Abstract
Groundwater quality in the approximately 2,170-square-mile
Western San Joaquin Valley (WSJV) study unit was
investigated by the U.S. Geological Survey (USGS) from
March to July 2010, as part of the California State Water
Resources Control Board (SWRCB) Groundwater Ambient
Monitoring and Assessment (GAMA) Program’s Priority
Basin Project (PBP). 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 WSJV study unit was the twenty-ninth study
unit to be sampled as part of the GAMA-PBP.
The GAMA Western San Joaquin Valley study was
designed to provide a spatially unbiased assessment of
untreated-groundwater quality in the primary aquifer
system, and to facilitate statistically consistent comparisons
of untreated groundwater quality throughout California.
The primary aquifer system is defined as parts of aquifers
corresponding to the perforation intervals of wells listed in
the California Department of Public Health (CDPH) database
for the WSJV study unit. Groundwater quality in the primary
aquifer system may differ from the quality in the shallower
or deeper water-bearing zones; shallow groundwater may be
more vulnerable to surficial contamination.
In the WSJV study unit, groundwater samples were
collected from 58 wells in 2 study areas (Delta-Mendota
subbasin and Westside subbasin) in Stanislaus, Merced,
Madera, Fresno, and Kings Counties. Thirty-nine of the wells
were selected by using a spatially distributed, randomized
grid-based method to provide statistical representation of the
study unit (grid wells), and 19 wells were selected to aid in the
understanding of aquifer-system flow and related groundwater-quality
issues (understanding wells).
The groundwater samples were analyzed for
organic constituents (volatile organic compounds
[VOCs], low-level fumigants, and pesticides and
pesticide degradates), constituents of special interest
(perchlorate, N-nitrosodimethylamine [NDMA], and
1,2,3-trichloropropane [1,2,3-TCP]), and naturally occurring
inorganic constituents (trace elements, nutrients, dissolved
organic carbon [DOC], major and minor ions, silica, total
dissolved solids [TDS], alkalinity, total arsenic and iron
[unfiltered] and arsenic, chromium, and iron species [filtered]).
Isotopic tracers (stable isotopes of hydrogen, oxygen, and
boron in water, stable isotopes of nitrogen and oxygen in
dissolved nitrate, stable isotopes of sulfur in dissolved sulfate,
isotopic ratios of strontium in water, stable isotopes of carbon
in dissolved inorganic carbon, activities of tritium, and
carbon-14 abundance), dissolved standard gases (methane,
carbon dioxide, nitrogen, oxygen, and argon), and dissolved
noble gases (argon, helium-4, krypton, neon, and xenon)
were measured to help identify sources and ages of sampled
groundwater. In total, 245 constituents and 8 water-quality
indicators were measured.
Quality-control samples (blanks, replicates, or matrix
spikes) were collected at 16 percent of the wells in the
WSJV study unit, and the results for these samples were
used to evaluate the quality of the data from 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
all were within acceptable limits of variability. Matrix-spike
recoveries were within the acceptable range (70 to
130 percent) for approximately 87 percent of the compounds.
This study did not evaluate the quality of water delivered
to consumers. After withdrawal, groundwater typically is
treated, disinfected, and (or) blended with other waters to
maintain water quality. Regulatory benchmarks apply to
water that is delivered 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 detected in groundwater
samples from the 39 grid wells were detected at concentrations
less than health-based benchmarks. Detections of organic and
special-interest constituents from grid wells sampled in the
WSJV study unit also were less than health-based benchmarks.
In total, VOCs were detected in 12 of the 39 grid wells
sampled (approximately 31 percent), pesticides and pesticide
degradates were detected in 9 grid wells (approximately
23 percent), and perchlorate was detected in 15 grid wells
(approximately 38 percent).
Trace elements, major and minor ions, and nutrients
were sampled for at 39 grid wells; most concentrations were
less than health-based benchmarks. Exceptions include two
detections of arsenic greater than the USEPA maximum
contaminant level (MCL-US) of 10 micrograms per liter
(μg/L), 20 detections of boron greater than the CDPH
notification level (NL-CA) of 1,000 μg/L, 2 detections of
molybdenum greater than the USEPA lifetime health advisory
level (HAL-US) of 40 μg/L, 1 detection of selenium greater
than the MCL-US of 50 μg/L, 2 detections of strontium
greater than the HAL-US of 4,000 μg/L, and 3 detections of
nitrate greater than the MCL-US of 10 μg/L.
Results for inorganic constituents with non-health-based
benchmarks (iron, manganese, chloride, sulfate, and
TDS) showed that iron concentrations greater than the
CDPH secondary maximum contaminant level (SMCL-CA)
of 300 μg/L were detected in five grid wells. Manganese
concentrations greater than the SMCL-CA of 50 μg/L
were detected in 16 grid wells. Chloride concentrations
greater than the recommended SMCL-CA benchmark of
250 milligrams per liter (mg/L) were detected in 14 grid
wells, and concentrations in 5 of these wells also were greater
than the upper SMCL-CA benchmark of 500 mg/L. Sulfate
concentrations greater than the recommended SMCL-CA
benchmark of 250 mg/L were measured in 21 grid wells,
and concentrations in 13 of these wells also were greater
than the SMCL-CA upper benchmark of 500 mg/L. TDS
concentrations greater than the SMCL-CA recommended
benchmark of 500 mg/L were measured in 36 grid wells, and
concentrations in 20 of these wells also were greater than the
SMCL-CA upper benchmark of 1,000 mg/L.