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By Justin T. Kulongoski, Kenneth Belitz, and Barbara J. Dawson
U.S. GEOLOGICAL SURVEY
Data Series 167
Sacramento, California 2006
Complete accessible text of report (16.9 MB PDF)
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Ground-water quality in the ~1,000 square-mile (mi2) North San Francisco Bay study unit was investigated from August to November, 2004, as part of the California Groundwater Ambient Monitoring and Assessment (GAMA) program. Samples were collected from 89 public-supply wells, 7 hydrothermal wells, and 1 hydrothermal spring in Napa, Sonoma and Marin Counties. Eighty-four of the public-supply wells sampled were selected to provide a spatially distributed, randomized monitoring network for statistical calculations. The study was designed to provide a spatially-unbiased assessment of raw ground-water quality within the study unit, as well as a statistically-consistent basis for comparing water quality in different study units.
Ground-water samples were analyzed for major and minor ions, trace elements, nutrients, volatile organic compounds, pesticides and (or) pesticide degradates, waste-water indicators, dissolved methane, nitrogen, carbon dioxide and noble gases (in collaboration with Lawrence Livermore National Laboratory). Naturally occurring isotopes (tritium, carbon-14, oxygen-18, deuterium and helium-4) also were measured in these samples to help identify the source and age of the sampled ground water. Results show that no anthropogenic constituents were detected at concentrations higher than those levels set for regulatory purposes, and relatively few naturally-occurring constituents were detected at concentrations greater than regulatory levels.
In this study, 21 of the 88 volatile organic compounds (VOCs) and gasoline additives and (or) oxygenates investigated were detected in ground-water samples, however, none were above established maximum contaminant levels (MCL). Thirty-two percent of the randomized wells sampled during the North San Francisco Bay GAMA study had at least a single detection of a VOC or gasoline additive and (or) oxygenate. The most frequently detected compounds were chloroform, found in 12 of 84 randomized wells sampled; carbon disulfide, found in 8 of 84 randomized wells sampled; and toluene, found in 4 of 84 randomized wells sampled. Trihalomethanes were the most frequently detected class of VOCs.
Nine of the 122 pesticides and (or) pesticide degradates investigated were detected in ground-water samples, however, none were above established MCLs. Seventeen percent of the randomized wells sampled during the North San Francisco Bay (NSF) GAMA study had at least a single detection of pesticide and pesticide degradate. Herbicides were the most frequently detected class of pesticides. The most frequently detected compound was simazine, found in 8 of the 84 of the randomized wells. Chlordiamino-s-triazine and deisopropyl atrazine were both found in 2 of the 84 randomized wells sampled.
Thirteen waste-water indicators were detected in ground-water samples in the NSF study unit. Twenty-two of the 84 randomized wells sampled for waste-water indicators had at least one detection. Isophorone was the most frequently detected in 6 out of 84 randomized wells. Bisphenol-A, caffeine, and indole each were detected in 3 out of 84 randomized wells.
Major and minor ions and dissolved solids (DS) samples were collected at 33 public-supply wells; 3 samples had DS concentrations above the secondary maximum contaminant level (SMCL) of 500 mg/L. Ground-water samples from 32 public-supply wells were analyzed for trace elements. Arsenic concentrations above the MCL of 10 μg/L were measured at 4 public-supply wells, Boron concentrations above the detection level for the purpose of reporting (DLR) of 100 μg/L were measured at 19 wells. Iron concentrations above the SMCL of 300 μg/L were measured at 7 wells, a lead concentration above the California notification level (NL) of 15 μg/L at one well, and manganese concentrations were above the SMCL of 50 μg/L at 17 wells. Vanadium concentrations above the detection level for the purpose of reporting (DLR) of 3 μg/L were measured at 9 public-supply wells; and chromium(VI) concentrations above the detection level for the purpose of reporting (DLR) of 1 μg/L were measured at 48 public-supply wells.
Radon-222 was detected in all 21 ground-water samples collected, with activities ranging from 210 to 1,500 picocuries per liter (pCi/L). Fifteen radon samples were above the proposed MCL of 300 pCi/L.
Microbial constituents were analyzed in 22 ground-water samples. Total coliform was detected in three wells. Counts ranged from 2 colonies per100 mL to 20 colonies per100 mL. MCLs for microbial constituents are based on reoccurring detection, and will be monitored during future sampling.
Abstract
Introduction
Purpose and Scope
Acknowledgements
Hydrologic Setting of the North San Francisco Bay GAMA Study Unit
Valley and Plains Study Area
Volcanic Highlands Study Area
Wilson Grove Formation Highlands Study Area
Hydrothermal Study
Methods
Sampling Design
Sample Collection
Sample Analysis
Data Reporting
Laboratory Reporting Conventions
Constituents on Multiple Analytical Schedules
Quality Control
Blanks
Replicates
Matrix Spikes
Surrogates
Results
Quality-Control Samples
Detections in Blanks
Equipment Blank
Field Blanks
Variability in Replicate Samples
Matrix Spike Recoveries
Surrogate Compound Recoveries
Ground-Water Quality
VOCs and Gasoline Additives and (or) Oxygenates
Pesticides and Pesticide Degradates
Waste-Water Indicators
Constituents of Special Interest
Nutrients and Dissolved Organic Carbon
Major and Minor Ions and Dissolved Solids
Trace Elements
Isotopes, Radioactivity, and Dissolved Gases
Microbial Constituents
Summary
References
Tables
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