By Brooke K. Mayer, PhD, PE

More than 90 percent of Americans are served by publicly or privately owned public water systems. These systems are defined as providing “water for human consumption through pipes or other constructed conveyances to at least 15 service connections or serving an average of at least 25 people for at least 60 days a year.”1 Figure 1 provides a brief snapshot of public water systems in the US. As of the end of 2020, small drinking water systems serving ≤10,000 people comprised 95 percent of the roughly 150,000 active public water systems.2 The majority of these systems were very small, serving ≤ 500 people. Note that “small systems” typically refers to those serving ≤10,000 people, although US EPA also defines a more granular classification system: very small systems serving ≤500 people, small systems serving 501 to 3,300 people, medium systems serving 3,301 to 10,000 people, large systems serving 10,001 to 100,000 people and very large systems serving > 100,000 people.

Figure 1. Overview statistics of public water systems in the United States, including distribution by system classification (community; non-transient, non-community and transient, non-community), source water (surface water [SW] or groundwater [GW]) and system size. Data and definitions are from US EPA.2,3

Current Safe Drinking Water Act regulations
Fortunately, US water systems are among the most reliable and effective in the world. US EPA’s Safe Drinking Water Act defines health-based, legally enforceable standards for approximately 90 contaminants in public water systems (including microorganisms, disinfectants, DBPs, inorganic chemicals, organic chemicals and radionuclides).4 Table 1 summarizes the drinking water rules grouped by contaminant type.4 In spite of tremendous success complying with these standards, infrequent drinking water violations do occur. Allaire et al. (2018)5 analyzed 17,900 community water systems over the period 1982-2015 and found that in any given year, 3–10 percent of the systems incurred health-based violations of the Safe Drinking Water Act, annually impacting an average of 19 million people.

Through their nationwide analyses, Allaire et al. (2018)5 and Rubin (2013)6 concurred that smaller systems were no more likely than larger systems to violate health-based standards. With the exception of very large systems (serving >100,000 people, which reported lower frequency violations), all community water systems experienced approximately the same frequency of health-related violations, with roughly 10 percent of the systems in each size category reporting violations.6 While small systems are no more likely than large systems to violate health-based standards, many small systems face additional unique financial and operational challenges in consistently providing safe, reliable drinking water. Such challenges can include lack of expertise to choose, operate and maintain systems, lack of financial resources, aging infrastructure, limited options for residuals disposal and limited resources for states to support large numbers of small systems.2

In particular, community water systems operated in rural counties (especially those relying on surface water sources) generally experience higher violation frequency compared to urban counties. For example, rates of total coliform violations were 56-percent higher in rural counties versus urban (and 76-percent higher across all violation categories). Low-income rural areas also experienced a larger compliance gap than higher-income rural communities. DBPs accounted for an inordinate fraction of the increased rural violations, with three times higher incidence compared to urban counties. This increased DBP formation may be attributed to higher water age associated with low-density housing in rural areas, in addition to less technical capacity and financial resources to upgrade infrastructure and perform frequent system flushes.5

Prior to 2003, total coliforms were the most common type of violation, while other violations (including DBPs, radionuclides and inorganic and organic chemicals) become more prevalent in subsequent years, likely as a result of new DBP and radionuclide regulations.5 Indeed, spikes in violations were observed immediately following implementation of new federal regulations such as the Disinfectants and Disinfection Byproducts Rules.5 During this post-regulatory implementation period, drinking water systems likely experience a dramatic learning curve and lag in adjustment.5 This emphasizes the importance of drinking water systems at all scales familiarizing themselves with the future regulatory horizon.

Contaminant Candidate List (CCL) and Unregulated Contaminant Monitoring Rule (UCMR)
As part of the 1996 amendments to the Safe Drinking Water Act, US EPA established an approach to assist with periodic reviews of “regulatory determination” for drinking water contaminants in light of the following three criteria7:

  • The contaminant may have an adverse effect on the health of persons;
  • The contaminant is known to occur or there is substantial likelihood the contaminant will occur in public water systems with a frequency and at levels of public health concern;
  • In the sole judgment of the Administrator, regulation of the contaminant presents a meaningful opportunity for health risk reductions for persons served by public water systems.

This procedure established the Contaminant Candidate List (CCL), which identifies chemical and biological contaminants as priority research targets as they are not yet regulated, but are known or believed to occur in public water systems. The CCL is updated approximately every five years based on public and expert input in light of current understanding of contaminant prevalence, health impacts, treatability, etc. The current version of the CCL (CCL 4) includes 97 chemicals or chemical groups (e.g., commercial chemicals, pesticides, toxins, DBPs and pharmaceuticals) and 12 microbial contaminants (including viruses, bacteria and protozoa). Nominations for the 5th CCL are now closed as US EPA develops the updated list for draft and public comment.7

In coordination with the CCL, the UCMR was developed for monitoring priority unregulated contaminants in drinking water (no more than 30 unregulated contaminants every five years). Under this program, priority unregulated contaminants are monitored for all large public water systems. Depending on the availability of appropriations and sufficient laboratory capacity, a representative sample of small systems serving <10,000 people is also monitored. Under the most recently completed UCMR (2018 – 2020), all large systems as well as 800 randomly selected small systems (serving 25 – 10,000 people) sourced with surface water or groundwater under the direct influence of surface water monitored for 10 cyanotoxins. Additionally, all large systems and a different group of 800 randomly selected small systems monitored for the 20 other UCMR 4 chemicals. US EPA covers the costs of UCMR sample analysis for systems serving <10,000 people.8,9

UCMR data are one of the primary inputs into the robust, national-level dataset, the National Contaminant Occurrence Database (NCOD). Community water systems must inform their consumers of UCMR monitoring results (average and range of detection), whereas non-community systems must inform consumers of data availability. Out of the 919,233 samples collected by 2,535 public water systems reported in the UCMR 4 Data Summary from April 2021,[10] nine percent were from small systems. In keeping with previous findings5,6, the relative number of small system exceedances (8.5 percent of the total) was essentially congruent with small system representation in the full dataset. The most commonly reported exceedances of the reference contaminant levels are shown in Figure 2.9

Figure 2. Exceedances of unregulated contaminant reference level (health-based concentrations to give context to UCMR measurements, not to be interpreted as regulatory or action levels). Data were derived from US EPA’s UCMR 4 for the period 2018-2020 (as of publication of the April 2021 data summary10). The data labels above each bar indicate the percentage of the total samples or the percentage of the small system (serving ≤ 10,000 people) samples exceeding the reference levels. The remainder of the 30 total UCMR contaminants were either not detected at levels above the maximum reporting level (1-butanol, 2-propen-1-ol, microcystin-LY, microcystin-LA, microcystin-LF, microcystin-LR, microcystin-RR, microcystin—YR, chloropyrifos, dimethipin, ethoprop, oxyfulorfen, tebuconazole, total pertmethrin and tribuofos) or do not have a health-based reference concentration (anatoxin-a, nodularin, HAA6Br, HAA9, germanium, butylated hydroxyanisole and o-toluiduine).

Looking forward to future data collection, US EPA published the UCMR 5 proposal to the Federal Register on March 11, with sample collection from January 2023 through December 2025. As proposed, all systems serving >3,000 people, as well as a representative sample of smaller systems, will collect samples for analysis. Small systems are required to notify US EPA by December 31, 2022 of sampling location and inventory changes and if they are unable to sample according to the established schedule.

On the horizon
The Safe Drinking Water Act stipulates that US EPA use current information such as that from the UCMR to make regulatory determinations on at least five contaminants from the current CCL. Based on current understanding of contaminants on the CCL 4, the agency has decided to move forward with national primary drinking water regulations for two per- and polyfluoroalkyl substances (PFAS): perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). The current US EPA advisory level for the sum of PFOS and PFOA is 70 ng/L. It was proposed that six other contaminants not be regulated: 1,1-dichloroethane, acetochlor, methyl bromide (bromomethane), metolachlor, nitrobenzene and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX).11

Moving forward, public water systems can begin preparations for these anticipated regulatory additions. Pre-emptive actions may help to flatten the future learning curve and facilitate more rapid adjustments to the changing regulatory landscape, which can be particularly important for small system compliance. Available resources include US EPA’s PFAS information webpage12, its presentation on PFAS and small systems13 as well as its small drinking water systems research webpage.14


  1. US EPA. Information about Public Water Systems. 2020. Accessed May 6, 2021.
  2. US EPA. 18th Annual EPA Drinking Water Workshop: Small System Challenges and Solutions. Published 2021. Accessed May 6, 2021.
  3. US EPA. Water System Facts and Figures, 2001. Published 2001. Accessed May 6, 2021.
  4.  US EPA. Drinking Water Regulations. sets legal limits on,that water systems must follow. Published 2017. Accessed May 6, 2021.
  5. Allaire M, Wu H, Lall U. National trends in drinking water quality violations. PNAS. 2018;115(9):2078-2083. doi:10.1073/pnas.1719805115
  6. Rubin SJ. Evaluating violations of drinking water regulations. J Am Water Works Assoc. 2013;105(3):51-52. doi:10.5942/jawwa.2013.105.0024
  7. US EPA. Contaminant Candidate List (CCL) and Regulatory Determination. Accessed May 6, 2021.
  8. US EPA. Fourth Unregulated Contaminant Monitoring Rule. Published 2021. Accessed May 6, 2021.
  9. US EPA. Learn About the Unregulated Contaminant Monitoring Rule. Published 2021. Accessed May 6, 2021.
  10. US EPA. Data Summary of The Fourth Unregulated Contaminant Monitoring Rule. 2021. Accessed May 6, 2021.
  11. US EPA. Regulatory Determination 4. Published 2021. Accessed May 6, 2021.
  12. US EPA. Per- and Polyfluoroalkyl Substances (PFAS). Published 2021. Accessed May 6, 2021.
  13. Speth T, Gillespie A, Impellitteri C, et al. US EPA’s Research on PFAS and Small Systems. 2019. Accessed May 6, 2021.
  14. US EPA. Small Drinking Water Systems Research. Published 2020. Accessed May 6, 2021.

About the author
Dr. Brooke K. Mayer is an Associate Professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds bachelors, masters and doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University. She is a registered Professional Engineer in the state of Arizona.


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