Geodetic Constraints on Mountain Bedrock Aquifer Flow and Diffusivity

Groundwater flowing through the fractured bedrock composing most mountain ranges has been increasingly recognized as a vital source of freshwater for both low-elevation communities and mountain ecosystems, maintaining streamflow and constituting a large portion of recharge to lowland aquifers used to support human activities. Despite the growing awareness of groundwater’s role in mountain hydrology and the potential impacts of climate change on mountain groundwater, it remains a challenge to study the dynamics of mountain aquifers, largely due to the low density of observational wells and challenges in characterizing the mountain block over large areas and depths. Here, we report on a new approach to characterize the flow and hydraulic properties of mountainous aquifers at a mountain range scale. We utilize high-precision Global Navigation Satellite Systems (GNSS) observations of vertical crustal displacement produced by the redistribution of freshwater on or near the Earth’s surface to estimate changes in groundwater storage within the Sierra Nevada and Cascades Range of the western United States with high spatial (10s of kilometer) and temporal (daily) resolution over the past two decades. We find that on average groundwater annual recharge is less than discharge, driving long-term declines in groundwater storage over the last 19 years. Furthermore, we find groundwater recharge to be up to 3x more variable than groundwater discharge in these mountainous areas, suggesting that mountain aquifers release a relatively constant amount of water to streams and adjacent lowland aquifers despite fluctuating recharge conditions. Utilizing identified periods of groundwater discharge, we characterize the hydraulic conductivity, storativity, and flow path length of these groundwater systems using fluid diffusion models in combination with our GNSS-inferred groundwater estimates. Our initial estimates of these parameters reveal relatively high values of bedrock conductivity (~1x10^-3-1x10^-4 m/s) relative to expected values based upon each region’s bedrock lithology, suggesting that areas with highly fractured bedrock as well as saprolite may exert a strong control on groundwater discharge at the mountain range scale. Furthermore, our results indicate that groundwater flow paths can span lengths on the order of 100s-1000s of meters, supporting the notion that groundwater can flow over extended areas supporting recharge at both a local and regional scales. Our work seeks to provide a new set of tools for hydrologists to investigate these often poorly understood systems.