Abstract
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 and
constituent detection frequency. 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 the water quality of different study
units.
Ground-water samples were analyzed for major and
minor ions, trace elements, nutrients, volatile organic compounds,
pesticides and 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
the samples to help identify the source and age of the 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,
detected concentrations were one-half to one-forty-thousandth
the maximum contaminant levels (MCL). Thirty-two percent
of the randomized wells sampled 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 the 84 randomized wells; carbon disulfide, found in 8
of the 84 randomized wells; and toluene, found in 4 of the 84
randomized wells. 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,
concentrations were one-seventieth to one-eight-hundredth the
MCLs. Seventeen percent of the randomized wells sampled
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 out of 63 compounds that may be indicative of
the prescence of waste-water were detected in ground-water
samples. Twenty-six percent of the randomized wells sampled
for waste-water indicators had at least one detection. Isophorone
was the most frequently detected in 6 of the 84 randomized
wells. Bisphenol-A, caffeine, and indole each were
detected in 3 of the 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 above the SMCL of 50
μg/L were measured at 17 wells. Vanadium concentrations
above the DLR of 3 μg/L were measured at 9 public-supply
wells; and chromium(VI) concentrations above the DLR of 1
μg/L were measured at 48 public-supply 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 above the SMCL of 50
μg/L were measured at 17 wells. Vanadium concentrations
above the DLR of 3 μg/L were measured at 9 public-supply
wells; and chromium(VI) concentrations above the DLR of 1
μg/L were measured at 48 public-supply wells.
Microbial constituents were analyzed in 22 ground-water
samples. Total coliform was detected in three wells. Counts
ranged from 2 colonies per 100 mL to 20 colonies per
100 mL. MCLs for microbial constituents are based on
reoccurring detection, and will be monitored during future
sampling.
