Identifying groundwater recharge connections in the Moscow (USA) sub-basin using isotopic tracers and a soil moisture routing model

Abstract

Globally, aquifers are suffering from large abstractions\rresulting in groundwater level declines. These declines\rcan be caused by excessive abstraction for drinking water,\rirrigation purposes or industrial use. Basaltic aquifers also face\rthese conflicts. A large flood basalt area (1.1 × 105 km2\r) can\rbe found in the Northwest of the USA. This Columbia River\rBasalt Group (CRBG) consists of a thick series of basalt flows\rof Miocene age. The two major hydrogeological units\r(Wanapum and Grand Ronde formations) are widely used\rfor water abstraction. The mean decline over recent decades\rhas been 0.6 m year−1\r. At present day, abstraction wells are\rdrying up, and base flow of rivers is reduced. At the eastern\rpart of CRBG, the Moscow sub-basin on the Idaho/\rWashington State border can be found. Although a thick poorly\rpermeable clay layer exists on top of the basalt aquifer,\rgroundwater level dynamics suggest that groundwater recharge\roccurs at certain locations. A set of wells and springs\rhas been monitored bi-weekly for 9 months for δ18O and δ2\rH.\rLarge isotopic fluctuations and d-excess values close to the\rmeteoric water line in some wells are indicating that recharge\roccurs at the granite/basalt interface through lateral flow paths\rin and below the clay. A soil moisture routing (SMR) model\rshowed that most recharge occurs on the granitic mountains.\rThe basaltic aquifer receives recharge from these sedimentary\rzones around the granite/basalt interface. The identification of\rthese types of areas is of major importance for future\rmanaged-aquifer recharge solutions to solve problems of\rgroundwater depletion.