Ground-water quality in the approximately 1,000-square-mile Upper Santa Ana Watershed study unit (USAW) was investigated from November 2006 through March 2007 as part of the Priority Basin Project of the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The GAMA Priority Basin project was developed in response to the Groundwater Quality Monitoring Act of 2001, and is being conducted by the U.S. Geological Survey (USGS) in cooperation with the California State Water Resources Control Board (SWRCB).

The Upper Santa Ana Watershed study was designed to provide a spatially unbiased assessment of raw ground-water quality within USAW, as well as a statistically consistent basis for comparing water quality throughout California. Samples were collected from 99 wells in Riverside and San Bernardino Counties. Ninety of the wells were selected using a spatially distributed, randomized grid-based method to provide statistical representation of the study unit (grid wells). Nine wells were selected to provide additional understanding of specific water-quality issues identified within the basin (understanding wells).

The ground-water samples were analyzed for a large number of organic constituents (volatile organic compounds [VOCs], pesticides and pesticide degradates, pharmaceutical compounds, and potential wastewater-indicator compounds), constituents of special interest (perchlorate, N-nitrosodimethylamine [NDMA], 1,4-dioxane, and 1,2,3-trichloropropane [1,2,3-TCP]), naturally occurring inorganic constituents (nutrients, major and minor ions, and trace elements), radioactive constituents, and microbial indicators. Naturally occurring isotopes (tritium, carbon-14, and stable isotopes of hydrogen and oxygen in water) and dissolved noble gases also were measured to help identify sources and ages of the sampled ground water. Dissolved gases, and isotopes of nitrogen gas and of dissolved nitrate also were measured in order to investigate the sources and occurrence of nitrate in the study unit. In total, nearly 400 constituents and water-quality indicators were investigated for this study.

This study did not attempt to evaluate the quality of water delivered to consumers; after withdrawal from the ground, water typically is treated, disinfected, and (or) blended with other waters to maintain acceptable water quality. Regulatory thresholds apply to treated water that is served to the consumer, not to raw ground water. However, to provide some context for the results, concentrations of constituents measured in the raw ground water were compared with regulatory and non-regulatory health-based thresholds established by the U.S. Environmental Protection Agency (USEPA) and the California Department of Public Health (CDPH) and thresholds established for aesthetic concerns (secondary maximum contaminant levels, SMCL-CA) by CDPH.

Volatile organic compounds (VOCs) were detected in more than 80 percent of USAW grid wells. Most VOCs detected were at concentrations far less than thresholds established for drinking water to protect human health; however, six wells had VOC concentrations above health-based thresholds. Twenty-four of the 85 VOCs investigated were detected in the study unit;11 were detected in more than 10 percent of the wells. The VOCs detected above health-based thresholds in at least one well were dibromochloropropane (DBCP), tetrachloroethene (PCE), trichloroethene (TCE), carbon tetrachloride, and 1,1-dichoroethene.

Pesticide compounds were detected in more than 75 percent of the grid wells. However, of the 134 different pesticide compounds investigated, 13 were detected at concentrations greater than their respective long-term method detection limits, and only 7 compounds (all herbicides or herbicide degradates) were detected in more than 10 percent of the wells. No pesticide compound was detected above its health-based threshold, although thresholds exist for fewer than half of the pesticide compounds investigated.

Samples were analyzed for a suite of 69 chemicals identified as “potential wastewater indicators.” However, data for this suite of chemicals are not presented in this report because inconsistent results for quality-control samples collected and analyzed using this analytical method suggested that the results were unreliable.

Samples were analyzed for 14 pharmaceutical compounds. Four compounds were detected in ground-water samples, but no compound was detected in more than five samples. All pharmaceutical compound concentrations were low compared to their typical prescription dosages.

Samples were analyzed for about 50 inorganic constituents. These include nutrients, major and minor ions, and trace elements, most of which occur naturally in ground water. Nitrate, a nutrient species of nitrogen, was detected at concentrations above its health-based threshold in 29 percent of the grid wells for which it was sampled. Concentrations of other forms of nitrogen detected were below health-based thresholds. Concentrations of total dissolved solids were greater than thresholds set for aesthetic concerns in nearly 40 percent of the wells for which total dissolved solids were sampled. Concentrations of chloride and sulfate were greater than such aesthetic thresholds in 3 wells and 1 well, respectively. Concentrations of all trace elements detected in samples from USAW wells were below health-based thresholds, except arsenic (above a threshold in 2 wells), boron (1 well), molybdenum (2 wells) and vanadium (2 wells). Concentrations of two additional trace elements—iron and manganese—were greater than their non-enforceable thresholds set for aesthetic concerns in 1 and 2 wells, respectively. Perchlorate, an inorganic constituent of special interest, was detected in two-thirds of the wells, and 11 wells contained perchlorate concentrations at or above its health-based threshold.

Radon-222 activities in more than half of the wells sampled for this radioactive isotope were above the proposed USEPA maximum contaminant level (MCL-US), but none were above the proposed alternative MCL-US. All other detections of radioactive constituents were below health-based thresholds except one gross-alpha measurement of 17.3 picocuries per liter (72-hour count). Total coliforms, a bacterial indicator, were detected in 1 out of 32 wells for which total coliforms were sampled.