Field estimates of groundwater circulation depths in two mountainous watersheds in the western US and the effect of deep circulation on solute concentrations in streamflow

Estimates of groundwater circulation depths based on field data are lacking. These data are critical to inform and refine hydrogeologic models of mountainous watersheds, and to quantify depth and time dependencies of weathering processes in watersheds. Here we test two competing hypotheses on the role of geology and geologic setting in deep groundwater circulation and the role of deep groundwater in the geochemical evolution of streams and springs. We test these hypotheses in two mountainous watersheds that have distinctly different geologic settings (one crystalline, metamorphic bedrock and the other volcanic bedrock). Estimated circulation depths for springs in both watersheds range from 0.6 to 1.6 km and may be as great as 2.5 km. These estimated groundwater circulation depths are much deeper than commonly modeled depths suggesting that we may be forcing groundwater flow paths too shallow in models. In addition, the spatial relationships of groundwater circulation depths are different between the two watersheds. Groundwater circulation depths in the crystalline bedrock watershed increase with decreasing elevation indicative of topography-driven groundwater flow. This relationship is not present in the volcanic bedrock watershed suggesting that both the source of fracturing (tectonic versus volcanic) and increased primary porosity in the volcanic bedrock play a role in deep groundwater circulation. The results from the crystalline bedrock watershed also indicate that relatively deep groundwater circulation can occur at local scales in headwater drainages less than 9.0 km(2) and at larger fractions than commonly perceived. Deep groundwater is a primary control on streamflow processes and solute concentrations in both watersheds. Plain Language Summary Groundwater is an important source of water for perennial streams and springs (perennial means that the streams and springs flow year-round). However, it's still unclear what pathways groundwater takes in the bedrock aquifer to reach these streams and springs. Some pathways are shallow and some are deep, we call this groundwater circulation. Estimates of circulation depths based on field data are lacking, so we have little information to inform, critique, or validate our hydrogeologic models of groundwater flow in watersheds. This is especially important in steep mountainous watersheds like the ones found in the western United States. The depth of circulation is important because it defines how long it will take for groundwater to reach a stream or spring, how it will change geochemically along the flowpaths, and how long it will take for landscape-scale perturbations such as droughts (reduced recharge) to propagate through the mountain aquifer and impact the perennial streams and springs. Our data reveal circulation depths exceeding 1 to 1.5 km suggesting that groundwater circulation in mountainous watersheds can be deeper than currently represented in models. This has important implications for understanding aquifer responses and response times to pertubations, and weathering fluxes from the landscape.