About 14 percent of groundwater basins have high concentrations of inorganic constituents; only 1 percent have high levels of man-made organic compounds
concentrations of naturally occurring inorganic constituents – including
arsenic, boron and lead – are found in about 14 percent of the primary aquifers
in California’s Napa, Sonoma and Marin counties, according to a U.S. Geological
Survey (USGS) report prepared in cooperation with the California Water
Resources Control Board. Primary aquifers are those that supply public drinking
concentration” means that a level is above a health-based benchmark, some of
which are regulatory. The USGS assessment analyzed untreated ground water from
wells, not water delivered to consumers through the tap. Regulatory benchmarks
are enforced only for water delivered to consumers.
detected above the U.S. Maximum Contaminant Level of 10 parts per billion in
about 10 percent of the primary aquifers. Boron was detected above the
California Notification Level, a non-regulatory benchmark of one part per million,
in about 4 percent of the primary aquifers. And lead was detected at high
concentrations in about 2 percent of the primary aquifers. The U.S. Action
Level for lead is 15 parts per billion.
concentrations of arsenic were attributed to the dissolution of sediments that
naturally contain arsenic. These processes are controlled by the amount of
oxygen dissolved in the ground water.
concentrations of boron were primarily associated with ground water mixing with
hydrothermal waters or high-salinity waters in the Napa-Sonoma lowlands.
of organic constituents – generally man-made compounds such as solvents and
pesticides – were above health-based standards in about 1 percent of the ground
“Status and Understanding of Groundwater Quality in the North San Francisco Bay
Groundwater Basins, 2004: California GAMA Priority Basin Project,” was
conducted under the State of California’s
Groundwater Ambient Monitoring and Assessment Program.
initial USGS well-sampling was conducted in 2004, and a data report released in
2006, this new interpretive report provides an assessment of the drinking water
aquifers, based on the USGS data and more than 300,000 records in a California
Department of Public Health database.
can be found on-line by followingthis
has been coordinating closely with water purveyors in the region. The owners of
wells sampled – cities, water agencies, private well owners – have received the
results of the study.
Of the 255
organic and special-interest constituents tested for, 26 were detected. Two
organic constituents were detected in 10 percent or more of the samples:
chloroform, a byproduct from the disinfection of water, and the herbicide
simazine. However, both were detected at very low concentrations – below one-tenth
of a health-based threshold. Nitrate was high in less than 1 percent of the
and simazine generally were detected in samples collected from shallow wells
located in urban or agricultural areas. In addition, age-dating of these
samples indicated that all or some of the ground water from these wells is less
than 50 years old. These results suggest
that chloroform and simazine in ground water are the result of human activities
in the last 50 years.
aquifer assessment reveals that the major threats to ground water quality are
naturally occurring trace elements, rather than man-made compounds associated
with human activities,” says Dr. Justin Kulongoski a research hydrologist and
senior author of the USGS report.
“The work done
by the Priority Basin Project in the North San Francisco Bay Area is
important because we are providing, for
the first time, a quantitative assessment of the extent to which deep ground water
may have high concentrations of both natural and man-made constituents,” says
co-author Dr. Kenneth Belitz, chief of USGS’ GAMA program. “This information
can be used by managers to insure that our drinking water supply remains safe.”
Water Resources Control Board’s GAMA Program is collaborating with the USGS to
monitor and assess water quality in 120 ground water basins across California over a
10-year period. The main goals of GAMA are to improve comprehensive statewide
ground water monitoring and to increase the availability of groundwater-quality
information to the public.
samples collected for this study contain fewer man-made compounds, at lower
concentrations, than many ground water basins studied by the USGS,” Kulongoski says.
“The concentrations of these constituents were typically one-half to less than
one-forty-thousandth of the levels utilized by the State of California’s Department of Public Health for
ability to detect the presence of man-made compounds in public-supply wells at
ultra-low concentrations is important for the protection of our water
resources,” Belitz notes. “Our goal is to understand how these compounds are
transported from the landscape and into the aquifer system.”
New Study Looks at Ground Water Quality in Bay Area
September 20, 2010