H138-0015
Quantifying river recharge rates based on tritium-helium groundwater ages
Monday, 14 December 2020
Poster
Ate Visser1, Laura Foglia2, Helen E Dahlke2, Amelia N Paukert Vankeuren3, Maribeth Kniffin4, Alisha Rodriguez4, Andrew Calderwood2, Amanda Deinhart5, Erik Oerter6, Kirsten Emilee Faulkner7, George Bennett7 and Jeff Dozier8, (1)Lawrence Livermore National Laboratory, Nuclear and Chemical Sciences Division, Livermore, CA, United States, (2)University of California Davis, Land, Air and Water Resources, Davis, CA, United States, (3)Lamont -Doherty Earth Observatory, Palisades, NY, United States, (4)University of California Davis, Dept. of Land, Air and Water Resources, Davis, CA, United States, (5)Lawrence Livermore National Laboratory, Livermore, CA, United States, (6)Lawrence Livermore National Laboratory, Physical and Life Sciences Directorate, Livermore, CA, United States, (7)USGS California Water Science Center Sacramento, Sacramento, CA, United States, (8)University of California, Mammoth Lakes, CA, United States
Abstract:
River water is an important source of groundwater recharge in the California Central Valley (USA). Groundwater resources alleviate long periods of drought and provide a seasonal buffer during the Mediterranean dry summer when future snow pack declines due to climate change. Quantifying recharge rates of river water, either natural river bank infiltration or managed aquifer recharge (MAR) during floods (Flood-MAR) or on agricultural land (Ag-MAR), is key to sustainable management of groundwater resources. Groundwater ages provide integrated estimates of river recharge rates over longer time periods and help us understand groundwater-surface water interactions.
We estimated river water recharge rates along a 15 km reach of the Cosumnes River (California) based on tritium-helium ages from two dozen monitoring, domestic, irrigation, and public supply wells. Stable isotopes mapped the extent of river water in the groundwater system and the direction of groundwater flow. Regional ambient groundwater is pre-modern (i.e. contains no detectable tritium) or fossil (i.e. contains terrigenic helium) indicating low recharge rates of recent local precipitation. We find Cosumnes water up to 20 years old in monitoring and irrigation wells up to 600 m north of the river. We also find evidence of recharge from Deer Creek, which runs parallel to the Cosumnes at 1.5 km. (The lower catchment elevation and consequently heavier isotopic signature allows us to discern Deer Creek from Cosumnes recharge.) South-east of the river, we find 30-40 year-old river water in wells at 1-3 km distance, mixed with local groundwater. Based on this preliminary data, we estimate a river recharge rate of 400-1,400 cubic meter per year per meter river reach. This corresponds to 1-5% of the 0.44 cubic kilometer average annual flow over the 15 km reach. These quantitative estimates of river water recharge will constrain the numerical groundwater flow model for this basin and aid groundwater managers in developing sustainability plans to balance groundwater pumping with recharge rates.
