ABSTRACT
Ground-water quality in the approximately 820 square-mile Coachella
Valley Study Unit (COA) was investigated during February and March 2007
as part of the Priority Basin Project of the Groundwater Ambient Monitoring
and Assessment (GAMA) Program. The GAMA Priority Basin Project was developed
in response to the Groundwater 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 study was designed to provide a spatially unbiased assessment of
raw ground water used for public-water supplies within the Coachella
Valley, and to facilitate statistically consistent comparisons of ground-water
quality throughout California. Samples were collected from 35 wells
in Riverside County. Nineteen of the wells were selected using a spatially
distributed, randomized grid-based method to provide statistical representation
of the study unit (grid wells). Sixteen additional wells were sampled
to evaluate changes in water chemistry along selected ground-water flow
paths, examine land use effects on ground-water quality, and to collect
water-quality data in areas where little exists. These wells were referred
to as “understanding wells.”
The ground-water samples were analyzed for a large number of organic
constituents (volatile organic compounds [VOC], pesticides and pesticide
degradates, pharmaceutical compounds, and potential wastewater-indicator
compounds), constituents of special interest (perchlorate and 1,2,3-trichloropropane
[1,2,3-TCP]), naturally occurring inorganic constituents (nutrients,
major and minor ions, and trace elements), radioactive constituents,
and microbial indicators. Naturally occurring isotopes (uranium, tritium,
carbon-14, and stable isotopes of hydrogen, oxygen, and boron), and
dissolved noble gases (the last in collaboration with Lawrence Livermore
National Laboratory) also were measured to help identify the source
and age of the sampled ground water.
A quality-control sample (blank, replicate, or matrix spike) was collected
at approximately one quarter of the wells, and the results for these
samples were used to evaluate the quality of the data for the ground-water
samples. Assessment of the quality-control information resulted in V-coding
less than 0.1 percent of the data collected.
This study did not attempt to evaluate the quality of water delivered
to consumers; after withdrawal from the ground, water typically is treated,
disinfected, and (or) blended with other waters to maintain acceptable
water quality. Regulatory thresholds apply to treated water that is
supplied 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 health-based thresholds established
by the U.S. Environmental Protection Agency (USEPA) and the California
Department of Public Health (CDPH) and thresholds established for aesthetic
purposes (secondary maximum contaminant levels, SMCL-CA) by CDPH.
Most constituents detected in ground-water samples were at concentrations
below drinking-water thresholds. Volatile organic compounds, pesticides,
and pesticide degradates were detected in less than one-third of the
grid well samples collected. All VOC and pesticide concentrations measured
were below health-based thresholds. Potential waste-water indicators
were detected in less than half of the wells sampled, and no detections
were above health-based thresholds. Perchlorate was detected in seven
grid wells; concentrations from two wells were above the CDPH maximum
contaminant level (MCL-CA). Most detections of trace elements in samples
collected from COA Study Unit wells were below water-quality thresholds.
Exceptions include five samples of arsenic that were above the USEPA
maximum contaminant level (MCL-US), two detections of boron above the
CDPH notification level (NL-CA), and two detections of molybdenum and
strontium above USEPA maximum contaminant levels (HAL-US). Concentrations
of nitrite plus nitrate as nitrogen in two understanding well samples
were above the USEPA maximum contaminant level (MCL-US). Activities
of radon-222 in samples from seven wells were above the proposed MCL-US
of 300 pCi/L; however, no samples had an activity above the proposed
alternative MCL-US of 4,000 pCi/L. Most samples collected in the COA
Study Unit had concentrations of major ions and total dissolved solids
below the non-enforceable thresholds set for aesthetic purposes. Major
ions detected at concentrations above the SMCL-CA thresholds included
chloride, fluoride, sulfate, manganese, and total dissolved solids.