2024-15807. National Primary Drinking Water Regulations; Announcement of the Results of EPA's Fourth Review of Existing Drinking Water Standards

Table 1—List of NPDWRs

Contaminants/parameters	MCLG (mg/L)  1 3	MCL or TT (mg/L)  2 3	Contaminants/parameters	MCLG (mg/L)  1 3	MCL or TT (mg/L)  2 3
Acrylamide	0	TT	Giardia lamblia  4	0	TT.
Alachlor	0	0.002	Glyphosate	0.7	0.7.
Alpha/photon emitters	0 (pCi/L)	15 (pCi/L)	Haloacetic acids (HAA5)	n/a 5	0.060.
Antimony	0.006	0.006	Heptachlor	0	0.0004.
Arsenic	0	0.010	Heptachlor epoxide	0	0.0002.
Asbestos	7 (million fibers/L)	7 (million fibers/L)	Heterotrophic bacteria 6	n/a	TT.
Atrazine	0.003	0.003	Hexachlorobenzene	0	0.001.
Barium	2	2	Hexachlorocyclopentadiene	0.05	0.05.
Benzene	0	0.005	Hexafluoropropylene oxide dimer acid (HFPO-DA)	10 (ppt)	10 (ppt).
Benzo[a]pyrene	0	0.0002	Lead	0	TT.
Beryllium	0.004	0.004	Legionella	0	TT.
Beta/photon emitters	0 (millirems/yr)	4 (millirems/yr)	Lindane	0.0002	0.0002.
Bromate	0	0.010	Mercury (inorganic)	0.002	0.002.
Cadmium	0.005	0.005	Methoxychlor	0.04	0.04.
Carbofuran	0.04	0.04	Monochlorobenzene (Chlorobenzene)	0.1	0.1.
Carbon tetrachloride	0	0.005	Nitrate (as N)	10	10.
Chloramines (as Cl 2 )	4	4.0	Nitrite (as N)	1	1.
Chlordane	0	0.002	Oxamyl (Vydate)	0.2	0.2.
Chlorine (as Cl 2 )	4	4.0	Pentachlorophenol	0	0.001.
Chlorine dioxide (as ClO 2 )	0.8	0.8	Perfluorohexane sulfonic acid (PFHxS)	10 (ppt)	10 (ppt).
Chlorite	0.8	1.0	Perfluorononanoic acid (PFNA)	10 (ppt)	10 (ppt).
Chromium (total)	0.1	0.1	Perfluorooctane sulfonic acid (PFOS)	0 (ppt)	4.0 (ppt).
Copper	1.3	TT	Perfluorooctanoic acid (PFOA)	0 (ppt)	4.0 (ppt).
Cryptosporidium	0	TT	PFAS Mixture (HFPO-DA, PFBS, PFHxS, & PFNA)	Hazard Index 12 of 1	Hazard Index of 1.
Cyanide (as free cyanide)	0.2	0.2	Picloram	0.5	0.5.
2,4-Dichlorophenoxyacetic acid (2,4-D)	0.07	0.07	Polychlorinated biphenyls (PCBs)	0	0.0005.
Dalapon	0.2	0.2	Radium 226/228 (combined)	0 (pCi/L)	5 (pCi/L).
Di(2-ethylhexyl)adipate (DEHA)	0.4	0.4	Selenium	0.05	0.05.
Di(2-ethylhexyl)phthalate (DEHP)	0	0.006	Simazine	0.004	0.004.
1,2-Dibromo-3- chloropropane (DBCP)	0	0.0002	Styrene	0.1	0.1.
1,2-Dichlorobenzene (o-Dichlorobenzene)	0.6	0.6	2,3,7,8-TCDD (Dioxin)	0	3 ×10 −8 .
1,4-Dichlorobenzene (p-Dichlorobenzene)	0.075	0.075	Tetrachloroethylene	0	0.005.
1,2-Dichloroethane (ethylene dichloride)	0	0.005	Thallium	0.0005	0.002.
1,1-Dichloroethylene	0.007	0.007	Toluene	1	1.
cis-1,2-Dichloroethylene	0.07	0.07	Total coliforms  7 8	n/a	TT.
trans-1,2-Dichloroethylene	0.1	0.1	Total Trihalomethanes (TTHM)	n/a 9	0.080.
Dichloromethane (methylene chloride)	0	0.005	Toxaphene	0	0.003.
1,2-Dichloropropane	0	0.005	2,4,5-TP (Silvex)	0.05	0.05.
Dinoseb	0.007	0.007	1,2,4-Trichlorobenzene	0.07	0.07.
Diquat	0.02	0.02	1,1,1-Trichloroethane	0.2	0.2.
E. coli	0	MCL,10 TT  8 11	1,1,2-Trichloroethane	0.003	0.005.
Endothall	0.1	0.1	Trichloroethylene	0	0.005.
Endrin	0.002	0.002	Turbidity 6	n/a	TT.
Epichlorohydrin	0	TT	Uranium	0	0.030.
Ethylbenzene	0.7	0.7	Vinyl Chloride	0	0.002.
Ethylene dibromide (EDB)	0	0.00005	Viruses	0	TT.
Fluoride	4.0	4.0	Xylenes (total)	10	10.
1  MCLG: the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur, allowing an adequate margin of safety. Maximum contaminant level goals are nonenforceable health goals.
2  MCL: the maximum level allowed of a contaminant in water which is delivered to any user of a public water system. TT: any action, process, or procedure required of the water system that leads to the reduction of the level of a contaminant in tap water that reaches the consumer.
3  Units are in milligrams per liter (mg/L) unless otherwise noted. Milligrams per liter are equivalent to parts per million. For chlorine, chloramines, and chlorine dioxide, values presented are MRDLG and MRDL.
4  The current preferred taxonomic name is Giardia duodenalis, with Giardia lamblia and Giardia intestinalis as synonymous names. However, Giardia lamblia was the name used to establish the MCLG in 1989. Elsewhere in this document, this pathogen will be referred to as Giardia spp. or simply Giardia unless discussing information on an individual species.
5  There is no MCLG for all five haloacetic acids. MCLGs for some of the individual contaminants are: dichloroacetic acid (zero), trichloroacetic acid (0.02 mg/L), and monochloroacetic acid (0.07 mg/L). Bromoacetic acid and dibromoacetic acid are regulated with this group but have no MCLGs.
6  Includes indicators that are used in lieu of direct measurements ( e.g., of heterotrophic bacteria, turbidity).
7  The Aircraft Drinking Water Rule (ADWR) 40 CFR part 141 subpart X, promulgated October 19, 2009, covers total coliforms and E. coli.
8  Under the RTCR, a PWS is required to conduct an assessment if it exceeded any of the TT triggers identified in 40 CFR 141.859(a). It is also required to correct any sanitary defects found through the assessment. 40 CFR 141.859(c).
9  There is no MCLG for total trihalomethanes (TTHM). MCLGs for some of the individual contaminants are: bromodichloromethane (zero), bromoform (zero), dibromochloromethane (0.06 mg/L), and chloroform (0.07 mg/L).
10  A PWS is in compliance with the E. coli MCL unless any of the conditions identified under 40 CFR 141.63(c) occur.
11  Under the GWR in 40 CFR 141.402, a ground water system that does not provide at least 4-log treatment of viruses and has a distribution system RTCR sample that tests positive for total coliform is required to conduct triggered source water monitoring to evaluate whether the total coliform presence in the distribution system is due to fecal contamination in the ground water source. The system must monitor for one of three State-specified fecal indicators ( i.e., E. coli, coliphage, or enterococci).
12  The Hazard Index is an approach that EPA uses to determine the health concerns associated with mixtures of certain PFAS in finished drinking water. The Hazard Index is made up of a sum of fractions. Each fraction compares the level of each PFAS measured in the water to the associated health-based water concentration.

IV. EPA's Protocol for Reviewing the NPDWRs Included in This Action

A. What was EPA's review process?

This section provides an overview of the process EPA used to review the NPDWRs discussed in this document. The protocol document, “EPA Protocol for the Fourth Review of Existing National Primary Drinking Water Regulations,” contains a detailed description of the process the agency used to review the NPDWRs (USEPA, 2024c). The foundation of this protocol was developed for the Six-Year Review 1 based on the recommendations of the National Drinking Water Advisory Council (NDWAC, 2000) and has undergone minor clarifications during each Six-Year Review cycle (USEPA, 2024c). Figure 1 presents an overview of the Six-Year Review protocol and the possible review outcomes.

The objective of the Six-Year Review process is to identify and prioritize NPDWRs for possible regulatory revision. The two major outcomes of the detailed review are either (1) the NPDWR is not appropriate for revision and no action is necessary at this time or (2) the NPDWR is a candidate for revision.

The reasons why EPA might list an NPDWR as “not appropriate for revision at this time” could include:

• Recently completed, ongoing, or pending regulatory action: The NPDWR was recently completed, is being reviewed under an ongoing action, or is subject to a pending action.
• Ongoing or planned health effects assessment: The contaminant or contaminants regulated by the NPDWR has an ongoing or planned health effects assessment.
• No new information: EPA did not identify any new relevant information for the contaminant since the last Six-Year Review that indicates changes to the NPDWR may be appropriate.
• Data gaps/emerging information: New information indicates a possible change to the MCLG and/or MCL but changes to the NPDWR are not appropriate due to data gaps and emerging information that needs to be evaluated.
• Low priority and/or no meaningful opportunity: New information indicates a possible change to the MCLG and/or MCL but changes to the NPDWR are not appropriate at this time due to one or more of the following reasons: (1) possible changes present negligible gains in public health protection; (2) possible changes present limited opportunity for cost savings while maintaining the same or greater level of health protection; and/or (3) possible changes are a low priority because of competing workload priorities, limited return on the administrative costs associated with rulemaking, and the burden on states and the regulated community associated with implementing any regulatory change that would result.

Alternatively, the reasons why an NPDWR could be listed as a candidate for revision are that the regulatory revision presents a meaningful opportunity to improve the level of public health protection, and/or achieve cost savings while maintaining or improving the level of public health protection.

Individual regulatory provisions that are evaluated as part of the Six-Year Review process include: MCLG, MCL, MRDLG, MRDL, TT, best available technology (BAT), and other requirements, such as monitoring requirements.

For example, the microbial regulations include TT requirements because no reliable, affordable, and technically feasible method is available to measure the microbial contaminants covered by those regulations. These TT requirements rely on the use of indicators that can be measured in drinking water, such as detection of total coliforms as an indicator of a potential pathway for pathogenic contamination in the distribution system. As part of the Six-Year Review 4, EPA evaluated new information related to the use of those indicators to determine if a meaningful opportunity to improve the level of public health protection exists. Results of EPA's review of the microbial regulations are presented in section V of this document.

Basic Principles

EPA applied several basic principles to the Six-Year Review process:

• The agency sought to avoid redundant review efforts. Because EPA has reviewed information for certain NPDWRs as part of recently completed, ongoing, or pending regulatory actions, these NPDWRs were not subject to detailed review under the Six-Year Review process.
• The agency does not believe it is appropriate to consider revisions to NPDWRs for contaminants with an ongoing or planned health effect assessment where the MCL is set equal to the MCLG or that were set at the level at which health risk reduction benefits were maximized at a cost justified by the benefits in accordance with SDWA section 1412(b)(6)(A)). This principle stems from the fact that any new health effects assessment may affect the MCL via a change in the MCLG or the assessment of the benefits associated with the MCL. EPA notes that these NPDWRs are not appropriate for revision and no action is necessary if the health effects assessment would not be completed during the review cycle.
• In evaluating the potential for new information to affect NPDWRs, EPA assumed no change to existing policies and procedures for developing NPDWRs. For example, in determining whether new information affected the feasibility of analytical methods for a contaminant, the agency assumed no change to current policies and procedures for calculating practical quantitation limits.
• EPA may consider whether there is new public health risk information to justify accelerating review and potential revision of a particular NPDWR before the next review cycle.

Procedures

EPA also applied the following procedures in the review process:

• EPA considered new information from health effects assessments that were completed by the information cutoff date. Assessments completed after this cutoff date will be reviewed by EPA during the next review cycle.
• During the review, EPA identified areas where relevant information, which is needed to determine whether a revision to an NPDWR may be appropriate, was either: inadequate, unavailable (i.e., data gaps), or emerging. To the extent EPA is able to fill data gaps or fully evaluate the emerging information, the agency will consider the information as part of the next review cycle.
• Finally, EPA assured that the scientific analyses supporting the review were consistent with the agency's peer review policy (USEPA, 2015a).

B. How did EPA conduct the review of the NPDWRs?

The protocol for the Six-Year Review 4 is organized as a series of questions to inform an assessment as to the appropriateness of revising an NPDWR. These questions are logically ordered into a decision tree. This section provides an overview of each of the review elements that EPA considered for each NPDWR during the Six-Year Review 4, including the following: initial review, health effects, analytical feasibility, occurrence and exposure, treatment feasibility, risk balancing, and other NPDWR revisions. The final review combines the findings from all these review elements to recommend whether an NPDWR is a candidate for revision. Further information about the review elements is described in the protocol document (USEPA, 2024c). The results of the Six-Year Review are presented in section V of this document.

1. Initial Review

EPA's initial review of all the contaminants included in the Six-Year Review 4 involved a simple identification of the NPDWRs that have either been recently completed or are being reviewed in an ongoing or pending action since the publication of Six-Year Review 3. In addition, the initial review also identified contaminants with ongoing health effects assessments that have an MCL equal to the MCLG. Excluding such contaminants from a more detailed review in the Six-Year Review 4 prevents duplicative agency efforts.

2. Health Effects

The principal objectives of the health effects review are to identify: (1) contaminants for which a new health effects assessment indicates that a change in the MCLG might be appropriate ( e.g., because of a change in cancer classification or a change in reference dose (RfD)), and (2) contaminants for which new health effects information indicates a need to initiate a new health effects assessment.

To meet the first objective, EPA reviewed the results of health effects assessments completed since promulgation of each NPDWR. To meet the second objective, the agency conducted a systematic literature search, to capture more recently published peer-reviewed studies on relevant health effects via the oral route of exposure for the general population as well as sensitive subpopulations including children. The results of the literature search were used to survey the health effects literature that has become available since the previous review cycle, identify any emerging issues for a contaminant, and identify data gaps to inform future health assessment nominations.

3. Analytical Feasibility

When establishing an NPDWR, EPA identifies a practical quantitation limit (PQL), which is the lowest achievable level of analytical quantitation during routine laboratory operating conditions within specified limits of precision and accuracy (50 FR 46880, USEPA, 1985). EPA has a separate process in place to approve new analytical methods for drinking water contaminants; therefore, review and approval of potential new methods is outside the scope of the Six-Year Review protocol. EPA recognizes, however, that the approval and adoption in recent years of new and/or improved analytical methods may enable laboratories to quantify contaminants at lower levels than was possible when NPDWRs were originally promulgated. This ability of laboratories to measure a contaminant at lower levels could affect its PQL, the value at which an MCL is set when it is limited by analytical feasibility. Therefore, the Six-Year Review process includes an examination of whether there have been changes in analytical feasibility that could possibly change the PQL for the subset of the NPDWRs that reach this stage of the review.

To determine if changes in analytical feasibility could possibly support changes to PQLs, EPA relied primarily on two approaches to develop estimated quantitation levels (EQLs), which are based on either (1) minimum reporting levels (MRLs) obtained as part of the Six-Year Review 4 Information Collection Request (ICR), or (2) method detection limits (MDLs) from EPA-approved laboratory protocols.

An MRL is the lowest level or contaminant concentration that a laboratory can reliably achieve within specified limits of precision and accuracy under routine laboratory operating conditions using a given method. The MRL values provide direct evidence from actual monitoring results about whether quantitation below the PQL using current analytical methods is feasible. An MDL is a measure of analytical sensitivity, representing the minimum reported concentration that can be distinguished from blank results with 99 percent confidence. MDLs have been used in the past to derive PQLs for regulated contaminants.

EPA used the EQL as a threshold for occurrence analysis to help the agency assess for a meaningful opportunity to improve public health protection. It should be noted, however, that the use of an EQL does not necessarily indicate the agency's intention to promulgate a revised MCL based on the new PQL. Any change in the PQL for a contaminant could be part of future rulemaking efforts if EPA decides to initiate a regulatory revision for the contaminant.

4. Occurrence and Exposure Analysis

EPA conducted the occurrence and exposure analysis in conjunction with other review elements to determine if an NPDWR revision would provide a meaningful opportunity to improve public health by:

• estimating the extent of contaminant occurrence,i.e., the number of PWSs in which contaminants occur at levels of interest (health-effects-based thresholds or analytical method limits), and;
• evaluating the number of people potentially exposed to contaminants at these levels.

To evaluate national contaminant occurrence under the Six-Year Review 4, EPA reviewed data from the Six-Year Review 4 ICR database (SYR 4 ICR database) and other relevant sources. EPA collected SDWA compliance monitoring data and treatment technique information through use of an ICR (84 FR 58381, USEPA, 2019). EPA requested that states, as well as Tribes and territories with primacy voluntarily submit their compliance monitoring data and treatment technique information for regulated contaminants in PWSs. Specifically, EPA requested the submission of compliance monitoring data, treatment technique information, and related details collected between January 2012 and December 2019 for regulated contaminants and related parameters ( e.g., water quality indicators). Forty-six states plus 13 other jurisdictions (Washington, DC, territories, and Tribes) provided data. The assembled data constitute the largest, most comprehensive set of drinking water compliance monitoring data and treatment technique information ever compiled and analyzed by EPA to inform decision making, containing almost 71 million analytical records from approximately 140,000 PWSs, serving approximately 301 million people nationally. Through extensive data management efforts, quality assurance evaluations, and communications with state data management staff, EPA established the SYR 4 ICR dataset (USEPA, 2019). The number of states and PWSs represented in the dataset varies across contaminants because of variability in state data submissions and contaminant monitoring schedules. EPA considers that these data are of sufficient quality to inform an understanding of the national occurrence of regulated contaminants and related parameters. Details of the data management and data quality assurance evaluations are available in the supporting document (USEPA, 2024d). The resulting database is available online on the Six-Year Review website at https://www.epa.gov/​dwsixyearreview.

5. Treatment Feasibility

An NPDWR either identifies an MCL or establishes enforceable TT requirements. When promulgating an MCL or enforceable treatment technique requirements, to determine feasibility, EPA identifies the best technology, treatment techniques, and other means which EPA finds, after examination for efficacy under field conditions and not solely under laboratory conditions, are available (taking cost into consideration). When promulgating an MCL, EPA also lists the technology, treatment techniques, or other means which are feasible for purposes of meeting the MCL. EPA reviews treatment feasibility to ascertain if available technologies meet BAT criteria for a hypothetical more stringent MCL, or if new information demonstrates an opportunity to improve public health protection through revision of an NPDWR TT requirement.

To be a BAT, the treatment technology must meet several criteria such as having demonstrated consistent removal of the target contaminant under field conditions. Although treatment feasibility and analytical feasibility are considered together in evaluating the technical feasibility requirement for an MCL, historically, treatment feasibility has not been a limiting factor for MCLs. The result of this review element is a determination of whether treatment feasibility would pose a limitation to revising an MCL or provide an opportunity to revise the NPDWR TT requirement.

6. Risk-Balancing

EPA reviews the risk-balancing analysis underlying some NPDWRs to examine how a potential regulatory revision would address tradeoffs in risks associated with different contaminants. Under this review, EPA considers whether a change to an MCL and/or TT will increase the public health risk posed by one or more contaminants, and, if so, the agency considers revisions that will balance overall risks. This review element is relevant only to the NPDWRs included in the microbial and disinfection byproduct (MDBP) rules, which were promulgated to address the need for risk-balancing between microbial and disinfection byproduct (DBP) requirements, and among differing types of DBPs. NPDWRs for microbials and disinfectants and DBPs were not reviewed during Six-Year Review 4 due to ongoing regulatory action initiated by Six-Year Review 3.

7. Other NPDWR Revisions

In addition to possible revisions to MCLGs, MCLs, and TTs, EPA evaluated whether other revisions are needed to other regulatory provisions in NPDWRs, such as monitoring and system reporting requirements. EPA focused this review element on issues that were not already being addressed through alternative mechanisms, such as a recently completed, ongoing, or pending regulatory action. EPA also reviewed implementation-related NPDWR concerns that were “ready” for rulemaking—that is, the problem to be resolved had been clearly identified, along with specific options to address the problem that could be shown to either clearly improve the level of public health protection or represent a meaningful opportunity for achieving cost savings while maintaining the same level of public health protection. The result of this review element is a determination regarding whether EPA should consider revisions to the monitoring and/or reporting requirements of an NPDWR.

V. Results of EPA's Review of NPDWRs

A. Overview of Six-Year Review 4 Results

Table 2 of this document, lists the results of EPA's review of the 88 NPDWRs assessed during Six-Year Review 4, along with the principal rationale for the review outcomes. Table 2 includes the 15 NPDWRs that have ongoing or pending regulatory actions.

Table 2—Summary of Six-Year Review 4 Results

Outcome			Regulated contaminants
Not Appropriate for Revision at this Time	Recently completed, ongoing or pending regulatory action.		Bromate Chloramines (as Cl 2 ) Chlorine Dioxide (as ClO 2 ) Chlorine (as Cl 2 ) Chlorite Copper Cryptosporidium (IE, LT1) 1 Giardia lamblia.	Haloacetic acids (HAA5). Heterotrophic bacteria. Lead. Legionella. Total Trihalomethanes (TTHM). Turbidity. Viruses (SWTR, IE, LT1).1
Not Appropriate for Revision at this Time	Health effects assessment in process or contaminant nominated for health assessment.		Alpha/photon emitters Arsenic Beta/photon emitters Chromium (total) Ethylbenzene	Mercury (inorganic). Polychlorinated biphenyls (PCBs). Radium 226/228 (combined). Uranium.
	No new information, NPDWR remains appropriate after review.		Asbestos Benzo(a)pyrene Chlorobenzene Dalapon Di(2-ethylhexyl)adipate (DEHA) Di(2-ethylhexyl)phthalate (DEHP) 1,2-Dibromo-3-chloropropane (DBCP)	trans-1,2-Dichloroethylene. Dinoseb. E. coli. Endrin. Ethylene dibromide. 2,4,5-TP (Silvex).
	New information, but no revision recommended because . . .	Low priority and/or no meaningful opportunity	Acrylamide Alachlor Antimony Atrazine Barium Benzene	Heptachlor. Heptachlor Epoxide. Hexachlorobenzene. Hexachlorocyclopentadiene. Lindane. Methoxychlor.
			Beryllium Cadmium Carbofuran Carbon Tetrachloride Chlordane Cryptosporidium (LT2)  1 1,2-Dichlorobenzene 1,4-Dichlorobenzene 1,2-Dichloroethane 1,1-Dichloroethylene cis-1,2-Dichloroethylene Dichloromethane	Oxamyl (Vydate). Pentachlorophenol. Picloram. Selenium. Simazine. Styrene. Tetrachloroethylene (PCE). Thallium. 1,2,4-Trichlorobenzene. 1,1,1-Trichloroethane. 1,1,2-Trichloroethane. Toluene.
			2,4-Dichlorophenoxyacetic acid (2,4-D) 1,2-Dichoropropane Dioxin (2,3,7,8-TCDD) Diquat. Endothall. Epichlorohydrin. Glyphosate.	Total Coliform. Toxaphene. Trichloroethylene (TCE). Vinyl Chloride. Xylenes.
		Emerging information and/or data gaps	Cyanide (as free cyanide) Fluoride.	Nitrate. Nitrite.
Candidate for Revision	New information.		None.
1  Regulation abbreviations: Aircraft Drinking Water Rule (ADWR), Ground Water Rule (GWR), Revised Total Coliform Rule (RTCR), Surface Water Treatment Rule (SWTR), Interim Enhanced Surface Water Treatment Rule (IE), Long Term 1 Enhanced Surface Water Treatment Rule (LT1), and Long Term 2 Enhanced Surface Water Treatment Rule (LT2).

EPA has identified no appropriate candidates for revision at this time.

EPA's Office of Ground Water and Drinking Water is currently engaged in several ongoing and potential regulatory actions, in addition to being involved in the efforts to successfully implement recently promulgated rules including:

• Developing a proposal to revise the Microbial and Disinfection By-Product Rules, including eight NPDWRs listed as candidates for revision in Six-Year Review 3 (85 FR 61680, USEPA, 2020a).
• On December 6, 2023, EPA published the proposed rule “National Primary Drinking Water for Lead and Copper: Improvements” (88 FR 84878, USEPA, 2023a).
• In January 2024, EPA announced its commitment to promulgate a National Primary Drinking Water Regulation for Perchlorate by May 2027.^[5]

• On April 26, 2024, EPA published the PFAS final rule “PFAS National Primary Drinking Water Regulation” (89 FR 32532, USEPA, 2024a).
• On May 24, 2024, EPA published the final rule “National Primary Drinking Water Regulations: Consumer Confidence Reports” (89 FR 45980, USEPA, 2024b).

Therefore, when evaluating the review results described in sections V.B and V.C of this document, EPA also considered competing workloads and potential diversion of resources from these other planned, ongoing, and pending higher priority efforts within the drinking water office.

B. Chemical Phase Rules/Radionuclides Rules

The NPDWRs for chemical contaminants, collectively called the Phase Rules, were promulgated between 1987 and 1992, following the 1986 SDWA amendments. In December 2000, EPA promulgated final radionuclide regulations, which had been issued as interim rules in July 1976.

1. Key Review Outcomes

EPA has decided that it is not appropriate at this time to revise any of the NPDWRs covered under the Phase or Radionuclides Rules (Table 2 of this document). These NPDWRs were determined not to be candidates for revision for one or more of the following reasons:

• ongoing/pending regulatory action warrants waiting for further review;
• no new information was identified to suggest possible changes in MCLG/MCL;
• new information did not present a meaningful opportunity for health risk reduction or cost savings while maintaining/improving public health protection;
• emerging information and/or data gaps create substantial uncertainty.

In addition, EPA is announcing that the NPDWRs for trichloroethylene (TCE) and tetrachloroethylene (PCE) are no longer candidates for revision at this time. In March 2010, as an outcome of the second cycle of Six-Year Review, EPA listed the TCE and PCE NPDWRs as candidates for revision (75 FR 15500, USEPA, 2010a). TCE and PCE were not reviewed under Six-Year Review 3 because regulatory revisions were being considered as part of plans to address regulated and unregulated Carcinogenic Volatile Organic Contaminants (cVOCs) in a group rule (75 FR 3525, January 21, 2010; 82 FR 3531, USEPA, 2017a). However, after evaluating currently available information for both of these chemicals, the EPA concludes that these NPDWRS are not appropriate for revision at this time because minimal reductions in health risks would be associated with any revisions to these regulations. Given resource limitations, competing workload priorities, and administrative costs and burden to states to adopt any regulatory changes associated with rulemakings, as well as limited potential health benefits, these NPDWRs are considered a low priority and are no longer candidates for revision at this time.

Section V.B.2 of this document describes the results of the review organized by each review element. Section V.B.3 of this document includes a description of the new information gathered by EPA for select contaminants that EPA determined are not candidates for revision at this time due to emerging information or data gaps or no meaningful opportunity for health risk reduction. The contaminants discussed in detail in section V.B.3 of this document are cyanide, fluoride, nitrate, nitrite, TCE, and PCE.

Review results organized by contaminant for the Chemical Phase and Radionuclides Rules can be found in the “Chemical Contaminant Summaries for the Fourth Six-Year Review of National Primary Drinking Water Regulations” (USEPA, 2024e).

2. Summary of Review Results

Initial Review

After conducting the initial review, as described in section IV.B.1 of this document, EPA identified two chemical contaminants (lead and copper) with NPDWRs that were considered as part of a recently completed action, and which are also currently part of an ongoing or pending regulatory action. EPA published the Lead and Copper Rule Revisions in January 2021 and published the proposed Lead and Copper Rule Improvements on December 6, 2023. EPA did not evaluate lead and copper in Six-Year Review 4 because such effort would be redundant with these recent and ongoing rulemakings. EPA also identified contaminants with ongoing or planned EPA health effects assessments. As of December 31, 2021, nine chemical or radiological contaminants reviewed had ongoing or planned formal EPA health effects assessments. Table 3 of this document below lists the contaminants with ongoing or planned EPA assessments at the time of the Six-Year Review 4 cutoff date and the current status of those reviews. EPA did not conduct a detailed review of these nine chemical and radiological contaminants under Six-Year Review 4.

Table 3—Six-Year Review Chemical/Radiological Contaminants With Ongoing or Planned EPA Health Assessments

Chemical/radionuclide	Status 1
Alpha/photon emitters	EPA Office of Air and Radiation (OAR) is conducting a review of alpha and beta photon emitters. Additional information about this effort can be found at in the Federal Register (87 FR 15988, USEPA, 2022a) or at: https://sab.epa.gov/​ords/​sab/​r/​sab_​apex/​sab_​bkup/​advisoryactivitydetail?​p18_​id=​2616&​clear=​18&​session=​8694491614209.
Arsenic	Inorganic arsenic is being assessed by the EPA IRIS Program. The assessment status can be found at: https://iris.epa.gov/​ChemicalLanding/​&substance_​nmbr=​278.
Beta/photon emitters	EPA/OAR is conducting a review of alpha and beta photon emitters. Additional information about this effort can be found at in the Federal Register (87 FR 15988, USEPA, 2022a) or at: https://sab.epa.gov/​ords/​sab/​r/​sab_​apex/​sab_​bkup/​advisoryactivitydetail?​p18_​id=​2616&​clear=​18&​session=​8694491614209.
Chromium VI (as part of total Cr)	Chromium VI is being assessed by the EPA IRIS Program. The assessment status can be found at: https://iris.epa.gov/​ChemicalLanding/​&substance_​nmbr=​144.
Ethylbenzene	Ethylbenzene is being assessed by the EPA IRIS Program. The assessment status can be found at: https://iris.epa.gov/​ChemicalLanding/​&substance_​nmbr=​51.
Mercury	Inorganic Mercury Salts is being assessed by the EPA IRIS Program. The Assessment status can be found at: https://iris.epa.gov/​ChemicalLanding/​&substance_​nmbr=​1522.
PCBs	PCBs are being assessed by the EPA IRIS Program. The assessment status can be found at: https://iris.epa.gov/​ChemicalLanding/​&substance_​nmbr=​294.
Radium 226/228	EPA/OAR is conducting a review of radium. Additional information about this effort can be found at in the Federal Register (87 FR 15988, USEPA, 2022a) or at: https://sab.epa.gov/​ords/​sab/​r/​sab_​apex/​sab_​bkup/​advisoryactivitydetail?​p18_​id=​2616&​clear=​18&​session=​8694491614209.
Uranium	Uranium is being assessed by the EPA IRIS Program. The assessment status can be found at: https://iris.epa.gov/​ChemicalLanding/​&substance_​nmbr=​259.
1  Additional information on the status of EPA IRIS Program assessments can be found in the EPA IRIS Program Outlooks at https://www.epa.gov/​iris/​iris-program-outlook.

Regarding the ongoing health assessment for Chromium VI (hexavalent chromium), on October 20, 2022 the EPA published its draft “IRIS Toxicological Review of Hexavalent Chromium [Cr(IV)]” (87 FR 63774, USEPA, 2022b). This draft health effects assessment, which includes a comprehensive evaluation of potential health effects, preliminarily categorizes hexavalent chromium as likely carcinogenic to humans via the oral exposure pathway. The final IRIS assessment was not available as of the publication of this document and for consideration as part of Six-Year Review 4. When this human health assessment is final, EPA will carefully review the conclusions and consider all relevant information to determine whether the NPDWR for chromium is a candidate for revision.

After the initial review was completed, EPA identified 71 chemical and radiological NPDWRs that were appropriate for detailed review.

Health Effects

The principal objectives of the health effects assessment review were to identify: (1) contaminants for which a new health effects assessment indicates that a change in MCLG might be appropriate ( e.g., because of a change in cancer classification or an RfD), and (2) contaminants for which the agency has identified new health effects information suggesting a need to initiate a new health effects assessment. For chemicals that were not excluded due to an ongoing or planned health effects assessment by EPA, a more detailed review was undertaken. Of the chemicals that underwent a more detailed review, EPA identified 29 contaminants for which an updated RfD and/or the cancer risk assessment (from oral exposure) or new relevant non-EPA assessments might support a change to the MCLG. These 29 chemicals were further evaluated as part of the Six-Year Review 4 to determine whether they were candidates for regulatory revision. Table 4 of this document lists the chemicals with available new health effects information and the sources of the relevant new information. As shown in this table, 15 chemical contaminants have information that could support a lower MCLG, and 14 contaminants have new information that could support a higher MCLG.

Table 4—Chemicals With New Health Assessments That Could Support a Change in MCLG

Chemical	Relevant new assessment
15 Contaminants with Potential to Decrease the MCLG
Antimony	CalEPA, 2016.
Cadmium	ATSDR, 2012.
Carbofuran	USEPA OPP, 2008.
Cyanide	USEPA IRIS, 2010b.
cis-1,2-Dichloroethylene	USEPA IRIS, 2010c.
Endothall	USEPA OPP, 2015b.
Fluoride	USEPA OW, 2010d.
Hexachlorocyclopentadiene	USEPA IRIS, 2001.
Methoxychlor	CalEPA, 2010a.
Oxamyl	USEPA OPP, 2017b.
Selenium	ATSDR, 2003.
Styrene	CalEPA, 2010b.
Toluene	Health Canada, 2014.
1,2,4-Trichlorobenzene	USEPA PPRTV, 2009a.
Xylenes	Health Canada, 2014.
14 Contaminants with Potential to Increase the MCLG
Alachlor	USEPA OPP, 2007a.
Atrazine	USEPA OPP, 2018a.
Barium	USEPA IRIS, 2005.
Beryllium	USEPA IRIS, 1998.
2,4-Dichlorophenoxy-acetic acid (2,4-D)	USEPA OPP, 2017c.
1,2-Dichlorobenzene	ATSDR, 2006.
1,4-Dichlorobenzene	ATSDR, 2006.
1,1-Dichloroethylene	USEPA IRIS, 2002.
Diquat	USEPA OPP, 2020b.
Glyphosate	USEPA OPP, 2017d.
Lindane	USEPA OPP, 2004.
Picloram	USEPA OPP, 2020c.
Simazine	USEPA OPP, 2018b.
1,1,1-Trichloroethane	USEPA IRIS, 2007b.

Details of the health effects assessment review of the chemical and radiological contaminants are documented in the “Results of the Health Effects Assessment for the Fourth Six-Year Review of Existing Chemical and Radionuclide National Primary Drinking Water Standards” (USEPA, 2024f).

Analytical Feasibility

EPA performed analytical feasibility analyses for the contaminants that reached this portion of the review. These contaminants included the 15 chemical contaminants identified under the health effects assessment review as having potential for a lower MCLG. EPA evaluated whether there were any analytical limitations to lowering the MCL to the potential MCLG. EPA also evaluated an additional 22 contaminants with MCLs higher than the current MCLGs due to analytical limitations at the time of rule promulgation. The document “Analytical Feasibility Support Document for the Fourth Six-Year Review of National Primary Drinking Water Regulations: Chemical Phase and Radionuclides Rules” (USEPA, 2024g) describes the process EPA used to evaluate whether changes in PQL are possible in those instances where the MCL may be limited by analytical feasibility.

Table 5 of this document shows the outcomes of EPA's analytical feasibility review for two general categories of drinking water contaminants: (1) contaminants where health effects assessments indicate potential for lower MCLGs, and (2) contaminants where existing MCLs were limited by analytical feasibility at the time of promulgation and new information indicates a potential to reduce the PQL.

• A health effects assessment indicates potential for lower MCLG. This category includes the 15 contaminants identified in the health effects review as having potential for a lower MCLG. EPA reviewed the available information to determine if analytical feasibility could limit the potential for MCL revisions. The current PQL is not a limiting factor for seven of the 15 contaminants identified by the health effects review as potential candidates for lower MCLGs (cis-1,2-dichloroethylene, fluoride, hexachlorocyclopentadiene, oxamyl, selenium, toluene, and xylenes). For the remaining eight contaminants, the current PQL is higher than the potential new MCLG, so EPA evaluated whether there is an opportunity to lower the PQL. The evaluations indicated that all but one contaminant (antimony) have potential for a lower PQL, although not to the potential MCLG. Consequently, analytical feasibility may limit potential MCL revisions for the remaining seven contaminants (Table 5 of this document).
• Existing MCLs are based on analytical feasibility. This category includes 22 contaminants with existing MCLs that are greater than the associated MCLGs due to analytical constraints at the time of rule promulgation. Two of the contaminants (thallium and 1,1,2-trichloroethane) are non-carcinogenic and have a non-zero MCLG, and the remaining 20 contaminants are carcinogens with MCLGs equal to zero. EPA evaluated whether the PQL could be lowered for each of these contaminants. The evaluations indicated that all but five (benzo[a]pyrene, DBCP, DEHP, ethylene dibromide, PCBs) of the 22 contaminants evaluated have potential for a lower PQL (Table 5 of this document).

Where analytical feasibility evaluations indicated the potential for a PQL reduction, Table 5 of this document lists the type of data that support this conclusion. The types of data considered include laboratory proficiency tests (PT), method detection limits (MDL) from EPA-approved methods, and minimum reporting level (MRL) from the SYR 4 ICR dataset. The methods to evaluate each of these data types to identify potential to reduce PQLs are described in the analytical feasibility support document (USEPA, 2024g). Where the evaluations indicated that the current PQL remained appropriate, Table 5 shows of this document “Data do not support PQL reduction.” EPA found information supporting potentially lower MCLs for 31 out of 37 contaminants evaluated.

Table 5—Analytical Feasibility Reassessment Results

Contaminant	Current PQL (µg/L)	Analytical feasibility reassessment result (and source of new information) 1
15 Contaminants Identified Under the Health Effects Review as Having Potential for Lower MCLG
Antimony	6	Data do not support PQL reduction.
Cadmium	2	PQL reduction supported (MDL, MRL).
Carbofuran	7	PQL reduction supported (MDL).
Cis-1,2-dichloroethylene	5	PQL not limiting.
Cyanide	100	PQL reduction supported (MDL).
Endothall	90	PQL reduction supported (MDL, MRL).
Fluoride	500	PQL not limiting.
Hexachlorocyclopentadiene	1	PQL not limiting.
Methoxychlor	10	PQL reduction supported (MDL, MRL, PT).
Oxamyl	20	PQL not limiting.
Selenium	10	PQL not limiting.
Styrene	5	PQL reduction supported (MDL, MRL, PT).
Toluene	5	PQL not limiting.
Xylenes	5	PQL not limiting.
1,2,4-Trichlorobenzene	5	PQL reduction supported (MDL, MRL, PT).
22 Contaminants with MCLs Limited by Analytical Feasibility and Higher than MCLGs
Benzene	5	PQL reduction supported (MDL, MRL, PT).
Benzo[a]pyrene	0.2	Data do not support PQL reduction.
Carbon tetrachloride	5	PQL reduction supported (MDL, MRL, PT).
Chlordane	2	PQL reduction supported (MDL).
1,2-Dibromo-3-chloropropane (DBCP)	0.2	Data do not support PQL reduction.
1,2-Dichloroethane	5	PQL reduction supported (MDL, MRL, PT).
Dichloromethane	5	PQL reduction supported (MDL, MRL, PT).
1,2-Dichloropropane	5	PQL reduction supported (MDL, MRL, PT).
Di(2-ethylhexyl)phthalate (DEHP)	5	Data do not support PQL reduction.
Ethylene dibromide	0.05	Data do not support PQL reduction.
Heptachlor	0.4	PQL reduction supported (MDL).
Heptachlor epoxide	0.2	PQL reduction supported (MDL).
Hexachlorobenzene	1	PQL reduction supported (MDL, MRL).
Pentachlorophenol	1	PQL reduction supported (MDL).
PCBs	0.5	Data do not support PQL reduction.
2,3,7,8-TCDD (dioxin)	0.00003	PQL reduction supported (MRL).
Tetrachloroethylene	5	PQL reduction supported (MDL, MRL).
Thallium	2	PQL reduction supported (MRL).
Toxaphene	3	PQL reduction supported (MRL, PT).
1,1,2-Trichloroethane	5	PQL reduction supported (MDL, MRL, PT).
Trichloroethylene	5	PQL reduction supported (MDL, MRL, PT).
Vinyl chloride	2	PQL reduction supported (MDL, MRL, PT).
1  The information source codes refer to the method detection limit (MDL), minimum reporting level (MRL), and proficiency testing (PT) data analyses. See USEPA (2024g) for further information.

Occurrence and Exposure

Using the SYR 4 ICR database, EPA conducted an assessment to evaluate national occurrence of regulated contaminants and estimate the potential population exposed to these contaminants. The details of the current chemical occurrence analysis are documented in the “Analysis of Regulated Contaminant Occurrence Data from Public Water Systems in Support of the Fourth Six-Year Review of National Primary Drinking Water Regulations: Chemical Phase Rules and Radionuclides Rules” (USEPA, 2024h). Based on quantitative benchmarks which were identified in the health effects and analytical feasibility analyses, EPA conducted the occurrence and exposure analysis for 31 contaminants.

This analysis shows that 27 of the 31 contaminants assessed rarely occur at levels above the identified benchmark ( e.g., potential MCLG or PQL). For these 27 contaminants, monitoring results only exceeded benchmarks in a very small percentage ( i.e., less than 0.5 percent) of systems, which serve a very small percentage of the population, indicating that revisions to NPDWRs are unlikely to provide a meaningful opportunity to improve public health protection at the national level. Therefore, these 27 contaminants were not further considered as candidates for regulatory revision. The other four contaminants (cyanide, fluoride, TCE, and PCE) occurred at rates ranging from 0.57 to 9.1 percent of systems within the SYR 4 ICR dataset and 3.4 to 6.3 percent of the population served by those systems. Additional considerations for cyanide, fluoride, TCE, and PCE are discussed in section V.B.3 of this document. Table 6 of this document lists the numerical benchmarks used to conduct the occurrence analysis, the total number of systems with mean concentrations exceeding a benchmark, and the estimated population served by those systems. These average concentration-based evaluations are intended to inform the Six-Year Review, not to assess compliance with regulatory standards.

Table 6—Occurrence and Potential Exposure Analysis for Chemical NPDWRs

Contaminant	Current MCL (ug/L)	Benchmark 1 (ug/L)	Number (and percentage) of systems with a mean concentration 2 higher than benchmark	Population served by systems with a mean concentration higher than benchmark (and percentage of total population)
Contaminants Identified Under the Health Effects Review as Having Potential for Lower MCLG
Cadmium	5	1	182 (0.36%)	430,823 (0.16%)
Carbofuran	40	5	3  7 (0.02%)	3  49,409 (0.02%)
Cyanide	200	50	328 (0.85%)	8,134,220 (3.43%)
cis-1,2-Dichloroethylene	70	10	7 (0.01%)	42,215 (0.02%)
Endothall	100	50	0	0
Fluoride 4	4,000	900	4,479 (9.05%)	17,058,830 (6.30%)
Hexachlorocyclopentadiene	50	40	0	0
Methoxychlor	40	1	1 (<0.01%)	22,536 (0.01%)
Oxamyl	200	9	3  7 (0.02%)	3  52,677 (0.02%)
Selenium	50	30	91 (0.18%)	84,988 (0.03%)
Styrene	100	0.5	89 (0.17%)	27,473 (0.01%)
Toluene	1,000	60	14 (0.03%)	5,256 (<0.01%)
1,2,4-Trichlorobenzene	70	0.5	15 (0.03%)	126,201 (0.05%)
Xylenes (total)	10,000	80	23 (0.05%)	34,728 (0.01%)
Contaminants with MCLs Higher than MCLGs (Limited by Analytical Feasibility)
Benzene	5	0.5	83 (0.16%)	319,633 (0.12%)
Carbon tetrachloride	5	0.5	90 (0.17%)	766,891 (0.28%)
Chlordane	2	1	1 (<0.01%)	240 (<0.01%)
1,2-Dichloroethane	5	0.5	60 (0.11%)	181,041 (0.07%)
Dichloromethane	5	0.5	215 (0.41%)	360,289 (0.13%)
1,2-Dichloropropane	5	0.5	41 (0.08%)	34,800 (0.01%)
Heptachlor	0.4	0.1	1 (<0.01%)	900 (<0.01%)
Heptachlor epoxide	0.2	0.1	3 (0.01%)	32,710 (0.01%)
Hexachlorobenzene	1	0.1	6 (0.02%)	17,278 (0.01%)
Pentachlorophenol	1	0.9	0	0
2,3,7,8-TCDD (dioxin)	0.00003	0.000005	7 (0.11%)	2,311 (<0.01%)
Tetrachloroethylene (PCE)	5	0.5	432 (0.83%)	15,811,810 (5.76%)
Thallium	2	1	71 (0.14%)	57,541 (0.02%)
Toxaphene	3	1	2 (0.01%)	335 (<0.01%)
1,1,2-Trichloroethane	5	3	2 (<0.01%)	50 (<0.01%)
Trichloroethylene (TCE)	5	0.5	297 (0.57%)	12,755,926 (4.65%)
Vinyl chloride	2	0.5	24 (0.05%)	307,275 (0.11%)
1  Benchmark screening levels were set to either potential maximum contaminant level goals (MCLGs) or estimated quantitation levels (EQLs), depending on the contaminant. For more information see USEPA (2024g).
2  Results are based on long-term means generated by substituting one-half the MRL for each non-detection record. For results based on substituting the value of the full MRL or zero see USEPA (2024h).
3  Oxamyl and carbofuran have health endpoints associated with acute exposure and are not appropriate for long-term mean estimates. Results show the number of systems with at least one detection exceeding the benchmark.
4  Estimates represent naturally occurring fluoride concentrations. Quality assurance steps were taken to exclude samples from fluoridated water systems. See USEPA (2024i) for details.

In addition, EPA performed a source water occurrence analysis for the 15 chemical contaminants in which updated health effects assessments indicated the possibility to increase ( i.e., render less stringent) the MCLG values. EPA conducted this analysis to assess for meaningful opportunity to achieve cost savings while maintaining or improving the level of public health protection. The data available to characterize contaminant occurrence was limited because a comprehensive dataset to characterize drinking water source quality is not available. Data from the U.S. Geological Survey (USGS) National Water Quality Assessment program and the U.S. Department of Agriculture Pesticide Data Program water monitoring survey provide useful insights into potential contaminant occurrence in source water. The analysis of the available contaminant occurrence data for potential drinking water sources indicated relatively low contaminant occurrence in the concentration ranges of interest, and consequently, no meaningful opportunity for system cost savings by increasing the MCLG and MCL for these 15 contaminants. The results of this analysis were documented in “Occurrence Analysis for Potential Source Waters for the Fourth Six-Year Review of National Primary Drinking Water Regulations” (USEPA, 2024j).

Treatment Feasibility

Currently, all of the MCLs for chemical and radiological contaminants are either (1) set equal to the MCLGs, (2) limited by analytical feasibility, or (3) set at the level at which health risk reduction benefits were maximized at a cost justified by the benefits; none are currently limited by treatment feasibility. EPA considers treatment feasibility after identifying contaminants with the potential to lower the MCLG/MCL that constitute a meaningful opportunity to improve public health. No such contaminants were identified in the occurrence and exposure analysis described above.

Treatment techniques were promulgated for two of the chemical and radiological contaminants that were subject to a detailed review in Six-Year Review 4. Acrylamide and epichlorohydrin occur in drinking water as treatment impurities and are primarily introduced as residuals in polymers and copolymers used for water treatment. There are no standardized analytical methods for their measurement in water; instead of sampling, water systems must certify to the State in writing that they use products meeting the specifications in the NPDWR. To evaluate the potential to revise the NPDWRs for these contaminants, EPA obtained data from NSF on analyses for approval of products against NSF/ANSI Standard 60, which are based on EPA's regulation. NSF certification data shows that manufactured products contain acrylamide and epichlorohydrin impurity levels far below the current regulatory standard. Specifically, the mean residual acrylamide concentration of certified products is one-fifth of the current regulatory level and the 90th percentile is one-half. There were no samples with detections of residual epichlorohydrin. The available data indicates that the majority of tested products already pose lower health risks than required under the current TT, and therefore, revisions are a low priority. EPA is not listing acrylamide and epichlorohydrin as candidates for revision at this time. See USEPA (2024k) for details.

Other Regulatory Revisions

In addition to possible revisions to MCLGs, MCLs, and TTs, as a part of the Six-Year Review 4, EPA considered whether other regulatory revisions to NPDWRs are needed to address implementation issues, such as revisions to monitoring and system reporting requirements. EPA used the protocol to evaluate which implementation issues to consider (USEPA, 2024c). EPA's protocol focused on items that were not already being addressed, or had not yet been addressed, through alternative mechanisms ( e.g., as a part of a recent or ongoing rulemaking).

EPA compiled information on implementation-related issues associated with the Chemical Phase Rules. EPA also identified unresolved implementation issues and concerns from previous Six-Year Reviews. The complete list of implementation issues related to the Phase and Radionuclides Rules is presented in “Consideration of Other Regulatory Revisions in Support of the Fourth Six-Year Review of the National Primary Drinking Water Regulations: Chemical Phase Rules and Radionuclides Rules” (USEPA, 2024l).

The agency focused on the following five implementation issues in the Six-Year Review 4:

• Use of an alternative MCL for nitrate in Noncommunity Water Systems (NCWSs)
• Frequency of nitrate monitoring in Transient Noncommunity Water Systems (TNCWS)
• Frequency of nitrite monitoring
• Total nitrate-nitrogen plus nitrite-nitrogen MCL
• Total cyanide screening for free cyanide

Table 7 of this document provides a brief description of the five issues and identified potential ways of addressing them. Please see section V.B.3. of this document for a discussion of these contaminants and their review outcomes. Please see USEPA (2024l) for a more detailed description and estimated scope of these issues.

Table 7—Chemical Rule Implementation Issues Identified That Fall Within the Scope of an NPDWR Review

Implementation issue	Description of issue
Nitrate Alternative MCL in Non-community Water Systems	EPA evaluated the possibility of removing or further restricting the options for some NCWSs to use an alternative nitrate-nitrogen MCL of up to 20 mg/L. The nitrate-nitrogen MCL specified for PWSs in 40 CFR 141.62 is 10 mg/L and is based on the critical health endpoint of methemoglobinemia in children under six months of age. 40 CFR 141.11 provides States the discretion to use an alternative MCL of 20 mg/L for non-community water systems (NCWS). This alternative MCL is allowed under certain conditions—including that water would be unavailable to children under six months of age.
	Monitoring requirements for nitrate-nitrogen are specified in the introductory text to 40 CFR 141.23, which states that “Non-transient, non-community water systems shall conduct monitoring to determine compliance with the maximum contaminant levels specified in § 141.62 in accordance with this section. Transient, non-community water systems (TNCWS) shall conduct monitoring to determine compliance with the nitrate and nitrite MCL in §§ 141.11 and 141.62 (as appropriate) in accordance with this section.”
	Potential concerns with the current rule provisions were identified as:
	• The alternative MCL does not address any nitrate-induced health concerns beyond methemoglobinemia and
	• While § 141.11 allows the use of the alternative MCL by all eligible NCWS, § 141.23 implies that only TNCWS, a subcategory of NCWS, are eligible to use the alternative MCL.
	To determine the scope of this issue, the agency reviewed state drinking water regulations and analyzed SYR 4 ICR nitrate compliance data and identified nominal application of the alternative nitrate MCL by NCWSs. In addition, the nitrate and nitrite human health assessments are currently being evaluated by the EPA IRIS program. An updated assessment could inform the potential health effects of nitrate exposure to levels between 10 and 20 mg/L on adult populations. EPA will consider all available and updated human health assessments as it conducts future cycles of the six-year review.
Nitrate Monitoring Frequency in Transient Noncommunity Water Systems	Currently, community water systems (CWSs) and NTNCWSs are required to monitor for nitrate quarterly if a sample is greater than or equal to 50 percent of the nitrate MCL (§ 141.23). TNCWSs are required to monitor for nitrate annually (§ 141.23(d)(4)). In the preamble to the 1991 final Phase II rule, the agency describes TNCWSs as being subject to the quarterly monitoring requirement stating that “EPA has decided to retain the 50 percent trigger for increased nitrate monitoring in the case of nitrate and also to extend this requirement to TWSs” (56 FR 3566, USEPA, 1991).
	EPA notes the conflict between the regulatory text and the preamble. To evaluate whether it may be appropriate to revise the nitrate NPDWR, the agency analyzed compliance monitoring data collected under the SYR 4 ICR. EPA found that while the majority of TNCWSs that reported detections equal or greater than 50 percent of the nitrate MCL did not conduct quarterly monitoring afterward, the number of these systems appears relatively small. Due to the limited scope of this issue, EPA is not revising the monitoring requirements at this time but will consider monitoring requirements if NPDWRs are revised in the future.
Nitrite Monitoring Frequency	According to 40 CFR 141.23(e)(1), all PWSs were required to monitor for nitrite once between January 1, 1993, and December 31, 1995. If this initial sample was less than 50 percent of the MCL (10 mg/L), systems “shall monitor at the frequency specified by the State“. Though the nitrite monitoring frequency is not explicitly stated in the CFR, EPA's guidance provides that this frequency should be at least once every 9-year compliance cycle (USEPA, 2020d). EPA is aware that some States may not require systems to conduct routine nitrite monitoring when sample results are less than 50 percent of the MCL. Because sample results below the MCL are not reported to EPA, the scope of this issue is uncertain.
	To address this uncertainty, EPA analyzed State regulations and nitrite compliance monitoring data to characterize the frequency of nitrite monitoring. Results indicated that a majority of systems monitored for nitrite at least once during the last 9-year compliance cycle (2011-2019). EPA intends to work with States to encourage more systems to sample for nitrite at least once during each 9-year compliance cycle.
Total Nitrate and Nitrite Analysis for Nitrate MCL Monitoring	In 40 CFR 141.62, the MCL for nitrate is specified as 10 mg/L and the MCL for total nitrate and nitrite is also specified as 10 mg/L. Sampling and analytical requirements as specified in 40 CFR 141.23, however, only included nitrate and left total nitrate and nitrite monitoring up to the discretion of States. Using Safe Drinking Water Information System (SDWIS) compliance data, EPA is aware that at least half of the States allow total nitrate/nitrite analysis to determine compliance with the nitrate MCL.
	To characterize monitoring practices for the nitrate MCL, the Agency analyzed Six-Year Review 4 compliance monitoring data for both nitrate and total nitrate/nitrite. This evaluation aims to serve as a baseline to assess nitrate monitoring practices in the future, in response to the 2020 EPA guidance outlining best practices when using total nitrate/nitrite analysis for monitoring compliance with the nitrate MCL. EPA is not revising the monitoring requirements at this time but will consider monitoring requirements in § 141.23 if NPDWRs are revised in the future, to incorporate best practices similar to those described in recent guidance (USEPA, 2020e).