Geostatistical Analysis of Tritium, 3H/3He Age and Noble Gas Derived Parameters in California Groundwater

Tuesday, 16 December 2014: 9:45 AM
Ate Visser1, Michael J Singleton1, Jean E Moran2, Miranda S Fram3, Justin T Kulongoski4 and Bradley K Esser1, (1)Lawrence Livermore National Laboratory, Livermore, CA, United States, (2)California State University East Bay, Hayward, CA, United States, (3)USGS California Water Science Center Sacramento, Sacramento, CA, United States, (4)USGS California Water Science Center San Diego, San Diego, CA, United States
Key characteristics of California groundwater systems related to aquifer vulnerability, sustainability, recharge locations and mechanisms, and anthropogenic impact on recharge, are revealed in a spatial geostatistical analysis of the data set of tritium, dissolved noble gas and helium isotope analyses collected for the California State Water Resources Control Board's Groundwater Ambient Monitoring and Assessment (GAMA) and California Aquifer Susceptibility (CAS) programs.

Over 4,000 tritium and noble gas analyses are available from wells across California. 25% of the analyzed samples contained less than 1 pCi/L indicating recharge occurred before 1950. The correlation length of tritium concentration is 120 km.

Nearly 50% of the wells show a significant component of terrigenic helium. Over 50% of these samples show a terrigenic helium isotope ratio (Rter) that is significantly higher than the radiogenic helium isotope ratio (Rrad = 2×10-8). Rter values of more than three times the atmospheric isotope ratio (Ra = 1.384×10-6) are associated with known faults and volcanic provinces in Northern California. In the Central Valley, Rter varies from radiogenic to 2.25 Ra, complicating 3H/3He dating. The Rter was mapped by kriging, showing a correlation length of less than 50 km. The local predicted Rter was used to separate tritiogenic from atmospheric and terrigenic 3He.

Regional groundwater recharge areas, indicated by young groundwater ages, are located in the southern Santa Clara Basin and in the upper LA basin and in the eastern San Joaquin Valley and along unlined canals carrying Colorado River water. Recharge in California is dominated by agricultural return flows, river recharge and managed aquifer recharge rather than precipitation excess. Combined application of noble gases and other groundwater tracers reveal the impact of engineered groundwater recharge and prove invaluable for the study of complex groundwater systems.

This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-658313