MOUNTAIN-BLOCK RECHARGE FROM ALPINE KARST

Mountain-block recharge (MBR) is an important component in the hydrologic budget of valley-fill aquifers, yet is inherently difficult to estimate. Karst solution enhancement of carbonate mountain blocks further complicates flow dynamics and precludes traditional modeling endeavors. The karstified Bear River Range in northern Utah contains the headwaters for the Logan River, a documented losing stream. Incremental downstream flow measurements quantify losses to groundwater and provide a minimum estimate for MBR to Cache Valley. Preliminary correlation and integration of losses with in-channel discharge yielded a volume of nearly 1878 acre-feet that could annually flow along the axis of the Logan Peak Syncline to recharge the adjacent valley-fill aquifer. While a volume this small suggests that mountain-block recharge is not the primary source of rejuvenation for the valley-fill aquifer, it does support inferences from an earlier MODFLOW numerical model that permitted groundwater to cross the mountain-front fault. Master recession curve analysis of the Logan River suggests that the saturated zone of the alpine karst aquifer quickly transmits recharge, which may explain the hydraulic response of stimulated MBR in the numerical model. The existence and possible magnitude of mountain-block recharge from alpine karst bears implications for estimating valley-fill aquifer hydrologic budgets and predicting effects of drought on groundwater resources.