Seasonal variation of high elevation groundwater recharge as indicator of climate response

•Aquifer recharge is occurring primarily in the lower elevations of Martis Valley.•Wells exhibit seasonal variations in recharge conditions and groundwater ages.•Wells with high seasonal variability are most vulnerable to the effects of climate change.

SummaryHigh elevation groundwater basins in the western United States are facing changes in the amount and timing of snowmelt due to climate change. The objective of this study is to examine seasonal variability in a high elevation aquifer (Martis Valley Watershed near Truckee, CA) by analyzing (1) tritium and helium isotopes to determine groundwater sources and age, (2) dissolved noble gases to determine recharge temperatures and excess air concentrations. Recharge temperatures calculated at pressures corresponding to well head elevations are similar to mean annual air temperatures at lower elevations of the watershed, suggesting that most recharge is occurring at these elevations, after equilibrating in the vadose zone. The groundwater flow depth required to increase the water temperature from the recharge temperature to the discharge temperature was calculated for each well assuming a typical geothermal gradient. Groundwater samples contain large amounts of excess helium from terrigenic sources, including mantle helium and radiogenic helium. Terrigenic helium and tritium concentrations are used to determine the amount of mixing between the younger and older groundwater sources. Many of the wells sampled show a mix of groundwater ages ranging from >1000s of years old to groundwater with tritium concentrations that are in agreement with tritium in modern day precipitation. Higher seasonal variability found in wells with younger groundwater and shallower flow depths indicates that the recent recharge most vulnerable to climate impacts helps to supplement the older, less sustainable waters in the aquifer during periods of increased production.