Groundwater quality in the 633-square-mile Northern Coast Ranges (NOCO) study unit was investigated by the U.S. Geological Survey (USGS) from June to November 2009, as part of the California State Water Resources Control Board (SWRCB) Groundwater Ambient Monitoring and Assessment (GAMA) Program’s Priority Basin Project (PBP) and the U.S. Geological Survey National Water-Quality Assessment Program (NAWQA). The GAMA-PBP was developed in response to the California Groundwater Quality Monitoring Act of 2001 and is being conducted in collaboration with the SWRCB and Lawrence Livermore National Laboratory (LLNL). The NOCO study unit was the thirtieth study unit to be sampled as part of the GAMA-PBP.

The GAMA Northern Coast Ranges study 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) are defined as that part of the aquifer corresponding to the perforation intervals of wells listed in the California Department of Public Health (CDPH) database for the NOCO 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 NOCO study unit, groundwater samples were collected from 58 wells in 2 study areas (Interior Basins and Coastal Basins) in Napa, Lake, Mendocino, Glenn, Humboldt, and Del Norte Counties. The 58 wells were selected by using a spatially distributed, randomized grid-based method to provide statistical representation of the study areas. GAMA-PBP wells sampled as part of the spatially-distributed, randomized grid-cell network are referred to as “grid wells.”

The groundwater samples were analyzed for organic and special-interest constituents (volatile organic compounds [VOC], pesticides and pesticide degradates, and perchlorate), naturally occurring inorganic constituents (trace elements, nutrients, dissolved organic carbon [DOC], major and minor ions, silica, total dissolved solids [TDS], and alkalinity), radioactive constituents (radon-222, radium isotopes, gross alpha and gross beta radioactivity, lead-210, and polonium-210), and microbial indicators (F-specific and somatic coliphage, Escherichia coli [E. coli] and total coliform). Naturally occurring isotopes (stable isotopes of hydrogen and oxygen in water, stable isotopes of carbon in dissolved inorganic carbon, activities of tritium, and carbon-14 abundance), and dissolved noble gases also were measured to identify the sources and ages of the sampled groundwater. In total, 239 constituents and 12 field water-quality indicators were measured.

Three types of quality-control samples (blanks, replicates, and matrix-spikes) were collected at up to 12 percent of the wells in the NOCO study unit, and the results for these samples were used to evaluate the quality of the data for the groundwater samples. 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. Replicate samples generally were within the limits of acceptable analytical reproducibility. Matrix-spike recoveries were within the acceptable range (70 to 130 percent) for approximately 89 percent of the compounds.

This study did not attempt to evaluate the quality of 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 benchmarks 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 benchmarks established by the U.S. Environmental Protection Agency (USEPA) and CDPH, and to non-regulatory benchmarks established for aesthetic concerns by CDPH. Comparisons between data collected for this study and benchmarks for drinking water are for illustrative purposes only and are not indicative of compliance or non-compliance with those benchmarks.

Most inorganic constituents that were detected in groundwater samples from the 58 grid wells in the NOCO study unit were detected at concentrations less than drinking-water benchmarks. In addition, all detections of organic and special-interest constituents from NOCO study-unit well samples were less than health-based benchmarks.

In total, VOCs were detected in 22 of the 58 wells sampled (approximately 38 percent), and pesticides and pesticide degradates were detected in 6 wells (approximately 10 percent) in the NOCO study unit. In the Interior Basins (NOCO-IN) study area, VOCs were detected in 13 of the 28 wells sampled (approximately 46 percent), and pesticides and pesticide degradates were detected in 5 wells (approximately 18 percent). In the Coastal Basins (NOCO-CO) study area, VOCs were detected in 9 of the 30 wells sampled (approximately 30 percent), and pesticide degradates were detected in 1 well.

Results for trace elements, major ions, and TDS with non-regulatory benchmarks set for aesthetic concerns from the 28 NOCO-IN study-area wells showed that iron concentrations greater than the CDPH secondary maximum contaminant level (SMCL-CA) of 300 μg/L were detected in 7 wells. Manganese concentrations greater than the SMCL-CA of 50 μg/L were detected in 12 wells. TDS concentrations greater than the SMCL-CA recommended benchmark of 500 mg/L were measured in 4 of the 28 NOCO-IN study-area wells.

In the 30 NOCO-CO study-area wells, iron concentrations greater than the SMCL-CA were detected in 8 wells, and manganese concentrations greater than the SMCL-CA were detected in 11 wells.

Microbial indicators (viral and bacterial) were sampled for at 57 wells in the NOCO study unit. One or more microbial indicator was detected in 22 wells (11 of 28 NOCO-IN study-area wells and 11 of 29 NOCO-CO study-area wells).