Direct Observation of the Depth of Active Groundwater Circulation in an Alpine Watershed

Author(s): Manning, AH; Ball, LB; Wanty, RB; Williams, KH | Abstract: The depth of active groundwater circulation is a fundamental control on stream flows and chemistry in mountain watersheds, yet it remains challenging to characterize and is rarely well constrained. We collected hydraulic conductivity, hydraulic head, temperature, chemical, noble gas, and 3H/3He groundwater age data from discrete levels in two boreholes 46 and 81nm deep in an alpine watershed, in combination with chemical and age data from shallow groundwater discharge, to discern groundwater flow rates at different depths and directly observe active and inactive groundwater. Vertical head gradients are steep (average of 0.4) and thermal profiles are consistent with typical linear conductive continental geotherms. Groundwater deeper than ∼20nm is distinct from shallow groundwater and creek water in its chemistry, noble gas signature, and age (dominantly g65nyears compared to l9nyears). Together these results suggest low vertical groundwater flow velocities and a relatively shallow active circulation depth of ∼20nm. This hypothesis is tested with a simple 2-D numerical fluid flow and heat transport model representing a hillslope transect through the two boreholes. The modeling indicates that the subhorizontally bedded sedimentary rocks underlying the basin are highly anisotropic with low vertical hydraulic conductivity, and at most ∼10% of bedrock recharge (equivalent to l2% of stream baseflow) flows below a depth of 20nm. The study demonstrates the considerable value of discrete-depth hydrogeologic, chemical, and age data for determining active circulation depth, and illustrates an approach for maximizing the utility of individual boreholes drilled for mountain bedrock aquifer characterization.