Population Served by Community Water Systems with No Reported Violations of Health-Based Standards

  • Introduction
    • Community water systems (CWS) are public water systems that supply water to the same population year-round. In fiscal year (FY) 2021, more than 315 million Americans (U.S. EPA, 2021)—roughly 95 percent of the U.S. population (U.S. Census Bureau, 2021)—got at least some of their drinking water from a CWS. This indicator presents the percentage of Americans served by CWS for which states reported no violations of EPA health-based standards for more than 90 contaminants (U.S. EPA, 2022a).

      Health-based standards include Maximum Contaminant Levels (MCLs), Maximum Residual Disinfection Levels (MRDLs), and Treatment Techniques (TTs). An MCL is the highest level of a contaminant that is allowed in drinking water. An MRDL is the highest level of a disinfectant allowed in drinking water (U.S. EPA, 2022c). A TT is an enforceable procedure or level of technological performance which public water systems must follow to ensure control of a contaminant (U.S. EPA, 2022b). TTs are adopted where it is not economically or technologically feasible to ascertain the level of a contaminant, such as microbes, where even single organisms that occur unpredictably or episodically can cause adverse health effects. Compliance with TTs may require a variety of actions to protect public health, such as assessment of the system, filtration and disinfection, and optimized corrosion control (U.S. EPA, 2022b).

      This indicator tracks the population served by CWS for which no violations of health-based standards were reported to EPA annually for the period from FY 1993 to FY 2021, the latest year for which data are available. Results are reported as a percentage of the overall population served by CWS, both nationally and by EPA Region. This indicator also reports the number of persons served by systems with reported violations of standards covering surface water treatment, microbial contaminants (microorganisms that can cause disease), disinfection byproducts (chemicals that may form when disinfectants, such as chlorine, react with naturally occurring materials in water and may pose health risks), and other contaminants. The indicator is based on violations reported quarterly by EPA, states, territories, and the Navajo Nation, who each review monitoring results for the CWS that they oversee.


  • What the Data Show
    • Of the population served by CWS nationally, the percentage served by systems for which no health-based violations were reported for the entire year increased overall from 79 percent in 1993 to 92 percent in FY 2021 (Exhibit 1). Drinking water regulations have changed in recent years. This indicator is based on reported violations of the standards in effect in any given year.

      When results are broken down by EPA Region, some variability over time is evident (Exhibit 2). Between FY 1998 and FY 2021, most Regions were consistently above the national percentage. Only Region 2 remained consistently below the national percentage over the entire period of record, largely because of a small number of public water systems serving large populations.

      In FY 2021, reported violations involving surface water treatment rules were responsible for exceeding health-based standards for 15.6 million people (4.9 percent of the population served by CWS nationally). Reported violations of the health-based disinfection byproducts rules (Stage 2) affected 4.3 million people (1.4 percent of the CWS-served population) (Exhibit 3).

  • Limitations
      • Non-community water systems (typically small systems) that serve only transient populations such as restaurants or campgrounds, or serving those in a non-domestic setting for only part of their day (e.g., a school, hospital, or office building), are not included in population served figures.
      • Domestic (home) use of drinking water supplied by private wells is not included. More than 13 million households get at least some of their drinking water from private wells (U.S. EPA, 2022d).
      • Bottled water, which is regulated by standards set by the Food and Drug Administration, is not included.
      • National statistics based on population served can be volatile, because a single very large system can sway the results by up to 2 to 3 percent. This effect becomes more pronounced when statistics are broken down at the regional level, and still more so for a single rule.
      • Some factors may lead to overstating the extent of population served by systems that violate standards. For example, the entire population served by each system in violation is reported, even though only part of the total population served may actually receive water that is out of compliance. SDWIS data does not indicate whether any, part, or all of the population served by a system receives water in violation. Therefore, there is no way to know how many, if any, people are actually drinking water in violation. In addition, violations stated on an annual basis may suggest a longer duration of violation than may be the case, as some violations may be as brief as an hour or a day.
      • Other factors may lead to understating the population served by systems that violate standards. For instance, CWS that purchase water from other CWS are not always required to sample for all contaminants themselves.
      • Under-reporting and late reporting of violations by states to EPA affect the ability to accurately report the national violations total.
      • Data reviews and other quality assurance analyses indicate that the most widespread data quality problem is under-reporting of monitoring violations. Even though these violations are separate from the health-based violations covered by this indicator, failures to monitor could mask violations of TTs, MRDLs, and MCLs.


  • Data Sources
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