Constraining the Depth and Temporal Distribution of Subsurface Flow and Transport Along a Hillslope Transect

Subsurface flow of water in mountainous watersheds influences rates of river discharge and water quality, elemental cycling, and rock weathering. While subsurface water and solute fluxes are recognized to be important, little information is available on their spatial and temporal distributions. In order to improve understanding of subsurface distributions of fluxes, we are conducting studies along a lower montane hillslope that drains into the East River, a tributary to the Colorado River (Colorado, USA). At four stations along a 190 m transect, 10 m deep boreholes were drilled to obtain samples of soil, and underlying weathered and fractured Mancos Shale. Vertical profiles of hydraulic potentials and pore waters are being obtained to determine directions of fluxes and pore water chemistry. Lacking evidence for significant overland flow, the approach taken for this analysis estimates net recharge as the difference between precipitation and estimated evapotranspiration. Based on net recharge along the transect and subsurface influxes from upslope terrain, combined with measured hydraulic potentials and hydraulic conductivities, we estimated the hillslope's depth- and time-resolved contributions of flow and transport to the river. This analysis indicates that the transmissive fractured shale zone extends substantially deeper than 10 m below the surface. The measurements and calculations conducted for a sequence of substantially above-average and below-average snowpack years (water years 2017 and 2018, respectively) show that snowmelt recharge within the transect generated pulses of downslope fluxes through the weathered shale zone that amounted to about a third of the peak flow, and that downslope fluxes through the soil were only significant water year 2017. Subsurface solute fluxes into the river are being estimated based on the geochemically distinct chemistries of pore waters in the soil and weathered shale zone relative to the fractured shale.