Groundwater quality in the Northern San Joaquin Valley region of California was studied as part of California State Water Resources Control Board (SWRCB) Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project (GAMA-PBP). The GAMA-PBP made a spatially unbiased assessment of the aquifer system used for domestic drinking-water supply in the study region and compared the results to the aquifer system used for public drinking-water supply. These assessments characterized the quality of raw groundwater to evaluate ambient conditions in regional aquifers and not the quality of treated drinking water. The study included two components: (1) a status assessment presenting study results summarizing the status of groundwater quality used for domestic supply in the Northern San Joaquin Valley and (2) a comparative assessment of groundwater resources used for domestic and public drinking-water supply in the study region.

The status assessment was based on data collected by the GAMA-PBP from 45 sites in the Northern San Joaquin Valley domestic-supply aquifer assessment study unit during 2019. To contextualize water-quality results, concentrations of water-quality constituents in ambient groundwater were compared to regulatory and non-regulatory benchmarks used by the State of California and Federal agencies as health-based or aesthetic standards for public drinking water. A grid-based method to estimate aquifer-scale proportions of groundwater resources with concentrations approaching or exceeding benchmark thresholds was used in the status assessment. This method provides spatially unbiased results and allows inter-comparability with similar groundwater-quality assessments. A spatially weighted method was used to calculate aquifer-scale proportions for public-supply wells within the domestic assessment grid network using contemporaneous regulatory compliance monitoring data. Differences among aquifer-scale proportions for constituents exceeding regulatory and non-regulatory benchmarks in domestic- and public-supply aquifers were quantitatively evaluated. Factors influencing the vertical and lateral distribution of key contaminants of concern (nitrate, fumigants, and arsenic) across overlapping aquifer systems used for domestic and public drinking-water supply were also evaluated.

Status assessment results indicated inorganic and organic constituents with health-based benchmarks were present at high relative concentrations (RCs), meaning they exceeded a benchmark threshold, in 20 and 9 percent of the domestic-supply aquifer system in the Northern San Joaquin Valley, respectively. Inorganic constituents with health-based benchmarks present at high RCs included nitrate and arsenic. The only organic constituents with health-based benchmarks present at high RCs were the fumigants 1,2-dibromo-3-chloropropane (DBCP) and 1,2,3-trichloropropane (1,2,3-TCP). Inorganic constituents with aesthetic-based benchmarks were present at high RCs in 13 percent of the domestic-supply aquifer system in the Northern San Joaquin Valley and included iron and manganese. Microbial indicators (total coliform bacteria and Enterococci) were present in 18 and 2 percent of the domestic-supply aquifer system in the Northern San Joaquin Valley, respectively.

Comparative assessment results indicated inorganic and organic constituents with health-based benchmarks were present at high RCs in 13 and 6 percent of the public-supply aquifer system in the Northern San Joaquin Valley, respectively. Inorganic constituents with aesthetic-based benchmarks were present at high RCs in 22 percent of the public-supply aquifer system in the Northern San Joaquin Valley. There were no significant differences among high RC proportions for individual water-quality constituents, except for nitrate, which was greater in the domestic- compared to public-supply aquifer system in the Northern San Joaquin Valley. The most prevalent constituents with health-based benchmarks contributing to high RC proportions in the public-supply aquifer system were arsenic and fumigants, including DBCP and 1,2,3-TCP.

Analysis of construction data for wells included in the comparative assessment indicated that, although depth to top of perforations are comparable for domestic and public-supply wells in the Northern San Joaquin Valley (median depth about 60 meters [m]), public-supply wells have longer perforation intervals and extend to deeper parts of the aquifer system than domestic wells that typically draw exclusively from the shallower aquifer system in the upper 80 m of unconsolidated sediments. Analysis of the vertical and lateral distribution of constituents of interest (nitrate, fumigants, and arsenic) across domestic- and public-supply aquifers indicated that nitrate is prevalent in shallow aquifers throughout the Northern San Joaquin Valley but is potentially diluted by mixing with deeper, older groundwater at long-screened public-supply wells. Fumigants were prevalent in areas of urban and agricultural land use in the western part of the Northern San Joaquin Valley, particularly in areas near Lodi, California, but 1,2,3-TCP was more widespread than DBCP and was detected in shallow and deeper parts of the aquifer system, potentially because of its recalcitrance in groundwater and ability to be detected at low concentrations. Arsenic was most prevalent in the western part of the Northern San Joaquin Valley with proximity to deltaic sediments and was detected at high RCs in wells tapping shallow and deep parts of the aquifer system.