Groundwater quality in the approximately 460-square-mile San Fernando–San Gabriel (FG) study unit was investigated as part of the Priority Basin Project of the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The study area is in Los Angeles County and includes Tertiary-Quaternary sedimentary basins situated within the Transverse Ranges of southern California. The GAMA Priority Basin Project is being conducted by the California State Water Resources Control Board in collaboration with the U.S. Geological Survey (USGS) and the Lawrence Livermore National Laboratory.

The GAMA FG study was designed to provide a spatially unbiased assessment of the quality of untreated (raw) groundwater in the primary aquifer systems (hereinafter referred to as primary aquifers) throughout California. The assessment is based on water-quality and ancillary data collected in 2005 by the USGS from 35 wells and on water‑quality data from the California Department of Public Health (CDPH) database. The primary aquifers were defined by the depth interval of the wells listed in the CDPH database for the FG study unit. The quality of groundwater in primary aquifers may be different from that in the shallower or deeper water-bearing zones; shallow groundwater may be more vulnerable to surficial contamination.

This study assesses the status of the current quality of the groundwater resource by using data from samples analyzed for volatile organic compounds (VOCs), pesticides, and naturally occurring inorganic constituents, such as major ions and trace elements. This status assessment is intended to characterize the quality of groundwater resources in the primary aquifers of the FG study unit, not the treated drinking water delivered to consumers by water purveyors.

Relative-concentrations (sample concentration divided by the health- or aesthetic-based benchmark concentration) were used for evaluating groundwater quality for those constituents that have Federal and (or) California regulatory or non-regulatory benchmarks for drinking-water quality. A relative-concentration greater than (>) 1.0 indicates a concentration greater than a benchmark, and less than or equal to (≤) 1.0 indicates a concentration equal to or less than a benchmark. Relative-concentrations of organic and special-interest constituents [perchlorate, N-nitrosodimethylamine (NDMA), 1,4-dioxane, and 1,2,3-trichloropropane (1,2,3-TCP)] were classified as “high” (relative-concentration >1.0), “moderate” (0.1< relative-concentration ≤ 1.0), or “low” (relative-concentration ≤ 0.1). Relative-concentrations of inorganic constituents were classified as “high” (relative‑concentration > 1.0), “moderate” (0.5 < relativeconcentration ≤ 1.0), or “low” (relative‑concentration ≤ 0.5).

Aquifer-scale proportion was used as the primary metric in the status assessment for evaluating regional-scale groundwater quality. High aquifer-scale proportion is defined as the percentage of the area of the primary aquifers with a relative-concentration greater than 1.0 for a particular constituent or class of constituents; percentage is based on an areal rather than a volumetric basis. Moderate and low aquifer-scale proportions were defined as the percentage of the primary aquifers with moderate and low relative-concentrations, respectively. Two statistical approaches—grid-based and spatially weighted—were used to evaluate aquifer-scale proportions for individual constituents and classes of constituents. Grid-based and spatially weighted estimates were comparable in the FG study unit (within 90-percent confidence intervals).

Inorganic constituents with human-health benchmarks were detected at high relative-concentrations in 9.1 percent of the primary aquifers and moderate in 33.3 percent. High aquifer-scale proportion of inorganic constituents primarily reflected high aquifer-scale proportions of nitrate (8.8 percent). The inorganic constituents with secondary maximum contaminant levels (SMCLs), iron, sulfate, and total dissolved solids (TDS) had relative-concentrations that were high in 3.2 percent and moderate in 18.2 percent of the primary aquifers.

Relative-concentrations of organic constituents (one or more) were high in 18.2 percent, and moderate in 42.9 percent, of the primary aquifers, based on the spatially weighted approach. The high aquifer-scale proportion of organic constituents primarily reflected high aquifer-scale proportions of trichloroethene (TCE; 14.8 percent), perchloroethene (PCE; 11.2 percent), and carbon tetrachloride (6.5 percent). Of the 212 organic and special-interest constituents analyzed, 66 constituents were detected. Chloroform, PCE, simazine, atrazine, and TCE were each detected in more than 50 percent of the 35 grid wells. Bromodichloromethane, cis-1,2-dichloroethene, 1,1-dichloroethane, perchlorate, carbon tetrachloride, and 1,1-dichloroethene were detected in more than 30 percent of the grid wells. Methyl tert-butyl ether (MTBE), prometon, and diuron were detected in more than 20 percent of the grid wells, and CFC-12, bromacil, carbon disulfide, 1,1,1-trichloroethane (TCA), CFC-113, tebuthiuron, dibromochloromethane, and CFC-11 were detected in more than 10 percent of the grid wells. However, perchlorate, diuron, and bromacil were sampled only in a subset of 11 wells, not in all 35 grid wells. Perchlorate and NDMA were detected at high relative-concentrations in 11.2 percent and 5.2 percent of the primary aquifers, respectively, based on the spatially weighted approach. Pharmaceutical compounds were not detected at concentrations greater than or equal to method detection limits in the study unit.