General Publications June 30, 2023

“D.C. Circ's Perchlorate Ruling Means Regulatory Restart,” Law360, June 30, 2023.

Extracted from Law360

On May 9, in National Resources Defense Council v. Regan, the U.S. Court of Appeals for the District of Columbia Circuit overturned the U.S. Environmental Protection Agency's 2020 determination withdrawing its decision to regulate perchlorate under the Safe Drinking Water Act, or SDWA.[1]

The D.C. Circuit's ruling will reopen a decadeslong debate over the regulation of perchlorate in drinking water, resulting in more time and resources spent on analyzing the chemical. The ruling could also lead to the proposal of lower maximum contaminant levels, or MCLs, than previously proposed.


Perchlorate is both a naturally occurring and manufactured chemical.[2] The U.S. Department of Defense and the aerospace industry have used ammonium perchlorate as an oxidizer in rocket testing since the 1940s.[3]

The pharmaceutical industry uses potassium perchlorate to treat hyperthyroidism, because the chemical blocks the thyroid's uptake of iodide.[4]

However, high levels of perchlorate may affect sensitive populations, such as pregnant women, by decreasing iodide uptake in a way that decreases the production of the hormones needed for fetal brain development.[5]

In 1997, perchlorate was first detected in the nation's water supplies. The EPA based its 2011 determination to regulate perchlorate on data showing over 4% of water systems tested detected perchlorate, and 5.2 million to 16 million people may be exposed to perchlorate in drinking water.

Studies since then have shown a decline in perchlorate in water systems.[6]

Long-Standing Debate Over Perchlorate Regulation

Perchlorate regulation has been debated since 1998, when the EPA first added perchlorate to its SDWA contaminant candidate list and initiated studies on the chemical.

In 2008, after 10 years of studies on perchlorate's health effects, the EPA declined to regulate perchlorate, determining that it "would not present a meaningful opportunity for health risk reduction" at the low levels found in water systems.[7]

California became the first state to regulate perchlorate, in 2004, setting its MCL at 6 micrograms per liter.[8] It was followed by Massachusetts in 2006, which set its MCL at 2 μg/L.[9] Thus far, no other states have followed suit.

In 2011, the EPA changed course, determining that it would regulate the chemical.[10] The EPA proposed to set the MCL at 56 μg/L, and requested comments on three alternatives: 18 μg/L, 90 μg/L or withdrawing its determination to regulate perchlorate.[11]

In 2020, after considering comments and studies on perchlorate, the EPA issued a final determination not to regulate perchlorate, concluding that the chemical did not occur at high enough levels, and with great enough frequency, to pose a health risk.

The Natural Resources Defense Council petitioned the D.C. Circuit to review the EPA's action, arguing that the EPA lacked the authority to withdraw a regulatory determination made under the SDWA — and that even if the EPA had that authority, it acted arbitrarily and capriciously by doing so.

The D.C. Circuit's Ruling

On May 9, the D.C. Circuit vacated the EPA's withdrawal of its determination to regulate perchlorate, and remanded any further proceedings to the agency.

In its opinion, the D.C. Circuit held that, because the EPA decided to regulate perchlorate in 2011, the agency was bound to adhere to that determination by setting regulatory standards for the chemical, and "lacks authority to withdraw that determination and decide what it 'shall not' regulate."[12]

Implications of the D.C. Circuit's Ruling

The D.C. Circuit's order has several implications. First, the EPA will again propose MCLs for perchlorate, despite the agency's prior determination to not regulate the chemical.

In 2020, the EPA concluded that "perchlorate does not occur at a frequency and at levels of public health concern," and that its regulation "does not present a meaningful opportunity for health risk reduction."[13] The determination was supported by studies showing that human exposure to perchlorate at the California and Massachusetts MCLs and higher did not have negative health impacts.

One study examined people who consumed drinking water that contained 4 μg/L to 24 μg/L, and found "no evidence that perchlorate-containing drinking water at the given level increased the prevalence of acquired hypothyroidism or of any other thyroid condition."[14]

The EPA cited studies showing decreased levels of perchlorate in water systems. For example, in a study of the Lower Colorado River, perchlorate concentrations before 2002 ranged from 4 parts per billion to 9 ppb, but most concentrations were between 1 ppb and 2 ppb after 2009.[15]

The EPA also supported its determination with data showing that there were only 15 water systems — 0.03% of all water systems in the U.S. — that detected perchlorate in drinking water above the lowest proposed MCL of 18 µg/L, and only one system that detected levels above 90 µg/L.[16]

The EPA will now return to square one of the regulatory process, resulting in continued debate and uncertainty over the status of perchlorate regulation.

A second implication of the ruling is that it will result in more time and resources spent on perchlorate, which is inefficient, given how much is known about perchlorate relative to other emerging chemicals.

Per- and polyfluoroalkyl substances, for example, are a large class of emerging contaminants that may be in some drinking water sources. Not enough is known about PFAS, and regulation of them is evolving.[17] Rather than studying PFAS, which are lesser-known chemicals, scientists and regulators will now have to continue to focus on perchlorate.

A third implication is that industries should carefully note which MCLs the EPA proposes, because the levels will determine the scale of cleanup costs. If the EPA follows California's most recent level of 1 µg/L — a level recently appealed and upheld by an appeals court[18] — it could cost billions of dollars to remove trace amounts of perchlorate from water systems.

MCLs are also used in determining cleanup levels under the Comprehensive Environmental Response, Compensation, and Liability Act, so perchlorate regulation could lead to more Superfund sites.[19] Cleanup costs will fall on taxpayers, because many perchlorate-containing sites are attributed to the Department of Defense, given its long-standing use of the chemical.[20]

Private industry should be prepared for compliance costs, and use the notice-and-comment period to advocate for reasonable, science-based MCLs.

[1] National Resources Defense Council v. Regan , No. 20-1335 (D.C. Cir. May 9, 2023), available at$file/20-1335-1998466.pdf.

[2] Jeffrey Dintzer and Clayton Namuo, EPA's Pass On Perchlorate Regulation Reduces Lawsuit Risk, Law360 (July 20, 2020), available at

[3] EPA, Technical Fact Sheet – Perchlorate (January 2014), available at

[4] National Institutes of Health, Perchlorate clinical pharmacology and human health: a review, available at

[5] Jeffrey Dintzer and Clayton Namuo, EPA's Pass On Perchlorate Regulation Reduces Lawsuit Risk, Law360 (July 20, 2020), available at

[6] EPA, Reductions of Perchlorate in Drinking Water (May 2020), available at

[7] Drinking Water: Preliminary Regulatory Determination on Perchlorate, 73 Fed. Reg. 60,262, 60,269, 60,280-81 (Oct. 10, 2008), available at

[8] See

[9] See

[10] Drinking Water: Regulatory Determination on Perchlorate, 76 Fed. Reg. 7762 (Feb. 11, 2011), available at

[11] National Primary Drinking Water Regulations: Perchlorate, 84 Fed. Reg. 30,524 (June 26, 2019), available at

[12] National Resources Defense Council v. Regan, No. 20-1335 (D.C. Cir. May 9, 2023), available at$file/20-1335-1998466.pdf.

[13] Drinking Water: Final Action on Perchlorate, 85 Fed. Reg. 43,990 (July 21, 2020), available at

[14] For a summary of this study and others, see Jeffrey Dintzer and Clayton Namuo, EPA's Pass On Perchlorate Regulation Reduces Lawsuit Risk, Law360 (July 20, 2020), available at

[15] For more information on this and other studies on the decreasing quantities of perchlorate in water, see Environmental Protection Agency, Reductions of Perchlorate in Drinking Water (May 2020), available at

[16] Drinking Water: Final Action of Perchlorate, 85 Fed. Reg. 43,990 (July 21, 2020), available at

[17] Alston & Bird PFAS Primer, available at; see also Jeffrey Dintzer and Gregory Berlin, Preparation for EPA Regulation of PFAS Starts Now, Law360 (Feb. 23, 2023),; Jeffrey Dintzer, Gregory Berlin and Shannon Vreeland, EPA's New PFAS Listings Raise Enforcement, Litigation Risks, Law360 (June 28, 2022), available at; Jeffrey Dintzer and Gregory Berlin, EPA Gets Serious on PFAS, Plans to Remove De Minimis Exception, Bloomberg Law (April 14, 2022), available at

[18] California appellate judge confirms state agency's limits on perchlorate, Courthouse News Service (June 11, 2023), available at

[19] EPA, Regional Screening Levels, available at

[20] EPA, Technical Fact Sheet – Perchlorate (January 2014), available at

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