Monterey Bay and Salinas Valley Groundwater Quality Assessment Released
This USGS report can be found online.
A summary fact sheet of this report can also be found online.
MONTEREY COUNTY, Calif. – High concentrations of natural and man-made compounds were found in aquifers used for drinking supply beneath valleys surrounding Monterey Bay, and the Salinas Valley. Scientists determined that concentrations of these substances in untreated groundwater are above state and federal health standards for drinking water. Scientists analyzed untreated groundwater from wells, not treated tap water, which may be disinfected, filtered, mixed, and/or exposed to the atmosphere to create safe levels for consumption before it is delivered to consumers.
Nitrate was the predominant constituent affecting groundwater quality in the study area, occurring in high concentrations in about 8% of the aquifer system at depths tapped by public supply wells. The quality of groundwater at shallower depths can differ from groundwater at deeper depths. The presence of nitrate in groundwater can be affected by both natural and human factors. Nitrate from human sources include fertilizers applied for agriculture and landscape maintenance, and septic systems. Concentrations are considered “high” if they are above Federal and California regulatory benchmarks for protecting human health.
“This aquifer assessment reveals that, of the factors affecting groundwater quality, nitrate is the constituent that most frequently exists at high concentrations in the primary aquifer system used for public supply (about 8%), while naturally occurring trace elements occur at high concentrations in about 6% of the primary aquifer system. In contrast, trace elements occur at high concentrations in 14% of the primary aquifer system in the North San Francisco Bay study unit.” said Dr. Justin Kulongoski, a hydrologist and author of the U.S. Geological Survey report prepared in collaboration with the California State Water Resources Control Board.
In the Monterey-Salinas study areas, high concentrations of nitrate were found in the shallower wells sampled (less than 400 feet depth) that are located in agricultural and urban areas. The groundwater with high and moderate concentrations of nitrate also were predominantly classified as modern age or mixed age, which indicates that a component of the groundwater was recharged in the past 50 years.
The naturally occurring trace elements arsenic, boron and molybdenum were also found in high concentrations in 2.8, 2, and 2.9% of the aquifer system respectively. Overall, one or more trace elements were found in 6% of the aquifer system. Trace and minor elements are naturally present in the minerals in rocks and soils, and in the water that comes into contact with those materials.
Radioactive constituents were present at high concentrations in 2% of the primary aquifer system, and 10% at moderate concentrations. Most of the radioactivity in groundwater comes from decay of naturally occurring isotopes of uranium and thorium present in minerals in the aquifer sediments.
Concentrations of organic constituents, generally man-made compounds such as solvents and gasoline additives were found at high concentrations in less than 1% of the aquifer system, while trihalomethanes and insecticides were detected only at moderate concentrations (lower than a health-based benchmark but more than one-tenth of the benchmark) in about 1 percent of drinking-water aquifers. Solvents are used for a number of purposes, including manufacturing and cleaning. Gasoline additives increase the efficiency of fuel combustion. Trihalomethanes form during municipal disinfection of water supplies, and may enter groundwater during landscape irrigation, and insecticides are used to control pests.
Of the 205 organic constituents tested for in this study, 32 were detected. One organic constituent, the herbicide simazine, was detected in 18 percent of samples, but was detected at low concentrations.
Some naturally occurring constituents that affect the aesthetic properties of water, but do not have health-based benchmarks associated with them, were found in Monterey-Salinas groundwater. Total dissolved solids, an indicator of salinity, was found at high concentrations in 9% of the aquifer system. Iron and manganese were found in high concentrations in 21% of the aquifer system. These constituents affect the taste, color, and odor of water, or may create nuisance problems, such as scaling and staining.
The report provides an assessment of groundwater quality in the Monterey-Salinas study area aquifers used for drinking water. Scientific analysis was based on USGS data collected from 97 wells and comprehensive review of more than 500,000 water quality records in a California Department of Public Health database.
“The work done by the Priority Basin Project in the Monterey Bay and Salinas Valley groundwater basins is important because we are providing, for the first time, a quantitative assessment of the extent to which deeper groundwater may have high concentrations of both natural and man-made constituents,” said co-author Dr. Kenneth Belitz, chief of USGS’ Groundwater Ambient Monitoring and Assessment program. “This information can be used by managers to insure that our drinking water supply remains safe.”
The State Water Resources Control Board’s GAMA program is collaborating with the USGS to monitor and assess water quality in 120 groundwater basins and in areas outside of basins across California over a ten-year period. The main goals of GAMA are to improve comprehensive statewide groundwater monitoring and to increase the availability of groundwater-quality information to the public.
The report was prepared in cooperation with the California State Water Resources Control Board and can found at http://pubs.usgs.gov/sir/2011/5058. Accompanying non-technical summaries can be found at http://pubs.usgs.gov/fs/2011/3089.
Links and contacts within this release are valid at the time of publication.
###
USGS provides science for a changing world. Visit USGS.gov, and follow us on Twitter @USGSwaterCA, and @USGS and our other social media channels.
