ABSTRACT

Groundwater quality in the approximately 766-square-mile South Coast Range–Coastal (SCRC) study unit was investigated from May to December 2008, as part of the Priority Basins Project of the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The GAMA Priority Basins Project was developed in response to legislative mandates (Supplemental Report of the 1999 Budget Act 1999-00 Fiscal Year; and, the Groundwater Quality Monitoring Act of 2001 [Sections 10780-10782.3 of the California Water Code, Assembly Bill 599]) to assess and monitor the quality of groundwater in California, and is being conducted by the U.S. Geological Survey (USGS) in cooperation with the California State Water Resources Control Board (SWRCB). The SCRC study unit was the 25th study unit to be sampled as part of the GAMA Priority Basins Project.

The SCRC study unit was designed to provide a spatially unbiased assessment of untreated groundwater quality in the primary aquifer systems and to facilitate statistically consistent comparisons of untreated groundwater quality throughout California. The primary aquifer systems (hereinafter referred to as primary aquifers) were defined as that part of the aquifer corresponding to the perforation interval of wells listed in the California Department of Public Health (CDPH) database for the SCRC study unit. The quality of groundwater in shallow or deep water-bearing zones may differ from the quality of groundwater in the primary aquifers; shallow groundwater may be more vulnerable to surficial contamination. In the SCRC study unit, groundwater samples were collected from 70 wells in two study areas (Basins and Uplands) in Santa Barbara and San Luis Obispo Counties. Fifty-five of the wells were selected using a spatially distributed, randomized grid-based method to provide statistical representation of the study unit (grid wells), and 15 wells were selected to aid in evaluation of specific water-quality issues (understanding wells). In addition to the 70 wells sampled, 3 surface-water samples were collected in streams near 2 of the sampled wells in order to better comprehend the interaction between groundwater and surface water in the area.

The groundwater samples were analyzed for organic constituents (volatile organic compounds [VOC], pesticides and pesticide degradates, polar pesticides and metabolites, and pharmaceutical compounds), constituents of special interest (perchlorate, N-nitrosodimethylamine [NDMA], and 1,2,3-TCP), naturally occurring inorganic constituents (trace elements, nutrients, dissolved organic carbon [DOC], major and minor ions, silica, total dissolved solids [TDS], and alkalinity), and radioactive constituents (gross alpha and gross beta radioactivity). Naturally occurring isotopes (stable isotopes of hydrogen and oxygen in water, stable isotopes of nitrogen and oxygen in dissolved nitrate, stable isotopes of sulfur in dissolved sulfate, stable isotopes of carbon in dissolved inorganic carbon, activities of tritium, and carbon-14 abundance), and dissolved gases (including noble gases) also were measured to help identify the sources and ages of the sampled groundwater. In total, 298 constituents and field water-quality indicators were investigated. Three types of quality-control samples (blanks, replicates, and matrix-spikes) were collected at approximately 3 to 12 percent of the wells in the SCRC study unit, and the results for these samples were used to evaluate the quality of the data for the groundwater samples. Field blanks rarely contained detectable concentrations of any constituent, suggesting that contamination from sample collection procedures was not a significant source of bias in the data for the groundwater samples. Differences between replicate samples generally were less than 10 percent relative and/or standard deviation, indicating acceptable analytical reproducibility. Matrix-spike recoveries were within the acceptable range (70 to 130 percent) for approximately 84 percent of the compounds.

This study did not attempt to evaluate the quality of drinking water delivered to consumers; after withdrawal from the ground, untreated groundwater typically is treated, disinfected, and/or blended with other waters to maintain water quality. Regulatory thresholds apply to water that is served to the consumer, not to untreated groundwater. However, to provide some context for the results, concentrations of constituents measured in the untreated groundwater were compared with regulatory and non-regulatory health-based thresholds established by the U.S. Environmental Protection Agency (USEPA) and CDPH, and to non-regulatory thresholds established for aesthetic concerns by CDPH. Comparisons between data collected for this study and thresholds for drinking water are for illustrative purposes only and are not indicative of compliance or noncompliance with those thresholds. Most organic and inorganic constituents that were detected in groundwater samples from the 55 grid wells in the SCRC study unit were detected at concentrations less than drinking-water thresholds. In addition, all detections of organic constituents in SCRC grid well samples were less than health-based thresholds. In total, VOCs were detected in 33 percent of the 55 grid wells sampled and pesticides and pesticide degradates were detected in 27 percent of grid wells sampled in the SCRC study unit. In the Basins study area, VOCs and pesticides and pesticide degradates were detected in approximately 33 percent of the 39 grid wells. In the Uplands study area, VOCs were detected in approximately 31 percent and pesticides and pesticide degradates were detected in approximately 13 percent of the 16 grid wells. Trace elements and minor ions were sampled for at 32 grid wells and nutrients at 33 grid wells in the SCRC study unit, and most detections were less than health-based thresholds. Exceptions in the Basins study area include one detection of arsenic greater than the USEPA maximum contaminant level (MCL-US) of 10 µg/L and three detections of nitrite plus nitrate, as nitrogen (NO2-+NO3-) greater than the MCL-US of 10 mg/L. Exceptions in the Uplands study area include two detections of arsenic greater than the MCL-US and eight detections of molybdenum greater than the USEPA lifetime health advisory level (HAL-US) of 40 µg/L. All detections of major and minor ions and gross alpha and gross beta radioactivity from the SCRC grid wells were less than health-based thresholds.

Results for trace elements, major ions, and TDS with non-enforceable thresholds set for aesthetic concerns from 16 Basins study area grid wells showed that iron concentrations greater than the CDPH secondary maximum contaminant level (SMCL-CA) of 300 µg/L were detected in grid wells. Manganese concentrations greater than the SMCL-CA of 50 µg/L were detected in six grid wells.

Chloride concentrations greater than the recommended SMCL-CA threshold of 250 mg/L were detected in one grid well. Sulfate concentrations greater than the recommended SMCL-CA threshold of 250 mg/L were measured in 12 grid wells and 3 of these wells also were greater than the upper SMCL-CA threshold of 500 mg/L. TDS concentrations greater than the SMCL-CA recommended threshold of 500 mg/L were measured in 14 of the 16 Basins study area grid wells and concentrations in 5 of these wells also were greater than the SMCL-CA upper threshold of 1,000 mg/L.

In the Uplands study area, iron concentrations greater than the SMCL-CA were detected in 2 of 16 grid wells and manganese concentrations greater than the SMCL-CA were detected in 3 grid wells. TDS and sulfate concentrations greater than the recommended SMCL-CA thresholds were detected in 11 and 2 grid wells, respectively, but none of these concentrations were greater than the SMCL-CA upper thresholds.

First posted June 18, 2010