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Last Edited: 4/12/2011

Drinking Water Monitoring and Regulation

The drinking water standards are set at levels necessary to protect the public from acute and chronic health risks associated with consuming contaminants in drinking water supplies. These limits are known as Maximum Contaminant Levels (MCLs).  Water systems are required to serve drinking water that meets all drinking water standards, as well as to conduct routine sampling and analysis of their drinking water supplies to certify compliance.

Learn more about how drinking water quality is regulated and monitored:


Drinking Water Standards

Community water systems are required to provide drinking water that meets standards established by the US Environmental Protection Agency (EPA) under the Safe Drinking Water Act.  The EPA establishes drinking water standards for individual contaminants and groups of contaminants.  Typically, EPA establishes Maximum Contaminant Levels (MCLs) and associated compliance monitoring requirements.  

When it is not feasible to measure a contaminant in drinking water, EPA establishes drinking water Treatment Technique Requirements (TTR), based on indicators of water quality.  These are requirements about the type of treatment required and measures of how well these treatment processes are working. Compliance with MCLs and TTRs is the basis of determining whether the drinking water meets public health standards.

In establishing a MCL or TTR, EPA evaluates

  • studies on health effects (toxicology and epidemiology)
  • studies on the occurrence of the contaminant in water
  • studies on the effectiveness and cost of treatment available to remove the contaminant

Based on analysis of this information, EPA sets a MCL Goal. This goal is set at a level at which no known or anticipated adverse effects on the health of persons occur and allows an adequate margin of safety. EPA also sets an enforceable standard, which is either a MCL or a TTR. The enforceable standard is set as close to the MCL Goal as technologically and economically feasible.

MCLs and TTRs apply to all community water systems.  However, the associated monitoring requirements vary for the following reasons:

  • Monitoring requirements are contaminant- or contaminant group- specific
  • Regulations and state drinking water agencies specify sample location and acceptable analytical methods
  • The frequency of monitoring for a contaminant may vary based on the type of source water and based on the results of previous samples
  • Surface water systems typically monitor more frequently than groundwater systems because the occurrence of contaminants is more variable over time
  • Systems that do not detect contaminants or detect them only at very low levels compared to the MCL can monitor less frequently
  • Monitoring may also vary based on service population size and water treatment used

Drinking water standards and monitoring requirements are not static. For example, new regulations may be developed for previously unregulated contaminants. Also, EPA must periodically review and, if necessary, revise existing regulations.  Revisions are based on new information on health effects, treatment, analytical methods, and contaminant occurrence.

Drinking water standards – the maximum contaminant levels (MCLs) – can be found in Title 22 of the California Code of Regulations . Primary MCLs address health concerns. Secondary MCLs address esthetics, such as taste and odor. Arsenic, nitrate, and disinfection byproducts have primary MCLs.

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Challenges of Complex Water Delivery Pathways

Since water quality standards were created to keep contaminants under an MCL, information about population-level exposures is not typically reportable, nor centralized. Water quality data does not tell us where water flows after sampling (see Figure 1).

water delivery pathways

Figure 1. Water delivery pathways, including water sources, treatment areas, water quality samples and recipient consumers.

Additional challenges for utilizing water quality data include:

  • Locations of water quality samples can also affect the data. Samples can be taken at the beginning, end, or in the middle of any community water system.
  • Frequency of testing in water systems plays a role in exposure assessment since different water systems may be tested with different frequencies throughout the year.
  • Lack of geographic data also plays a role.  It is often unknown where the service areas of water systems are, and it is difficult to gauge the population distribution of Community Water Systems (CWSs).
  • Lack of data on consumption is another challenge.  Water consumption patterns (bottled, filter, tap) in a population are largely unknown.  Bottled and vended water is not subject to the US EPA drinking water regulations. In California, bottled and vended water are regulated as food, under California Department of Public Health’s Food and Drug Branch purview.

Due to the above challenges, the ability to generate estimates of exposures to contaminants in drinking water depends to a great extent on the availability and quality of water data. Most of this data is collected for compliance and regulatory purposes, not for health tracking. Consequently, this site identifies the data gaps to a larger degree than providing accurate assessment of exposures and population at risk. For more discussion on the challenges and data limitations, see Drinking Water Measures and Limitations.

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Population Served by Community Water Systems

Community water systems (CWS) are required to report to the State the number of people they serve.  CWSs must estimate this number by one of three methods: 

  • using the US Census
  • multiplying the number of service connections by 3.3
  • determining the total number of certain dwelling units, mobile home park spaces, individual businesses, and billing units served by the water system and multiplying this total by 2.8 (California Code of Regulations, Title 22, Article 2, Section 64412)

Also, a single service connection may be a large entity (such as an institutional facility with year-round residents) with a single water meter, where water is provided to hundreds of individuals a day. The reported estimates of population served by CWSs in California are often inaccurate and at times overestimated as compared to the US Census data.

According to Drinking Water Program’s 2006 annual compliance report, about 97% of Californians (approximately 36.6 million people) received their water from public water systems.  The remaining residents received from private wells or other sources.  This figure is likely innacurate, since there is no data source that can provide a precise estimate. 

The US Geological Survey (USGS) Water-Use Report estimated that in 2000, 89% of the California population received drinking water from the public water supply.  Within the United States, the range of populations served by community water systems as the primary drinking water source in homes varies from 97% to as low as 45%.

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