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Data Series 387

Data Series 387
Version 1.1, March 2009

Ground-Water Quality Data in the Coastal Los Angeles Basin Study Unit, 2006: Results from the California GAMA Program

In cooperation with the California State Water Resources Control Board.

By Timothy M. Mathany, Michael Land, and Kenneth Belitz

Cover Photo


Ground-water quality in the approximately 860 square-mile Coastal Los Angeles Basin study unit (CLAB) was investigated from June to November of 2006 as part of the Statewide Basin Assessment Project of the Ground-Water Ambient Monitoring and Assessment (GAMA) Program. The GAMA Statewide Basin Assessment was developed in response to the Ground-Water 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 Coastal Los Angeles Basin study was designed to provide a spatially unbiased assessment of raw ground-water quality within CLAB, as well as a statistically consistent basis for comparing water quality throughout California. Samples were collected from 69 wells in Los Angeles and Orange Counties. Fifty-five of the wells were selected using a spatially distributed, randomized grid-based method to provide statistical representation of the study area (“grid wells”). Fourteen additional wells were selected to evaluate changes in ground-water chemistry or to gain a greater understanding of the ground-water quality within a specific portion of the Coastal Los Angeles Basin study unit ("understanding wells").

Ground-water samples were analyzed for: a large number of synthetic organic constituents [volatile organic compounds (VOCs), gasoline oxygenates and their degradates, pesticides, polar pesticides, and pesticide degradates, pharmaceutical compounds, and potential wastewater-indicators]; constituents of special interest [perchlorate, N-nitrosodimethylamine (NDMA), 1,4-dioxane, and 1,2,3-trichloropropane (1,2,3-TCP)]; inorganic constituents that can occur naturally [nutrients, major and minor ions, and trace elements]; radioactive constituents [gross-alpha and gross-beta radiation, radium isotopes, and radon-222]; and microbial indicators. Naturally occurring isotopes [stable isotopic ratios of hydrogen and oxygen, and activities of tritium and carbon-14] and dissolved noble gases also were measured to help identify the sources and ages of the sampled ground water.

Quality-control samples (blanks, replicates, and samples for matrix spikes) were collected at approximately one-fourth of the wells, and the results for these samples were used to evaluate the quality of the data for the ground-water samples. Field blanks rarely contained detectable concentrations of any constituent, suggesting that contamination was not a significant source of bias. Differences between replicate samples were within acceptable ranges, indicating acceptably low variability. Matrix spike recoveries were within acceptable ranges for most compounds. Assessment of the quality-control information resulted in applying “V” codes to approximately 0.1 percent of the data collected for ground-water samples (meaning a constituent was detected in blanks as well as the corresponding environmental data).

This study did not attempt to evaluate the quality of drinking water delivered to consumers; after withdrawal from the ground, water typically is treated, disinfected, and (or) blended with other waters to maintain acceptable drinking-water quality. Regulatory thresholds are applied to the treated drinking 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), California Department of Public Health (CDPH, formerly California Department of Health Services [CADHS]) and thresholds established for aesthetic concerns (secondary maximum contaminant levels, SMCL-CA) by CDPH. Comparisons between data collected for this study and drinking-water thresholds are for illustrative purposes only, and are not indicative of compliance or non-compliance with those thresholds.

VOCs were detected in almost three-quarters of the grid wells, and pesticides and pesticide degradates were detected in 42 percent of the grid wells. Potential wastewater indicators were detected in 44 percent of the grid wells. All of the detections of these organic compounds in samples from CLAB grid wells were below health-based thresholds, with the exception of tetrachloromethane (carbon tetrachloride), a VOC, which was detected above the MCL-CA. In CLAB understanding wells, there were two detections of trichloroethene (TCE) and one detection of perchloroethene (PCE) above the MCL-US.

Nutrient and trace-element concentrations in the CLAB grid wells were below health-based thresholds. There were two detections of boron above the NL-CA of 1,000 µg/L in the CLAB understanding wells. Activities of radioactive constituents in water samples collected in CLAB grid wells were below health-based thresholds, with the exception of two detections of radon-222 that were above the proposed MCL-US of 300 pCi/L; however, none of the samples had an activity above the proposed alternative MCL-US of 4,000 pCi/L. Total coliforms were detected at one of CLAB’s understanding wells. Most of the samples from CLAB grid wells had concentrations of major elements and total dissolved solids below the non-enforceable thresholds set for aesthetic concerns. Four grid wells had total dissolved solids concentrations above the SMCL-CA recommended threshold (SMCL-CA threshold for total dissolved solids has a recommended value of 500 mg/L, and an upper value of 1,000 mg/L). There were two detections of manganese, and four detections of iron in CLAB grid wells above their respective SMCL-CAs, and a single detection of arsenic above the MCL-US. Two understanding wells had concentrations of chloride and sulfate above the recommended SMCL-CA (both have a recommended threshold value of 250 mg/L, an upper value of 500 mgL).


Hydrogeologic Setting
References Cited

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