotopes were used to independently assess contrasting recharge models by comparing ¹⁸O/¹⁶O and D/H ratios of late-Holocene shallow ground water and deep ground water. Linear regression of local precipitation ratios yields δD = 6.9 δ¹⁸O-18.5. There is no evidence of fractionation of ground water ratios by recharge processes or water-rock interactions. Deep basalt ground water δ¹⁸O values are depleted by 0.4 to 4.9 per mil relative to shallow, recently recharged ground waters and have δ¹⁸O values statistically distinct from waters sampled from other stratigraphic units. These findings suggest that the deep waters in the basin were not precipitated under current climate conditions and that aquifer recharge rates to the deep basalt aquifer are substantially lower than have been recently estimated. This in turn suggests that a sustainable ground water exploitation scheme must reduce reliance on the deep ground water resource.