On April 5 and 7, 2022, the State Water Resources Control Board (Board) will be holding public workshops to present information and solicit public input regarding a proposed administrative draft of a hexavalent chromium (chromium-6) maximum contaminant level (MCL). MCLs are drinking water standards with which public water systems must comply. The workshops, and administrative draft of the MCL, will help inform the Board’s formal rulemaking, expected to begin later this year. If adopted, the MCL would be the first drinking water standard for chromium-6 in the nation. ^[1]

Background

California Health and Safety Code (HSC) section 116365(a) requires the Board to establish an MCL at a level as close to the public health goal (PHG) as is technologically and economically feasible. The PHG is the concentration of a contaminant in drinking water that is not anticipated to cause or contribute to adverse health effects.

In 2011, the Office of Environmental Health Hazard Assessments (OEHHA) established a hexavalent chromium PHG of 0.02 parts per billion (ppb) based on cancer risk. In 2014, the California Department of Public Health established an MCL of 10 ppb (0.010 mg/L) for chromium-6. ^[2] In 2017, the Superior Court of California invalidated that MCL, finding the state had failed to consider the economic feasibility of compliance, and directing the state to withdraw the MCL and establish a new one.

Proposal

The Board’s new draft chromium-6 MCL proposal provides:

• A proposed hexavalent chromium MCL of 10 ppb.

• A proposed hexavalent chromium detection limit for purposes of reporting (DLR) of 0.05 ppb. ^[3]

• A proposed compliance schedule, based on water system size, as follows:

  • Systems with 10,000 or more service connections will have a two-year compliance schedule;

  • Systems with 1,000 to 10,000 service connections will have a three-year compliance schedule; and

  • Systems with less than 1,000 service connections will have a four-year compliance schedule.

A primary drinking water standard must be set at a level that is both technologically and economically feasible. HSC 116365(a). The Board proposes the following three treatment technologies as “best available technologies” to meet the proposed MCL of 10 ug/L:

• Reduction coagulation filtration (RCF): reduces hexavalent chromium to trivalent chromium. Trivalent chromium has a very low solubility, which results in the formation of a precipitate that can be removed by filtration to result in hexavalent chromium concentrations less than 5 μg/L in finished water;

• Ion exchange: uses strong base resins to which the hexavalent chromium anion can adsorb, decreasing hexavalent chromium concentrations to less than 1 μg/L in finished water; and

• Reverse osmosis: can filter hexavalent chromium through membranes to less than 1 μg/L.

In its economic feasibility analysis, the Board estimates the following chromium-6 MCL compliance costs: ^[4]

• Total Average Annual Costs Per System: $82,711 to $15,865,599, depending on the type and size of the system ^[5]

• Average Annual Costs Per Person: $34 to $2,657, again, depending on the type and size of the system.

EPA Scientific Workshop

While California moves forward with a draft chromium-6 MCL, EPA may be moving in the same direction. Currently, EPA (like California) regulates only total chromium in drinking water. To inform its ongoing IRIS assessment of chromium-6, EPA convened a panel of experts representing scientific areas related to the reduction and absorption of ingested chromium-6 at a workshop held on two days in September 2021 ^[6] Additional information about the workshop is available here.

Next Steps

Oral comments on the Board’s proposed chromium-6 MCL may be made via Zoom link at either of the two workshops, on April 5 or 7, 2022. The Board will accept written comments until 12:00 p.m. noon (PT), Friday, April 29, 2022. The Board will accept additional comments once formal rulemaking begins later this year. Additional information about the Board’s chromium-6 MCL proposal is available here.

EPA has not announced any follow-up to the September 2021 scientific workshop.

ENDNOTES

[1] As discussed below, EPA held a scientific workshop in September 2021 as part of the Agency’s effort to develop an updated IRIS assessment of hexavalent chromium.

[2] The Department of Public Health’s Drinking Water Program was transferred to the State Water Board in 2014.

[3] DLRs are the designated minimum levels at or above which any analytical finding of a contaminant in drinking water resulting from monitoring must be reported to the Board.

[4] The Board based its cost estimates on strong base anion ion exchange technology because it is anticipated that this will be the most commonly used treatment for hexavalent chromium.

[5] Systems include those with less than 100 connections to those with more than 10,000 connections and include community water systems (CWS), nontransient noncommunity water systems (NTNCWS), transient noncommunity water systems (TNCWS), and wholesalers. The cost estimates are available here.

[6] IRIS is “Integrated Risk Information System.” An IRIS assessment includes the first two steps of the risk assessment process: hazard identification and dose-response assessment. An IRIS assessment may inform EPA’s regulatory decisions.