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MCL Review in Response to PHGs

March 1, 2008

Last Update: March 12, 2008

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.

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; (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.

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

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.  

Regulated chemicals' MCLs, DLRs, and PHGs are here (Excel)Opens in new window..

New PHGs in 2007

In 2007 OEHHA revised the PHG for glyphosate and established a PHG for polychlorinated biphenyls. 

MCL Review Status

From the list of contaminants PHGs (Excel)Opens in new window., those with PHGs established through 2007 were identified. 

  • Regulated contaminant with PHGs:  The selection process for MCLs for possible review first considers the regulated chemicals with PHGs.
    There are 80 contaminants with MCLs that have PHGs (Table 1) (Excel)Opens in new window..
  • Contaminants with MCLs greater than PHGs:  The selection process then identified contaminants with MCLs greater than PHGs.   Chemicals with MCLs below their PHGs 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.  Lead and copper do not have MCLs (they are covered by a different regulatory approach) and were not considered further.  Their MCL-like "action levels" (22 CCR §64678) are 7.5 and 4.3 times their PHGs, respectively. In addition, 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.
    There are 36 contaminants with MCLs greater than their PHGs (Table 2) (Excel)Opens in new window..   
  • 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 at or above the DLR is a "detection" for purposes of this step. 
    There are 17 contaminants with recent (2003-2006) detections (Table 3) (Excel)Opens in new window..
  • Contaminants for further review:  Of the 17 contaminants in Table 3, arsenic was excluded because a revised California MCL for arsenic is required by state statute, outside of this review process.  The state MCL cannot be less restrictive than the federal MCL of 10 micrograms per liter (µg/L).  The federal MCL became effective January 2006.  Radionuclides (radium-226, radium-228, and uranium) were also excluded because regulations for radionuclides underwent extensive revision in 2006, outside of this review process.  The remaining 13 contaminants in Table 3 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 (Table 4) (Excel)Opens in new window., and in terms of the criteria presented earlier.  

Comments on Individual Contaminants and Recommendations for Further Review

The 13 contaminants and the result of the evaluation are listed below.  The basis for the PHG enters into this discussion, so the following bullets provide some additional information

  • PHGs for Carcinogens:  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."
  • PHGs for Non-Carcinogens: 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.

Contaminants:

  • Benzene  - 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 MCL:  6.7 X 10-6. Detections (2003-2006):  4 above PHG and 4 above MCL (see Tab 4).  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.   
  • Beryllium - Basis for PHG:  Non-cancer effects, no observable adverse effects level (NOAEL) based upon gastrointestinal lesions in dogs fed beryllium in their diets.  Uncertainty factor (UF) at PHG:  PHG includes a 1,000-fold UF (including a 10-fold factor reflecting the possible carcinogenic potential from ingested beryllium). The MCL is 4 times the PHG. Detections (2003-2006): 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 - Basis for PHG:  Non-cancer effects, based upon tubular damage in human kidneys indicated by the presence of small proteins and other substances. UF at PHG:  PHG includes a 50-fold UF (including a 10-fold factor reflecting the possible carcinogenic potential from ingested cadmium). The MCL is 125 times the PHG. Detections (2003-2006): 12 above PHG and 3 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.  
  • Carbon tetrachloride - 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 (2003-2006): 51 above PHG and 51 above 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.
  • 1,2-Dibromo-3-chloropropane (DBCP) - Basis for PHG: Cancer risk level, 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 (2003-2006): 428 above the PHG, and 131 above the MCL, even though DBCP's use as a fumigant has been prohibited for many years. Although CDHS (now CDPH) was not aware of changes in treatment techniques or new evidence regarding risks to public health from DBCP in drinking water, given the number of detections and the 20-fold difference between the MCL the DLR, CDHS considered DBCP a candidate for possible MCL revision.  CDHS concluded in 2000 (PDF)Opens in new window. that reduction in the current MCL would not be economically feasible.
  • 1,2-Dichloropropane - Basis for PHG:  Cancer risk, based upon experimental studies in mice. Cancer risk at PHG: 1 X 10-6. The DLR is the same as the PHG. Cancer risk at MCL: 1 x 10-5. Detections (2003-2006): 12 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.
  • Ethylene dibromide (EDB) - Basis for PHG:  Cancer risk, based upon forestomach tumors produced 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 (2003-2006): 3 above PHG and 3 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 the relatively few detections and 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.
  • Mercury - Basis for PHG:  Non-cancer effects:  NOAEL is based on kidney toxicity in short term studies in rats. UF at the PHG:  PHG includes a 1,000-fold UF.  The DLR is below the PHG. The MCL is 1.7 times the PHG.  Detections (2003-2006): 2 above PHG and 1 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 - Basis for the PHG:  Non-cancer effects:  NOAEL based upon reproduction toxicity studies in rats. UF at the PHG:  PHG includes a 1,000-fold UF (including a 10-fold factor reflecting the possible carcinogenic potential from ingested nickel). The DLR is below the PHG. The MCL is 8.3 times the PHG. Detections (2003-2006): 24 above PHG and 4 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.
  • Tetrachloroethylene (Perchlorethylene, PCE) - 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 (2003-2006): 465 above PHG, and 152 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 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 - 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 (2003-2006):  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 benzene 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) - Basis for the PHG:  Cancer risk, based upon experimental studies in mice.  Cancer risk at PHG: 1 X 10-6. The DLR is below the PHG. Cancer risk at MCL: 6.3 x 10-6. Detections (2003-2006): 348 above PHG and 151 above MCL. In 2001, 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 (CDPH's prior organization) 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, as part of its update of reviews for chemicals with PHGs prepared in the first years of the program, based on availability of new data and significance as a drinking water contaminant. Thus, CDPH suspended its evaluation of TCE.
  • Vinyl chloride:  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 (2003-2006): 3 above PHG and 3 above 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.