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MCL Review in Response to PHGs

March 1, 2011

Health and Safety Code §116365(g) requires the Department, at least once every five years to review its MCLs.  In this review, CDPH's MCLs are to be consistent with criteria of §116365(a) and (b).  These 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, CDPH 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, CDPH is to identify each MCL it intends to review that year.

Click here (Excel)Opens in new window. for a list of regulated chemicals' MCLs and PHGs.

The Process of Review

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

The California Department of Health Services' (CDHS') Procedure for Reviewing Maximum Contaminant Levels (MCLs) for Possible Revision (PDF)Opens in new window. (August 1999), which was established after posting for public comment, is being used for reviews of MCLs for possible revision.  (CDHS was the precursor to CDPH.)

To assess chemical occurrence in drinking water sources, we obtained four years of recent analytical data from the Water Quality Monitoring (WQM) database and analyzed each chemical being considered for review.

For each chemical in WQM, CDPH has a standardized quantification level called the "detection level for purposes of reporting" (DLR). The DLR represents the level at which the CDPH is confident about the accuracy of the quantity of contaminant being reported. 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.

Since this process began, MCLs for these chemicals have been revised downward: atrazine, cyanide, ethylbenzene, oxamyl, methoxychlor, and 1,2,4-trichlorobenzene. 

MCL Review and Status

The steps in selecting contaminants for possible MCL review are as follows:

1. Regulated contaminants with PHGs - The selection process for MCLs for possible review first considers the regulated chemicals with PHGs. From the list of regulated contaminants, those with PHGs established through 2010 were identified.  

(OEHHA, at the request of CDPH, has also established PHGs for two as yet unregulated chemicals,  N-nitrosodimethylamine (NDMA) and 1,2,3-trichloropropane. CDPH has not yet proposed MCLs for these, and they are not considered further in this review.)

In 2010 OEHHA established new PHGs for selenium, styrene, benzo(a)pyrene, methoxychlor, and 2,3,7,8-TCDD (dioxin).  (see OEHHA's list of contaminants with PHGs)

As a result, through 2010, there are 86 contaminants with MCLs that have PHGs (Table 1) (Excel)Opens in new window..

2. Contaminants with MCLs greater than PHGs - The selection process then identified contaminants with MCLs greater than PHGs.  

There are 37 chemicals with MCLs below their PHGs that were not considered for further review, since their MCLs provide the same or more protection to the drinking water consumer as their PHGs.  For this step in the process, chemicals with an MCL 1.3 times the PHG were considered to have an MCL equivalent to the PHG:  i.e., diquat, 1,2-dichloroethane, dichloromethane, and heptachlor. 

In addition, 6 other chemicals were excluded at this stage:

Chemicals Regulated by the Lead and Copper Rule: Lead and copper do not have MCLs (they are covered by "action levels" and a different regulatory approach, as set forth in 22 CCR §64678) and were not considered further.  The PHG for lead is lower than its DLR; the action level is 3 times its DLR. The action level for copper is 4.3 times its PHG. 

Chemicals Used in Drinking Water Treatment to Provide a Public Health Benefit:  Aluminum and fluoride may also be added in drinking water treatment to provide a public health benefit, so they are not necessarily always "contaminants."  They were not considered further.  Their MCLs are 1.7 and 2 times their PHGs, respectively.

Chemicals That Are Byproducts of Disinfection Treatment:   Bromate and chlorite are byproducts of drinking water disinfection, which is required for public health protection.  They were not considered further.  The PHG for bromate is lower than its DLR; the MCL is 2 times its DLR.  The MCL for chlorite is 20 times its PHG.

When chemicals with MCLs below their PHGs and the additional chemicals identified above are excluded from the regulated contaminants in Step 1, there remain 43 contaminants with MCLs greater than their PHGs (Table 2) (Excel)Opens in new window..

3. Recent detections of contaminants with MCLs greater than PHGs - The selection process excludes contaminants with no recent detections at or above the DLR in at least one drinking water source.  Two detections in a source at or above the DLR is a "detection" for purposes of this step. 

There are 22 contaminants with recent (2006-2009) detections (Table 3) (Excel)Opens in new window..

4. Contaminants for further reviewOf the 22 contaminants detected, four were excluded from further review because they have recently revised MCLs:

Arsenic:  Arsenic was excluded because a revised California MCL for arsenic was required by state statute, outside of this review process.  The state MCL for arsenic became effective in 2008. 

Radionuclides:  Three detected radionuclides (radium-226, radium-228, and uranium) were excluded because regulations for radionuclides underwent extensive revision in 2006, outside of this review process. 

The remaining 18 contaminants were evaluated to determine chemicals for further review of the MCL.  They were considered in terms of  the number of sources (active and standby) with reported detections above the PHG or the MCL, and in terms of the criteria presented earlier.  They are discussed individually below.

Notes pertinent to individual chemicals:  (1) With regard to the basis for the PHG mentioned below, PHGs for cancer-causing substances are set at a level of 1 X 10-6, or up to one excess case of cancer per million people per 70-year lifetime exposure.  This is also called "de minimis" cancer risk. Public health and environmental regulatory agencies generally consider risks within the 10-6 to 10-4 cancer risk range to be "acceptable."  (2) For chemicals considered to be non-carcinogens, PHGs are set at a level equivalent to the no observed adverse effect level (NOAEL) divided by an uncertainty factor (UF) that reflects limitation in available scientific information related to the evaluation of effects.  For some contaminants, the UF may include an extra 10-fold factor to account for a possibility of cancer -- this would occur, for example, if the chemical is known to be carcinogenic when inhaled, but hasn't be found to be carcinogenic when ingested.

Inorganic Chemicals

  • Beryllium - MCL = 4 ppb; DLR = 1 ppb; PHG = 1 ppb.  Basis for PHG:  Non-cancer effects, NOAEL is based upon gastrointestinal lesions in dogs fed beryllium in their diets.  PHG includes a 1,000-fold UF (including a 10-fold factor reflecting the possible carcinogenic potential from ingested beryllium).  Detections (2006-2009):  1 above PHG, and 0 above MCL.  CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from beryllium in drinking water.   Thus, and considering the few detections, CDPH does not plan on further review of the beryllium MCL.
  • Cadmium - MCL = 5 ppb; DLR = 1 ppb; PHG = 0.04 ppb. Basis for PHG:  Non-cancer effects, based upon tubular damage in human kidneys indicated by the presence of small proteins and other substances. PHG includes a 50-fold UF (including a 10-fold factor reflecting the possible carcinogenic potential from ingested cadmium). Detections (2006-2009): 11 above PHG and 1 above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from cadmium in drinking water.   Thus, and considering the relatively few detections of cadmium and that the DLR is only 5 times lower than the MCL, CDPH does not plan on further review of the cadmium MCL. 
  • Mercury - MCL = 2 ppb; DLR = 1 ppb; PHG = 1.2 ppb. Basis for PHG:  Non-cancer effects:  NOAEL is based on kidney toxicity in short term studies in rats. PHG includes a 1,000-fold UF.   Detections (2006-2009): 4 above PHG and 2 above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from mercury in drinking water. Thus, and considering the few detections of mercury and that the MCL is 1.7 times the PHG, CDPH does not plan on further review of the mercury MCL.
  • Nickel - MCL = 100 ppb (no federal MCL); DLR = 10 ppb; PHG = 12 ppb. Basis for the PHG:  Non-cancer effects:  NOAEL based upon reproduction toxicity studies in rats. PHG includes a 1,000-fold UF (including a 10-fold factor reflecting the possible carcinogenic potential from ingested nickel). Detections (2006-2009): 41 above PHG and 2 above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from nickel in drinking water.  Thus, and considering the few detections, CDPH does not plan on further review of the nickel MCL.
  • Selenium - MCL = 50 ppb; DLR = 5 ppb; PHG = 30 ppb. Basis for the PHG:  Non-cancer effects:  NOAEL based upon hair loss and nail damage in people. PHG includes a 3-fold UF.  Detections (2006-2009): 22 above PHG and 14 above MCL. Selenium is an essential nutrient.  CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from selenium in drinking water.  Thus, and considering the few detections, CDPH does not plan on further review of the selenium MCL.
  • Thallium - MCL = 2 ppb; DLR = 1 ppb; PHG = 0.1 ppb. Basis for the PHG:  Non-cancer effects:  NOAEL based upon hair loss in rats. PHG includes a 3,000-fold UF.
    Detections (2006-2009): 4 above PHG and 1 above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from thallium in drinking water.  Thus, and considering the few detections, CDPH does not plan on further review of the thallium MCL.

Volatile Organic Chemicals

  • Benzene  - MCL = 1 ppb (federal MCL = 5 ppb); DLR = 0.5 ppb; PHG = 0.15 ppb.  Basis for PHG:  Cancer risk, based upon human data from workplace exposures. Cancer risk at PHG:  1 X 10-6. Cancer risk at DLR:  3.3 X 10-6.  Cancer risk at the 1-ppb MCL:  6.7 X 10-6.  Detections (2006-2009):  6 above PHG and 5 above MCL (3 above federal MCL).  CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from benzene in drinking water.  Thus, and considering the few detections and the constraints by the DLR, CDPH does not plan on further review of the benzene MCL. 
  • Carbon tetrachloride - MCL = 0.5 ppb (federal MCL = 5 ppb); DLR = 0.5 ppb; PHG = 0.1 ppb. Basis for PHG: Cancer risk, based upon experimental studies in mice. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR:  5 X 10-6. Cancer risk at MCL:  5 x 10-6. Detections (2006-2009): 51 above PHG and above MCL (8 above federal MCL). CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from carbon tetrachloride in drinking water.  Thus, and considering the constraints by the DLR (DLR = MCL), CDPH does not plan on further review of the carbon tetrachloride MCL.
  • 1,1-Dichloroethane - MCL = 5 ppb (there is no federal MCL); DLR = 0.5 ppb: PHG = 3 ppb. Basis for PHG: Cancer risk ,based upon experimental studies in rates.  Cancer risk at PHG: 1X10-6.  Cancer risk at MCL:  1.7 X 10-6 ppb. Detections (2006-2009): 1 above PHG and none above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from 1,1-dichloroethane in drinking water. Thus, and considering the few detections and that the MCL is only 1.7 times the PHG, CDPH does not plan on further review of the 1,1-dichloroethane MCL.
  • 1,2-Dichloropropane - MCL = 5 ppb; DLR = 0.5 ppb; PHG = 0.5 ppb. Basis for PHG:  Cancer risk, based upon experimental studies in mice. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR:  1 X 10-6. Cancer risk at MCL: 1 X 10-5. Detections (2006-2009): 14 above PHG and 0 above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from 1,2-dichloropropane in drinking water. Thus, and considering the few detections, CDPH does not plan on further review of the 1,2-dichloropropane MCL.  
  • 1,3-Dichloropropene - MCL = 0.5 ppb; DLR = 0.5 ppb; PHG = 0.2 ppb. Basis for PHG:  Cancer risk, based upon experimental studies in rodents. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR:  2.5 X 10-6. Cancer risk at MCL: 2.5 X 10-6. Detections (2006-2009): 3 above PHG and MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from 1,3-dichloropropene in drinking water. Thus, and considering the few detections, CDPH does not plan on further review of the 1,3-dichloropropene MCL.
  • Styrene - MCL = 100 ppb; DLR = 0.5 ppb; PHG = 0.007 ppb. Basis for the PHG:  Cancer risk, based upon laboratory studies in rodents.   Cancer risk at PHG: 1 X 10-6.  Cancer risk at DLR: 7.1 x 10-5.  Cancer risk at MCL: 1.4 X 10-2   Detections (2006-2009): 1 above PHG and 0 above MCL. CDPH is not aware of changes in treatment techniques related to styrene in drinking water.  However, there is new evidence regarding risks to public health from styrene in drinking water, as evidenced by the cancer-based PHG.  Thus, even though there is only a single detection, CDPH plans on further review of the styrene MCL, as resources allow.
  • Tetrachloroethylene (Perchlorethylene, PCE) - MCL = 5 ppb; DLR = 0.5 ppb; PHG = 0.06 ppb. Basis for the PHG: Cancer risk, based upon experimental studies in rodents. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 8.3 X 10-6. Cancer risk at MCL: 8.3 x 10-5. Detections (2006-2009): 406 above PHG, and 127 above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from PCE in drinking water. PCE is among the more frequently detected organic contaminants.  Even though there are constraints associated with the DLR, CDPH intends to examine the PCE detections, as resources allow, in order to develop a cost benefit analysis of possible MCL revisions.  This will likely be done along with a similar analysis for TCE, another frequently detected contaminant (see below).
  • 1,1,2-Trichloroethane - MCL = 5 ppb; DLR = 0.5 ppb; PHG = 0.3 ppb. Basis for PHG:  Cancer risk, based on experimental studies in mice. Cancer risk at PHG:  1 X 10-6.  Cancer risk at DLR:  1.7 X 10-6.  Cancer risk at MCL:  1.7 X 10-5. Detections (2006-2009):  3 above PHG and 0 above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from 1,1,2-trichloroethane in drinking water.  Thus, and considering the few detections, CDPH does not plan on further review of the 1,1,2-trichloroethane MCL. 
  • Trichloroethylene (TCE) - MCL = 5 ppb; DLR = 0.5 ppb; PHG = 1.7 ppb. Basis for the PHG:  Cancer risk, based upon experimental studies in mice.  Cancer risk at PHG: 1 X 10-6. Cancer risk at MCL: 2.9 x 10-6. Detections (2006-2009): 227 above PHG and 135 above MCL.  In 2001 when the PHG was 0.8 ppb, considering the number of TCE detections -- even though there was a lack of changes in treatment techniques and a lack of any new scientific evidence regarding risks to public health -- CDHS developed a draft cost benefit analysis (PDF)Opens in new window. of possible MCL revisions. No public comment period was scheduled for that document. In July 2004, OEHHA announced its plans to review the PHG for TCE and CDPH suspended its evaluation of TCE. 
  • Vinyl chloride - MCL = 0.5 ppb (federal MCL = 2 ppb); DLR = 0.5 ppb; PHG = 0.05 ppb. Basis for PHG:  Cancer risk, based upon experimental studies in mice.  Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 1 X 10-5. Cancer risk at MCL: 1 x 10-5. Detections (2006-2009): 2 above PHG and MCL (1 above federal MCL). CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from vinyl chloride in drinking water.  Thus, and considering the few detections as well as the constraints of the DLR, CDPH does not plan on further review of the vinyl chloride MCL. 

Synthetic Non-Volatile Organic Chemicals

  • 1,2-Dibromo-3-chloropropane (DBCP) - MCL = 0.2 ppb; DLR = 0.01 ppb; PHG = 0.0017 ppb. Basis for PHG: Cancer risk, based upon experimental studies in mice. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR:  5.9 X 10-6. Cancer risk at MCL:  1.2 x 10-4. Detections (2006-2009): 437 above PHG, and 139 above MCL, even though DBCP's use as a fumigant has been prohibited for many years. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from DBCP in drinking water.  Previously CDHS considered DBCP a candidate for possible MCL revision, given the number of detections and the 20-fold difference between MCL and DLR.  CDHS concluded (PDF)Opens in new window. that reduction of the current MCL would not be economically feasible.
  • Ethylene dibromide (EDB) - MCL = 0.05 ppb; DLR = 0.02 ppb; PHG = 0.01 ppb.  Basis for PHG:  Cancer risk, based upon forestomach tumors in experimental studies in rats and mice. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR:  2 X 10-6. Cancer risk at MCL: 5 X 10-6. Detections (2006-2009): 17 above PHG and 8 above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from EDB in drinking water. Thus, and considering that the MCL is just 2.5 times the DLR and 5 times the PHG, CDPH does not plan on further review of the EDB MCL.
 
 
Last modified on: 10/26/2011 12:48 PM