Groundwater quality in the San Francisco Bay groundwater basins, California

U.S. Geological Survey and the California State Water Resources Control Board

Groundwater provides more than 40 percent of California’s drinking water. To protect this vital resource, the State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State’s groundwater quality and increases public access to groundwater-quality information. Selected groundwater basins of the San Francisco Bay area constitute one of the study units being evaluated.

The San Francisco Bay Study Unit Overview of Water Quality

GAMA’s Priority Basin Project evaluates the quality of untreated groundwater. However, for context, benchmarks established for drinking-water quality are used for comparison. Benchmarks and definitions of high, moderate, and low concentrations are discussed in the inset box on page 3.

Many inorganic constituents occur naturally in groundwater. The concentrations of the inorganic constituents can be affected by natural processes as well as by human activities. In the San Francisco Bay study unit, one or more inorganic constituents were present at high concentrations in about 5% of the primary aquifer system and at moderate concentrations in about 25%.

Human-made organic constituents are found in products used in the home, business, industry, and agriculture. Organic constituents can enter the environment through normal usage, spills, or improper disposal. In this study unit, one or more organic constituents were present at high concentrations in less than 1% of the primary aquifer system and at moderate concentrations in about 12%.

RESULTS: Groundwater Quality in the San Francisco Bay Study Unit

INORGANIC CONSTITUENTS

minor elements

SPECIAL-INTEREST CONSTITUENT Trace and

radioactive constituents

Perchlorate Inorganic Constituents with Human-Health Benchmarks

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. In the San Francisco Bay study unit, barium was the only trace element detected at high concentrations (in about 3% of the primary aquifer system).

Radioactivity is the release of energy or energetic particles during spontaneous decay of unstable atoms. Most of the radioactivity in groundwater comes from the decay of naturally occurring isotopes of uranium and thorium in minerals in the sediments of the aquifer. Radioactive constituents were not detected at high concentrations in the primary aquifer system, and were detected at moderate concentrations in <1% based on the spatially weighted approach.

Nutrients, such as nitrate and nitrite, can be naturally present at low concentrations in groundwater. High and moderate concentrations generally occur as a result of human activities, such as applying fertilizer to crops. Livestock, when in concentrated numbers, and septic systems also produce nitrogenous waste that can leach into groundwater. Nutrients were present at high concentrations in about 2% of the primary aquifer system and at moderate concentrations in about 19% of the primary aquifer system.

Inorganic Constituents with Non-Health Benchmarks

(Not included in water-quality overview charts shown on the front page)

Some constituents affect the aesthetic properties of water, such as taste, color, and odor, or may create nuisance problems, such as scaling and staining. The State of California has a recommended and an upper limit for total dissolved solids (TDS) in drinking water. All groundwater naturally contains TDS as a result of the weathering and dissolution of minerals in the sediments. Anoxic conditions in groundwater (low amounts of dissolved oxygen) may result in release of manganese and iron from minerals into groundwater.

In the San Francisco Bay study unit, salinity indicator concentrations (TDS and/ or chloride) were high in about 7% of the primary aquifer system. About 37% of the primary aquifer system had moderate concentrations of salinity indicators. Iron and manganese are naturally occurring elements, and they were present (one or both) at high concentrations in about 12% of the primary aquifer system and at moderate concentrations in about 12% of the primary aquifer system.

Perchlorate

(Not included in water-quality overview charts shown on the front page)

Perchlorate is an inorganic constituent which has been regulated in California drinking water since 2007. It is an ingredient in rocket fuel, fireworks, and other products, may be present in some fertilizers, and occurs at low concentrations under natural conditions in groundwater. Perchlorate was not detected at high concentrations in the primary aquifer system and was detected at moderate concentrations in 42%.

RESULTS: Groundwater Quality in the San Francisco Bay Study Unit

BENCHMARKS FOR EVALUATING GROUNDWATER QUALITY

GAMA’s Priority Basin Project uses benchmarks established for drinking water to provide context for evaluating the quality of untreated groundwater. After withdrawal, groundwater may be disinfected, filtered, mixed, and exposed to the atmosphere before being delivered to consumers. Federal and California regulatory benchmarks for protecting human health (Maximum Contaminant Level, MCL) were used for the evaluation when available. Otherwise, non-regulatory benchmarks for protecting aesthetic properties (Secondary Maximum Contaminant Level, SMCL), such as taste and odor, and non-regulatory benchmarks for protecting human health (Notification Level, NL, and Lifetime Health Advisory, HAL) were used.

High, moderate, and low concentrations are defined relative to benchmarks

Concentrations are considered high if they are greater than a benchmark. For inorganic constituents, concentrations are moderate if they are greater than one-half of a benchmark. For organic and special-interest constituents, concentrations are moderate if they are greater than one-tenth of a benchmark; this lower threshold was used because organic constituents generally are less prevalent and have smaller concentrations relative to benchmarks than inorganic constituents. Low includes nondetections and values less than moderate concentrations. Methods for evaluating water quality are discussed in Parsons and others (2013).

Herbicides Organic Constituents

The Priority Basin Project uses laboratory methods that can detect low concentrations of volatile organic compounds (VOCs) and pesticides, far below human-health benchmarks. The presence of VOCs and pesticides detected at these low concentrations can be used to trace water from the landscape into the aquifer system.

Volatile Organic Compounds with Human-Health Benchmarks

VOCs are used in many household, commercial, industrial, and agricultural products and are characterized by their tendency to volatilize (evaporate) into the air.

Solvents are used for a number of purposes, including manufacturing and cleaning. In the San Francisco Bay study unit, solvents were present at high concentrations in less than (<) 1% of the primary aquifer system. The solvent detected at high concentrations was tetrachloroethene. Solvents were detected at moderate concentrations (mainly tetrachloroethene and trichloroethene) in about 5% of the primary aquifer system.

Trihalomethanes may form during municipal water purification and can enter groundwater by the infiltration of landscape irrigation water. Trihalomethanes were detected at moderate concentrations in about 2% of the primary aquifer system.

Other VOCs include organic synthesis reagents and gasoline additives. Other VOCs were detected at high concentrations in <1% of the primary aquifer system and at moderate concentrations in about 2% of the primary aquifer system. The compound detected at high concentrations was the gasoline additive methyl tert-butyl ether (MTBE).

Pesticides with Human-Health Benchmarks

Pesticides are applied to crops, gardens, lawns, around buildings, and along roads to help control unwanted vegetation (weeds), insects, fungi, and other pests. In the San Francisco Bay study unit, insecticides (including fumigants) were not detected at high or moderate concentrations in the primary aquifer system. Herbicides were detected at moderate concentrations in about 2% of the primary aquifer system, reflecting moderate concentrations of atrazine.

Salinity in the San Francisco Bay Study Unit

By Mary C. Parsons (formerly Mary C. Ray), Justin T. Kulongoski, and Kenneth Belitz

Priority Basin Assessments

GAMA’s Priority Basin Project (PBP) assesses water quality in that part of the aquifer system used for drinking water, primarily public supply. Water quality in the primary aquifer system may differ from water quality in shallower and deeper parts of the aquifers. GAMA’s Domestic Well Project assesses water quality in the shallower parts of the aquifer system. Ongoing PBP assessments are being conducted in more than 120 basins throughout California.

The PBP assessments are based on a comparison of constituent concentrations in untreated groundwater with benchmarks established for the protection of human health and for aesthetic concerns. The PBP does not evaluate the quality of drinking water delivered to consumers.

The PBP uses two scientific approaches for assessing groundwater quality. The first approach uses a network of wells to statistically assess the status of groundwater quality. The second approach combines water-quality, hydrologic, geographic, and other data to help assess the factors that affect water quality. In the San Francisco Bay study unit, data were collected by the PBP in 2007 and from the CDPH database for 2004–2007. The PBP includes chemical analyses not generally available as part of regulatory compliance monitoring, including measurements at concentrations much lower than human-health benchmarks, and measurement of constituents that can be used to trace the sources and movement of groundwater.

Where this page came from

This page was imported from U.S. Geological Survey. Published by the U.S. Geological Survey and, as a work of the United States government, in the public domain.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.