Glossary

1,2,3-Trichloropropane (1,2,3-TCP) is a manmade chemical found at industrial and hazardous waste sites. It has been used as a cleaning and degreasing solvent and in pesticide products. 1,2,3-TCP is suspected to cause cancer in humans based on evidence from animal studies. Human exposure can occur from drinking water containing 1,2,3-TCP. The established State Maximum Contaminant Level (MCL) in drinking water is 0.005 µg/L, or 5 parts per trillion. Public water systems statewide have been required to test for 1,2,3-TCP quarterly in their drinking water sources since January 2018. Based upon data collected by the State Water Board between 2007 and 2017, 395 public water supply wells (of 5,863 sampled) had detections above the MCL. Download a factsheet for more information (English / Español).

This layer shows the locations for all domestic wells, including those that may not be in current use, or are in an area with no population, or are within the boundaries of a CWS. These data came from the Department of Water Resources’ Online System for Well Completion Reports (OSWCR). Private domestic well location data is available at the section level. A section is a one-mile squared unit of the National Public Land Survey System (PLSS). Points representing wells are displayed at the center of PLSS sections. Visit the Methodology page to learn how the likely domestic well community layer was constructed.

The alluvial boundary defines the extent of the alluvial deposits in California’s Central Valley including the Sacramento, San Joaquin, and Tulare Lake groundwater basins as defined by California’s Department of Water Resources Bulletin 118 (Faunt 2012).  Alluvial deposits are generally shallow sand and gravel deposits laid down over time in a river channel or floodplain. The name “alluvial” refers to the loose, unlayered nature of the material – often silt, clay, sand, and gravel. The alluvial boundary is the geographic extent of the drought scenario analysis for private domestic wells and small community water systems in the Central Valley.

An aquifer is a geologic formation or structure that is water-bearing and can store and/or transmit water via wells and springs. Use of the term is usually restricted to those water-bearing formations capable of producing a usable supply for people’s needs.

Arsenic is a naturally occurring element in rock formations that can be released into groundwater. Human activities like industry and agriculture can release arsenic into the environment and cause it to move to new areas and/or become concentrated in water supplies. Exposure to arsenic is associated with cancers of the lung, liver, kidney and skin, and has been linked to diabetes and cardiovascular disease in humans. The current maximum contaminant level for arsenic in California is 10 µg/L.  Download a factsheet for more information (English / Español).

A Bulletin 118 Groundwater Basin is a groundwater basin defined and characterized by the Department of Water Resources’ Bulletin 118, the official publication on the occurrence and nature of groundwater statewide. Bulletin 118 also provides information and recommendations on groundwater management (see Bulletin 118).

CalEnviroscreen 3.0 (CES) is a screening tool used to identify communities that are disproportionately burdened by multiple sources of pollution and possess population characteristics that make them more sensitive to pollution. CES was developed by California’s Office of Environmental Health Hazard Assessment. Review OEHHA’s resources to learn more about CalEnviroScreen 3.0  and its water quality data.

A community water system (CWS) is a publicly regulated water supplier that serves at least 25 people year-round or has at least 15 service connections (i.e. households). It can be operated or managed by a city, county, water district,  investor-owned utility, mobile home park, etc.. This tool has information about 2,851 active community water systems (as of January 2019) as described in Data.

The Consumer Confidence Report (CCR) is the annual water quality report that community water systems are required to distribute to customers. CCR’s include important information about whether water supplied is at, above or below maximum contaminant levels (MCLs). The CCR for most community water systems in California can be obtained by visiting the Public Drinking Water Watch database or contacting the system directly.

This is the total cost of the mitigation measures for domestic wells, per drought scenario in the Groundwater Supply layers (see Gailey 2020 for more information).

This is the total cost of the mitigation measures for small community water system public supply wells, per drought scenario in the Groundwater Supply layer (see Gailey 2020 for more information).

Counties are local administrative units and have different drinking water and groundwater oversight responsibilities. Counties have existing and new responsibilities under the Sustainable Groundwater Management Act (SGMA). Each county has information about the number of water systems, disadvantaged communities (census places), groundwater basins, and groundwater sustainability agencies in the California Water Data Tool. Visit the Data page for more information.

A disadvantaged community (DAC) is one with an average Median Household Income (MHI) of less than 80% of California’s overall MHI (Public Resources Code). In 2021, the statewide MHI was $84,097: the calculated DAC threshold is $67,278. A severely disadvantaged community (SDAC) is one with an average MHI of less than 60% of California’s overall MHI: the calculated SDAC threshold is $50,458 (2021). 

A drought scenario in this tool is a scenario of groundwater level decline that is based on observations from the 2012 to 2016 drought. Groundwater level decline was based on a scaled version of the 2012 to 2016 drought (drought factors of 0.5, 0.75, and 1.0) for each drought scenario of 50%, 75%, and 100%, respectively. Impacts and costs to remediate impacts were calculated for small community water systems (<10,000 people served) and private domestic wells in the Central Valley (See Gailey 2020 for more information).

Groundwater is water that occurs beneath the land’s surface and fills the pore spaces of the alluvium, soil, or rock formation in which it is situated. It excludes soil moisture.

A groundwater basin is an aquifer or series of aquifers with reasonably well-defined boundaries. The hydrologic characteristics and defined basin boundaries of California’s groundwater basins are described in DWR’s Bulletin 118 (DWR Bulletin 118).

A Groundwater Sustainability Agency (GSA) is a self-determined groundwater management unit created as part of the implementation of the Sustainable Groundwater Management Act.

Groundwater Sustainability Plans (GSP) are plans developed by Groundwater Sustainability Agencies. GSPs outline local groundwater conditions and establish a clear and achievable path, including projects and management actions, to achieving sustainability.

Groundwater wells are a means of accessing water in underground aquifers. There are two main types of groundwater wells: driven and drilled wells.  Driven wells are built by driving a small-diameter pipe into soft earth, such as sand or gravel. A screen is usually attached to the bottom of the pipe to filter out sand and other particles. Driven wells use water sources close to the surface, are cased continuously and shallow (approximately 30 to 50 feet deep). Drilled wells use drilling equipment and can be drilled more than 1,000 feet deep.  Most modern wells are drilled, which requires a fairly complicated and expensive drill rig. In both types, a pump is often placed in the well at some depth to push the water up to the surface. Community water system supply wells (public supply wells) tend to be deeper than household wells (private domestic wells) (Learn More).

The boundary of a Groundwater Sustainability Agency (GSA) is formed in compliance with the State Groundwater Management Act (SGMA).  Data in this tool is based on the Exclusive GSA boundaries available from the Department of Water Resources.

Hexavalent Chromium or Chromium (VI) (Cr(VI) is a carcinogen that can be found in many drinking water systems. Exposure through skin contact, drinking, eating, or inhaling is associated with cancer, and liver and kidney damage. Animal studies show that drinking water containing Cr(VI) can cause cancers of the mouth and intestines. There is no federal maximum contaminant level (MCL) for Cr(VI), but California is in the process of establishing an MCL of 10 µg/L. Download a factsheet for more information (English / Español).

In 2012, California Governor Brown signed Assembly Bill 685, making California the first state in the nation to legislatively recognize the human right to water. The bill states that “every human being has the right to safe, clean, affordable and accessible water adequate for human consumption, cooking, and sanitary purposes”.

The Human Right to Water (HR2W) Portal was launched after California statutorily recognized that “every human being has the right to safe, clean, affordable, and accessible water adequate for human consumption, cooking, and sanitary purposes.” The HR2W Portal provides information and data about progress toward this goal and serves as the state’s primary location for information on water system performance measures, like compliance with the Safe Drinking Water Act (HR2W Portal).

An impacted private domestic well is one with at least one of four impacts per drought scenario. The impacts considered are: increased pumping lift, pump cavitation, well screen clogging and wells running dry.

An impacted small community water system public supply well is one with at least one of four impacts per drought scenario. The impacts considered are: increased pumping lift, pump cavitation, well screen clogging and wells running dry. 

This layer combines four data sources to identify likely private domestic well communities at the Public Land Survey System (PLSS) section level: 1) domestic well locations, 2) 2010 census block-level population estimates, 3) community water system locations, and 4) residential parcel locations. A likely domestic well community is defined as a section that has at least one domestic well, intersects a populated census block (2010) and a residential parcel, and is not served by a CWS.  Visit the Methodology page to learn how the likely domestic well community layer was constructed.

Limit of detection (LOD) is the lowest concentration of the substance being identified that can be reliably measured.  LOD refers to the detectable limit at which water quality contaminants can be reliably measured.

Lowering groundwater levels are defined as the lowering of the elevation of the groundwater table across a whole basin or locally. Groundwater elevation data is available from the Department of Water Resources and is measured as feet above or below mean sea level.

The maximum contaminant level (MCL) is the highest level of a contaminant allowed in drinking water set and enforced by the federal Environmental Protection Agency and the State Water Board. The MCL is set based on the best available treatment technology and cost considerations, therefore, the public health goal (PHG) may be lower than the formal regulatory and enforceable MCL standards (EPA 2019).

A maximum contaminant level (MCL) violation is issued when water from a community water system violates a Safe Drinking Water Act drinking water quality standard.

Median household income (MHI) values in this tool are the estimated 5-Year Average from the American Community Survey of the US Census (ACS). The Demographic layers (Census Tract, Block Group and Place) feature the period 2017-2021 5-year estimated MHI, and the Community Water System interactive layer includes demographics from the 2012-2016 ACS (More Info).

Micrograms (µg) per liter is a standard measurement used to measure how many micrograms of a contaminant are present in one liter of water: 1 µg is 0.001 milligrams.

Milligrams (mg) per liter is a standard measurement used to measure how many milligrams of a contaminant are present in one liter of water: one mg/L is equal to one-millionth of a liter, or one-part per million (ppm), and 1 mg is 1000 micrograms.

Nitrates are chemical compounds made up of nitrogen and oxygen. Nitrates are one of the most common kinds of groundwater contamination. Many fertilizers contain nitrogen and excess nitrate that can enter groundwater through runoff from agricultural areas, gardens, and lawns. Additional sources include animal and human waste and can enter groundwater from dairies, septic tanks, and sewer systems. Drinking water with high levels of nitrates can harm the respiratory and reproductive system, kidney, spleen, and thyroid. Nitrates are particularly harmful to infants. If there are high levels of nitrates in your drinking water, boiling, softening, and filtering will not reduce or remove nitrate, and may actually lead to higher concentrations. The current maximum contaminant level (MCL) for nitrate in California is 10 mg/L. Download a factsheet for more information English / Español).

Community water systems are classified based on the presence of certain source types (i.e. any surface water, or any groundwater influenced by surface water).  The primary water source of a community water system indicates the presence of a source type that determines the treatment and filtration requirements for that system. Because surface water and groundwater are treated differently, the primary water source type classification by state and federal agencies allows for higher monitoring and fulfills greater public health protection requirements. 

State and federal drinking water system databases define six categories of primary water sources: 1. groundwater under the direct influence of surface water; 2. purchased groundwater under the direct influence of surface water; 3. groundwater; 4. purchased groundwater; 5. surface water; and 6. purchased surface water (view full definition).

A private domestic well serves fewer than five connections (i.e., homes, apartments, etc.) or fewer than 25 individuals for more than 60 days of the year. These wells are not regulated under the Safe Drinking Water Act.

Public Health Goals (PHGs) are concentrations of contaminants in drinking water considered safe if ingested at that level continuously throughout life. PHGs are developed and published by the Office of Environmental Health Hazard Assessment (OEHHA) and are based solely on protecting public health. They do not take into account cost or the technology available to achieve that standard (OEHHA 2019). They are only goals and are not enforceable.

Public supply well location data is available at the section level. A section is a one-mile squared unit of the National Public Land Survey System (PLSS).

A public water system (PWS) is a water system that provides water for human consumption through 15 or more service connections and serves at least 25 or more people. It includes community water systems and other system classifications. For example, a PWS could serve households with year-round residents as well as restaurants, schools, hospitals, campgrounds, or gas stations (SWRCB 2019).

Race and ethnicity data in this tool is from the American Community Survey (ACS) of the US Census 5-Year Average. Each category is shown as a percent of the total population for each census geography (county, place, tract, block group) by eight different race/ethnicity categories. The interactive county layer and the census geographies in the demographic layers feature the 2017-2021 ACS 5-year estimated percentages. The interactive Community Water System layer shows data from the 2012-2016 ACS (More Info).

The Safe Drinking Water Act (SDWA) refers to federal and state laws that provide oversight for the drinking water quality of all water systems in the United States. It is enforced by the federal Environmental Protection Agency (EPA), which sets standards for drinking water quality and implement various technical and financial programs to ensure drinking water safety. In California, the SDWA enforcement authority is delegated to the State Water Resources Control Board by the federal EPA (EPA 2019).

The Public Land Survey System (PLSS) is one way to subdivide and describe public land in the United States. Public land is divided into 6×6 mile units called townships, which are subdivided into 36 1×1 mile grid squares, called sections. In the California Water Data Tool, domestic and public supply well locations are typically located at the center (or centroid) of each 1×1 mile section. This is the unit of analysis for the likely private domestic well community layer and the drought scenario analysis. 

A severely disadvantaged community (SDAC) is one with an average median household income (MHI) of less than 80% of California’s overall MHI.  In 2017, the statewide MHI was $67,169, therefore, the calculated SDAC thresholds is $40,301  (Public Resources Code).

A community water system (CWS) is a publicly regulated water supplier that serves at least 25 people year-round or has at least 15 service connections (i.e. households). A small community water system is a CWS that serves 10,000 people or fewer.

The boundaries of California’s 80 state assembly districts in the California Water Data tool are based on boundary lines published by the California Redistricting Commission. To find your assembly member, visit this site.

The boundaries of California’s 40 state senate districts in the California Water Data tool are based on boundary lines published by the California Redistricting Commission. To find your senator, visit this site.

A state small water system (SSWS) is a water system that does not meet the requirements of a public water system. Although it provides piped drinking water, it does not regularly serve drinking water to more than an average of 25 individuals daily for more than 60 days out of the year and has between five and fourteen service connections. The water quality of a SSWS is regulated differently than that of a public water system.

The California State Water Resources Control Board (State Water Board) is one of six branches of the California Environmental Protection Agency (EPA). The State Water Board oversees the allocation and use of the state’s water resources to various entities for agricultural irrigation, hydroelectric power generation, and municipal water supplies. It also enforces the Safe Drinking Water Act for public water systems.

Surface water is water obtained from streams, lakes, and reservoirs. Surface water used by community water systems is subject to the Surface Water Treatment Rule, which is intended to reduce illnesses caused by pathogens in drinking water through treatment and filtration requirements.

The Sustainable Groundwater Management Act (SGMA) was developed to ensure that groundwater is managed and used in a manner that does not cause undesirable results: chronic lowering of groundwater levels; reductions in groundwater storage; seawater intrusion; degraded water quality; land subsidence; and surface water depletions that have adverse impacts on beneficial users. Learn more SGMA related terminology from Community Water Center’s SGMA glossary, and how to get involved in local SGMA implementation at Getting Involved.

The Public Land Survey System (PLSS) is one way to subdivide and describe public land in the United States. Public land is divided into 6-mile square units called townships and townships are then further subdivided into 36 1×1 mile square sections. In the California Water Data Tool, township boundaries are included to indicate the spatial resolution of the water quality data that is used for the private domestic wells’ water quality information.

United States Congressional Districts. Updated 2022.

When a well is constructed, altered, or destroyed, a well completion report (WCR) is filed with the Department of Water Resources within 60 days of the completion of the work. Well drillers can submit WCRs with the Online System of Well Completion Reports (OSWCR). This OSWCR database is a foundational dataset for several layers in California Water Data Tool. The OSWCR contains both digitized completion reports from historical, paper records and more recently submitted online reports (More info).