California GAMA Special Study: Ion exchange and trace element surface complexation reactions associated with applied recharge of low-TDS water in the San Joaquin Valley, California

Abstract

Executive SummaryUnder the Special Studies portion of the State Water Resources Control Board's Groundwater Ambient Monitoring and Assessment (GAMA) program, water quality topics of statewide relevance are addressed through focused research studies carried out in collaboration with Lawrence Livermore National Laboratory. The study described here addresses changes in groundwater quality associated with managed aquifer recharge of high quality water.Managed aquifer recharge, including the storage of potable water in accessible aquifers as an alternative to surface reservoir storage, is a valuable water resource management tool for water purveyors. Managed recharge usually entails the infiltration of water that is out of chemical equilibrium with local aquifer minerals, potentially resulting in mineral reactions, redox reactions, and trace element mobilization that can affect water quality and local aquifer permeability. Improvements to recharge water quality (e.g., remineralization of organic material, removal of organic compounds, denitrification) have been observed with recharge of agricultural return water and wastewater. In contrast, water quality changes associated with managed recharge of high quality surface water, e.g. Sierra Nevada runoff containing very low total dissolved solids (TDS), have received less attention.The objective of this study was to understand the geochemical interactions of high quality, imported recharge water with underlying aquifer materials associated with a four-year-old managed aquifer recharge project underway near Stockton, California, U.S.A. in California's San Joaquin Valley. The study employed advanced analytical methods to delineate mixing of ambient groundwater and recharge water in the subsurface, including both an introduced dissolved gas tracer and groundwater age dating (He method). This information, used in concert with extensive major ion, trace element, and isotopic data, allowed development of a geochemical model that provides a framework for interpreting the geochemical response of the local groundwater system to recharge of imported water.The study site, operated by Stockton East Water District (SEWD), consists of ponds adjacent to agricultural fields and a surface water treatment plant. Approximately 7000 acre-ft of imported water from New Melones and New Hogan reservoirs are recharged annually. Wells adjacent to the pond had tracer detections 6 to 11 days after the peak tracer detection in the pond, while nested monitoring wells downgradient had detections after 17 days. Production wells screened below semi-confining units had no tracer detections and had groundwater ages indicating that produced water recharged at least several decades ago.Overall groundwater quality in the area is very good, with <17 mg/L nitrate (as NO 3 -), total organic carbon < 1 mg/L, and a low frequency of detection of low level volatile organic compounds. Major element chemistry of recharged water is controlled by 1) mixing between low TDS recharge water and higher TDS ambient groundwat...

Geostatistical analysis of tritium, groundwater age and other noble gas derived parameters in California

“…The Priority Basin program applied well selection on an irregular grid, to provide a spatially unbiased data set. The Priority Basin data set was supplemented with localized studies of groundwater systems, collected by Lawrence Livermore National Laboratory (LLNL) and local water district staff, as part of the California Aquifer Susceptibility (CAS) program (Moran and Halliwell, 2002;Moran et al, 2002a;Eaton et al, 2003;Moran and Halliwell, 2003;Moran et al, 2004Moran et al, , 2002bMoran et al, , 2005aMoran et al, , 2005bMoran et al, , 2005cCarle et al, 2006) and GAMA Special Studies (McNab et al, 2007;Singleton et al, 2007;Moran et al, 2009;McNab et al, 2010;Moran et al, 2011;Visser et al, 2012).…”

California GAMA Special Study: Geostatistical analysis of groundwater age and other noble gas derived parameters in California

“…The Priority Basin program applied well selection on an irregular grid, to provide a spatially unbiased data set. The Priority Basin data set was supplemented with localized studies of groundwater systems, collected by LLNL and local water district staff, as part of the California Aquifer Susceptibility (CAS) program [Carle et al, 2006;Eaton et al, 2003;Moran et al, 2005a;Moran and Halliwell, 2002;Moran and Halliwell, 2003;Moran et al, 2002a;Moran et al, 2004;2005b;Moran et al, 2002b;Moran et al, 2005c] and GAMA Special Studies [McNab Jr et al, 2010;McNab et al, 2007;Moran et al, 2011;Moran et al, 2009;Michael J. Singleton and Moran, 2010;Michael J. Singleton et al, 2010;Visser et al, 2012].…”