ROLE OF OLD-AGED GROUNDWATER IN A MOUNTAIN WATERSHED - APPLICATION OF ENVIRONMENTAL AGE TRACERS AND AN INTEGRATED HYDROLOGIC MODEL (Invited Presentation)

Headwater mountain systems provide an outsized proportion of the water resources for the Western United States. Regional aquifers can be supported by mountain front recharge and direct recharge from the mountain block; however, it remains uncertain how much groundwater is stored in headwater systems, the timescales over which groundwater moves through bedrock, its role in streamflow generation, and how it may be impacted by climate change. In this work we use observations of environmental tracers (³H and ⁴He) and high-performance, integrated hydrologic and particle tracking models to quantify groundwater recharge and stream discharge flux dynamics and age distributions in the East River Watershed, Colorado (USA). ³H and ⁴He observations from 10 wells completed in excess of 10 meters below land surface (mbls) in fractured Mancos shale reveal groundwater age distributions are generally characterized as a mixture of modern (<70 years) and pre-modern (>70 years) components. Modern water mixing fractions at all wells suggests that modern groundwater recharge circulates to considerable depths of at least ~30 mbls. The integrated hydrologic model ParFlow-CLM and Lagrangian particle tracking model EcoSLIM were used to simulate transient and spatially distributed groundwater age dynamics over the last 7-years. Groundwater discharge age distributions indicate an old-age fraction (>3 years) consistently supports streamflow, despite this period having large variation in annual snowpack. These model results suggest the release of old-aged groundwater is crucial to buffer streamflow, especially in the presence of low-snow years. This buffering of streamflow with groundwater is further illustrated by the model-based inverse relationship between annual streamflow efficiency (defined as the ratio of annual streamflow to annual precipitation) and groundwater storage efficiency (defined as the ratio of annual change in storage to annual precipitation). Importantly, both simulated and observed groundwater level timeseries show declines over the past 7 years, highlighting a potential unsustainable loss of groundwater storage with unknown impacts on future streamflow. Further understanding the processes dictating groundwater recharge, discharge, and storage changes in high-elevation mountains is crucial to manage downstream water resources.