High Levels of Naturally Occurring Elements Found in Sacramento Valley Aquifers

Some manmade compounds also were detected throughout valley

High concentrations of naturally occurring elements, including arsenic and boron, as well as human-introduced compounds such as nitrate, were found in three aquifers studied in the Sacramento Valley in California. Scientists determined that concentrations of these substances in untreated water exceed state and federal health standards for drinking water. Scientists analyzed untreated groundwater from wells.

The report provides an assessment of groundwater quality of the southern, middle and northern Sacramento Valley aquifers used for drinking water. High concentrations were more prevalent in the southern aquifer system that extends from Fairfield to Lincoln and includes the greater Sacramento metropolitan area than in the northern part that includes Redding, Red Bluff and Los Molinos. Scientific analysis was based on U.S. Geological Survey (USGS) data collected from 235 wells and comprehensive review of more than 1.5 million water quality records in a California Department of Public Health database.

Arsenic was detected above the U.S. Maximum Contaminant Level, set by the U.S. Environmental Protection Agency (EPA), of 10 parts per billion in 2%, 22% and 16% of drinking water aquifers in the northern, middle and southern parts of the valley, respectively. Most of the samples with high arsenic concentration were collected from wells close to the Sacramento and Feather rivers or near the margin of the Sacramento-San Joaquin river delta.

Boron was detected above the California Notification Level, set by the California Department of Public Health, a non-regulatory benchmark of 1 part per million, in about 19% of the southern and in about 6% of the middle parts of the valley. High concentrations of boron were found either near the margins of the delta, or near the coastal ranges on the western side of the valley.

Concentrations of organic constituents, generally manmade compounds such as solvents and pesticides, were above health-based benchmarks in less than 1% of drinking water aquifers. The solvent perchloroethene and the gasoline additives tert-butyl alcohol and benzene were the only organic constituents reported above heath-based benchmarks.

Of the 226 organic constituents tested for, 79 were detected. Six organic constituents were detected in 10% or more of the samples in at least one of the three study units. The six detected constituents include the following: chloroform, a byproduct from the disinfection of water; the solvents PCE and trichloroethene; and the herbicides atrazine, bentazon and simazine. The number of constituents detected in 10% or more of the samples was lowest in the northern part of the valley and highest in the southern part. Most detections had very low concentrations, below one-tenth of a health-based benchmark.

The State Water Resources Control Board’s Groundwater Ambient Monitoring and Assessment (GAMA) program is collaborating with the USGS to monitor and assess water quality in 120 groundwater 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.

U.S. Geological Survey