The MCL Review Process


Health and Safety Code §116365(g) requires the Water Board, at least once every five years, to review its MCLs. In the review, the State Water Board's MCLs are to be consistent with criteria of §116365(a) and (b). Those criteria state that the MCLs cannot be less stringent than federal MCLs, and must be as close as is technically and economically feasible to the public health goals (PHGs) established by the Office of Environmental Health Hazard Assessment (OEHHA). Consistent with those criteria, the State Water Board is to amend any standard if any of the following occur: (1) Changes in technology or treatment techniques that permit a materially greater protection of public health or attainment of the PHG, or (2) New scientific evidence indicates that the substance may present a materially different risk to public health than was previously determined. Each year by March 1, the State Water Board is to identify each MCL it intends to review that year.

For a list of all regulated chemicals' MCLs and PHGs, click here (PDF).

The Process of Review

The first step in the review process is an initial screening. The criteria for this screening include: (1) The relationship between the PHG and both federal and state MCLs (PDF); (2) any changes in technology or treatment techniques that permit a materially greater protection of public health or attainment of the public health goal; and (3) any new scientific evidence indicating that the substance might present a materially different risk to public health than was previously determined.

In addition, occurrence is assessed for each regulated contaminant in drinking water sources. The four most recent years of analytical data from the State Water Board’s Division of Drinking Water’s (DDW’s) Water Quality Monitoring (WQM) database is analyzed.

Laboratory findings are based on a standardized quantification level called the "detection level for purposes of reporting" (DLR). The DLR represents the level at which we are confident about the accuracy of the quantity of contaminant being reported by laboratories. Although any findings below DLRs are considered "non-detects" and are not technically required to be reported, some laboratories may on occasion report lower levels for chemicals. For some chemicals, the DLR affects the technical feasibility of revising the MCL, in that the limits on a chemical’s detectability by analytical laboratories also serve to limit the extent to which the MCL might be lowered.

2018 MCL Review

The MCL and PHG for each contaminant were compared. The PHG represents a contaminant level that poses no significant health risks, therefore when an MCL is at or below the PHG, a revision of the MCL will not offer any additional health benefit. Currently, there are 29 MCLs that achieve this level of public health protection.

Asbestos trans-1,2-Dichloroethylene Alachlor
Barium Ethylbenzene Bentazon
Cyanide Methyl tertiary butyl ether (MTBE) Dalapon
Nitrate (as N) Monochlorobenzene Di(2-ethylhexyl)phthalate (DEHP)
Nitrite (as N) Toluene Dinoseb
Nitrate + Nitrite 1,2,4-Trichlorobenzene Endothal
1,2-Dichlorobenzene 1,1,1-Trichloroethane (1,1,1-TCA) Glyphosate
1,4-Dichlorobenzene (p-DCB) Trichlorofluoromethane (Freon 11) Hexachlorocyclopentadiene
1,1-Dichloroethylene (1,1-DCE) 1,1,2-Trichloro-1,2,2-Trifluoroethane (Freon 113) Simazine
cis-1,2-Dichloroethylene Xylenes  

In many cases, MCL revision is not warranted since there is very little public health risk since exposure to the chemical is considered negligible.

The Division of Drinking Water (DDW) researched exposure risk based on the occurrence of the contaminant in drinking water sources. Water quality data from the past four years - 2013 to 2017 - were queried from DDW’s water quality database, and showed that 26 contaminants were not detectable in any of the regulated drinking water sources. For the purpose of this analysis, two detections in a source at or above the DLR was considered a confirmed "detection" Wells not associated with drinking water distribution with a status of abandoned, agricultural/irrigation, inactive, monitoring, pending, or purchased were not considered.


Beryllium Carbofuran
Tritium Chlordane
Strontium -90 2,4-Dichlorophenoxyacetic acid (2,4-D)
1,3-Dichloropropene Endrin
1,1,2,2-Tetrachloroethane Lindane
Heptachlor Molinate
Benzo(a)pyrene Oxamyl
Di(2-ethylhexyl)adipate Polychlorinated biphenyls (PCBs)
Diquat Thiobencarb
Heptachlor epoxide Toxaphene
Hexachlorobenzene Vinyl chloride
Methoxychlor 2,3,7,8-TCDD (dioxin)
Picloram 2,4,5-TP (Silvex)

The remaining 32 MCLs were reviewed in 2017, and DDW is not considering any MCL revisions for 2018.

MCL Reviews From Previous Years

In 2017, all of the regulated contaminants were screened. Perchlorate was considered a candidate for a potential MCL revision . No other regulated contaminant was considered for revision.

Information on MCLs and PHGs

More on Drinking Water Contaminants

Contacts and Other Information for Drinking Water System

Drinking Water Laws