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

Keywords: Climate impacts Groundwater hydrology Infiltration Water supply Isotopes s u m m a r y High 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. The western United States continues to rely heavily on groundwater to support population growth and agriculture making it critically important to understand how groundwater recharge will be impacted by predicted climate change (Green et al., 2011). Overall, regional groundwater recharge may increase or decrease as a result of higher predicted temperatures (Earman and Dettinger, 2007). However, groundwater recharge has been shown to be vulnerable to the effects of climate, especially in arid and semiarid regions Alpine and subalpine groundwater basins in California may be particularly affected since they receive most of their groundwater recharge from seasonal snowpack melting. Even modest increases in temperature predicted by climate change have the potential to cause a greater proportion of precipitation in Califor-nia to occur as rain, decrease the amount of snowpack in the Sierra Nevada, and shift the snowmelt hydrograph to an earlier and …