Dissolved gases and isotopes as tools for aquifer characterization in Martis Valley

The Martis Valley groundwater basin, near Lake Tahoe, is experiencing increasing water demand and likely changes in the amount and timing of snowmelt due to global warming. Groundwater is the exclusive water supply for drinking water in the town of Truckee and surrounding ski resorts and for golf course irrigation. The objective of this study is to provide insight into the age, source, and recharge conditions of the Martis Valley groundwater basin by sampling production wells for: 1) tritium and helium isotopes to determine groundwater sources and age, 2) dissolved noble gases to determine recharge temperatures and the amount of excess air and 3) stable isotopes to determine groundwater source. Sampling events took place during winter 2011-2012, June 2012, and September 2012 to assess seasonal variation in groundwater and potential vulnerability to climate change. Noble gas concentrations and the ratio 3He/4He were measured using cryogenic separation and static mass spectrometry at Lawrence Livermore National Laboratory. Tritium contents were determined by degassing and tritiogenic 3He accumulation, measured by static mass spectrometry. Water temperature was found to increase between snowmelt runoff and groundwater recharge as well as between groundwater recharge and discharge. Recharge temperatures were found to be similar to mean annual air temperatures at lower elevations of Martis Valley, suggesting that most recharge is occurring at lower elevations after equilibrating in the vadose zone. Mean integrated groundwater flow depth for each well was estimated by calculating the depth the water would need to reach in order to increase its recharge temperature to its discharge temperature using a standard geothermal gradient. Low levels of excess air found in groundwater suggest the bulk of recharge is occurring in the valley floor alluvium rather than through fractures in the mountain block. Many wells contained large amounts of excess helium from terrigenic sources, including mantle helium and radiogenic helium. The Polaris Fault was identified as a possible source of mantle helium. Mantle helium originating from the Polaris Fault can be used to trace groundwater flow directions and mixing of different groundwater sources. Terrigenic helium and tritium concentrations were used to determine the amount of mixing between the younger and older groundwater sources that were found in these long screened production wells. Recharge temperatures, amount of excess air, and tritium concentrations were found to vary between sampling events, especially in wells with younger groundwater (as indicated by higher tritium concentrations) and shallower flow depths (as indicated by small differences between recharge and discharge temperatures). These seasonal variations suggest that changes in the timing and amount of recharge under future warmer climate conditions will rather quickly impact at least a portion of the aquifer system in Martis Valley.