DISSOLVED NOBLE GASES AND EXCESS AIR IN CALIFORNIA GROUNDWATERS

The composition and concentration of dissolved gases in groundwater are useful for understanding both groundwater flow (e.g. age dating, recharge temperature) and geochemical reactions (e.g. denitrification).  Lawrence Livermore National Laboratory, in collaboration with the US Geological Survey, has analyzed dissolved gases in more than 1200 water supply wells across California as part of the Groundwater Ambient Monitoring and Assessment (GAMA) program of the California State Water Resources Control Board.  Samples were analyzed for noble gases (He, Ne, Ar, Kr, Xe) as well as tritium and delta ¹⁸O.  A noteworthy aspect of the data set is the large range in excess air concentrations.  The majority of high excess air samples are from wells in coastal basins, while wells in Central Valley basins show much lower values.  For example, significant fractions of wells within Orange County and Santa Clara County have excess air concentrations of >15 cm³/kg (some as high as 40 cm³/kg) while both Sacramento and San Joaquin County have mean excess air <5 cm³/kg.  Many high excess air values in the heavily urbanized coastal basins are attributed to artificial recharge; however high values are also found in areas of Santa Clara County without major artificial recharge facilities.  Groundwater recharged prior to the 1950s (i.e. tritium <1 pCi/L) generally contains lower amounts of excess air relative to more recently recharged water.  The presence of 15 cm³/kg of excess air introduces ~50% more dissolved oxygen to groundwater than equilibrium solubility alone.  Increased dissolved oxygen can greatly impact geochemical reactions such as denitrification and oxidation of organic carbon during groundwater recharge and transport.  

    This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.