Data Series 258

Data Series 258

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To assess the quality of ground water from public-supply wells and establish a program for monitoring trends in ground-water quality, the U.S. Geological Survey (USGS), in collaboration with the California State Water Resource Control Board and Lawrence Livermore National Laboratory (LLNL), implemented a statewide ground-water-quality monitoring and assessment program ( The USGS developed a comprehensive approach for this effort (Belitz and others, 2003; The Ground-Water Ambient Monitoring and Assessment (GAMA) program is a comprehensive assessment of Statewide ground-water quality designed to help better understand and identify risks to ground-water resources. The assessment will be based on ground-water samples collected at many locations across California in order to characterize its constituents and identify trends in water quality (for example, Wright and others, 2005; Kulongoski and others, 2006). The results of the sampling and analysis provide information for water agencies to address a variety of issues ranging in scale from local water supply to statewide resource management.

The GAMA program was developed in response to the Ground-Water Quality Monitoring Act of 2001 (CAL. WATER §§ 10780-10782.3): a public mandate to assess and monitor the quality of ground water used as public supply for municipalities in California. The goal of the Ground-Water Quality Monitoring Act of 2001 is to improve statewide ground-water monitoring and facilitate the availability of information about ground-water quality to the public.

The three main objectives of GAMA are (1) status, to assess the current quality of the ground-water resource; (2) trends, to detect changes in ground-water quality; and (3) understanding, to identify the natural and human factors affecting ground-water quality (Kulongoski and Belitz, 2004). This report will present an assessment of the quality of the ground-water resource (objective (1) – status) in the Monterey Bay and Salinas Valley GAMA study unit, while subsequent interpretive reports will address the trends and understanding listed in objectives (2) and (3).

The GAMA program is unique because the data collected during the study include analyses for an extensive number of chemical constituents, analyses that are not normally available. This broader understanding of ground-water composition will be especially useful for providing an early indication of changes in water chemistry. Additionally, the GAMA program will analyze this broader suite of constituents at detection limits that are lower than those currently required by the California Department of Health Services (CADHS). An understanding of the occurrence and distribution of these constituents is important for the long term management and protection of ground-water resources.

The range of hydrologic, geologic, and climatic conditions that exist in California must be considered in an assessment of ground-water quality (Belitz and others, 2003). To accomplish this, the State was partitioned into 10 hydrogeologic provinces, each with distinctive hydrologic, geologic, and climatic characteristics (fig. 1). The ground-water basins within these hydrologic provinces generally consist of relatively permeable, unconsolidated deposits of alluvial or volcanic origin (California Department of Water Resources, 2003). For the purpose of designing the GAMA program, ground-water basins were prioritized (for sampling) on the basis of the number of public-supply wells in the basin (Belitz and others, 2003). Secondary consideration was given to the amount of municipal ground-water use, agricultural pumping, the number of leaking underground fuel tanks, and pesticide application within a basin. Similar adjacent ground-water basins were then combined and designated as GAMA study units. The Monterey Bay and Salinas Valley GAMA study unit, hereafter referred to as the MS study unit, lies in the Southern Coast Ranges Hydrogeologic province (fig. 1), and contains eight ground-water basins that are considered high priority based on the number of public-supply wells, basin location, agricultural use, and pesticide applications within each basin (Belitz and others, 2003).

Purpose and Scope

The purpose of this report is to present the results of analyses for organic and inorganic constituents, microbial constituents, and water-quality indicators from ground-water samples collected in the MS study unit. Discussions of the factors that influence the distribution and occurrence of the compounds and microbial constituents detected in ground-water samples will be the subject of subsequent publications.

This study determined the chemical and biological constituents of untreated aquifer water. In order to provide context for these results, the analytical results reported in this study were compared to state and federal drinking water standards that apply to treated drinking water. Samples collected for this program do not represent the water delivered to consumers; after withdrawal from the ground, water typically is treated, disinfected, and (or) blended with other waters to maintain water quality. Regulatory thresholds are established by the United States Environmental Protection Agency (USEPA), the California Environmental Protection Agency (CAEPA), and (or) the California Department of Health Services (CADHS). Health-based regulatory thresholds include maximum contaminant levels (MCLs); health-based advisory levels (ALs), notification levels (NLs); or the USEPA Lifetime Health Advisory (HALs).

Non-enforceable thresholds established for aesthetic qualities (taste, odor, and color) include California secondary maximum contaminant levels (SMCLs-CA), and detection limits for the purposes of reporting (DLR) set by the CADHS for the purposes of tracking unregulated chemicals for which monitoring is required. The SMCLs-CA for chloride, sulfate, specific conductance, and total dissolved solids include recommended thresholds (same as the United States SMCLs), upper thresholds, and short term thresholds for each constituent. The data presented in this report are intended to characterize the quality of untreated ground-water resources within the study unit, not the treated drinking water delivered to consumers by water purveyors.

Detection frequencies, or the percentage of ground-water samples in which a constituent is observed, were reported for the VOCs, pesticides, and pesticide degradates. Detection frequencies are useful for determining trends in ground-water quality. Also presented in this report are the results and analyses of quality-control samples collected during the Monterey Bay and Salinas Valley GAMA study. Samples for pharmaceutical compounds were also collected as part of this study; however, the presentation of these results and their associated quality assurance/quality control are beyond the scope of this report and will be presented in a future report.


The authors thank the following agencies for their support: California Water Boards, California Department of Health Services (CADHS), California Department of Water Resources (DWR), and Lawrence Livermore National Laboratory (LLNL).

We also thank the cooperating well owners and water purveyors for their generosity in allowing the USGS to collect samples from their wells.

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