MTBE:  Regulations and Drinking Water Monitoring Results

Last Update:  November 20, 2008

Follow this link for information about the Drinking Water Treatment and Research Fund (DWTRF). The DWTRF addresses MTBE and other oxygenates in drinking water.

MTBE Drinking Water Standards

The gasoline additive methyl tertiary butyl ether (MTBE) is a regulated drinking water contaminant in California.  MTBE has:

• a primary MCL of 13 micrograms per liter (µg/L), established in 2000, that addresses health concerns, as explained in the final statement of reasons.   Its 1999 public health goal is 13 µg/L, corresponding to the de minimis cancer risk derived from animal studies.
• a secondary MCL of 5 µg/L, established in 1999.  The secondary MCL addresses taste and odor concerns.
• a detection limit for purposes of reporting (DLR) of 3 µg/L.  The DLR is the level at which CDPH is confident about the quantity being reported.  Results at or above the DLR are required to be reported to CDPH; some laboratories may report results at lower concentrations.

Monitoring Results

MTBE was found in some drinking water sources in the early and mid-1990s, including two wells under development but not put into use that had peak concentrations of 23 and 500 µg/L at the Presidio of San Francisco in 1990, and in  four wells in Santa Monica with peak concentrations of 87 to 610 µg/L in 1995-1996. 

Thereafter,  CDHS (the California Department of Health Services, now CDPH) adopted a 1997 regulation that included MTBE as an unregulated chemical for which monitoring was required to be done by certain public water systems.  Subsequently, required monitoring has been associated with compliance with MTBE's MCLs.

As of November 2008, the CDPH database from 1997 to the present includes MTBE analytical results reported for about 15,000 sources, where "sources" may include both raw and treated drinking water wells and surface water sources, distribution systems, blending reservoirs, and other sampled entities.  Active, standby, inactive, and abandoned or destroyed sources are included.

Nearly all of the results are non-detects.  However, 90 sources have reported MTBE detections (PDF) greater than the DLR.  The sources are in 33 counties, as shown in Table 1, which also shows 32 sources with a peak detection greater than MTBE's primary MCL, and 31 sources with a peak level greater than the secondary MCL but lower than the primary MCL.  Readers should be advised that public water systems may not serve drinking water that exceeds the primary MCL.  In addition, many of these sources are inactive, abandoned or destroyed. 

Since 1997, the highest findings have been 400 µg/L in Monterey County in 2004; 234 µg/L in Yuba County in 1999; and 112 µg/L in San Diego County in 2004.