Your County Government Community
Kids & Families
Environment Public Safety Jobs

Environmental Health

A to Z Services
About Us
Doing Business

Related Links

Air Resources Board
California Water Quality Control Board - Region 9 San Diego
Federal EPA
Federal EPA Brownfields
Federal EPA Region 9
Occupational Safety and Health Association
State of California Legislation Information
State Water Resources Control Board
Underground Storage Tank Program

Methy-tertiary-butyl-ether (MTBE)

Printer Friendly
Font Size Extra Large Font Size Large Font Size Default
Chemical structure of methyl-tertiary-butyl ether.

What you need to know about MTBE

What Must Be DoneMaximum Contaminant Levels, MCLs
Prioritization of MTBE
Status and Impact of State Ban
SWRCB Draft Guidelines
Physical Properties 

What You Should Know

Methyl-tertiary-butyl ether (MTBE) is a fuel oxygenate added to gasoline to reduce air pollution and increase octane ratings. Widespread use of this chemical since the 1970s has resulted in frequent detection of MTBE in shallow groundwater in San Diego County.

On March 25, 1999 Governor Davis issued Executed Order D-5-99, and signed Senate Bill 989 (Chapter 812, Statues of 1999) on October 8, 1999.  These documents recognized that Methyl-tertiary-butyl ether (MTBE) can cause significant adverse impacts to current and future beneficial uses of ground and surface water and direct the SWRCB to develop guidelines for investigation and cleanup of MTBE in vulnerable areas.

In accordance with the Health and Safety Code, Section 25299 which became effective January 1, 2000, all sites in San Diego County with MTBE contamination must be prioritized as to the potential threat the site may pose to beneficial ground and surface waters.

What Must Be Done

Groundwater resource areas in San Diego County are very diverse. These areas include: alluvial deposits along streams and rivers; sedimentary formations; and fractured crystalline rock. Consequently, the vulnerability of these resources must be evaluated on a site-by-site basis. The main criterion for determining vulnerability is whether the aquifer underlying a site or a surface water body near the site is or will be used as a source of drinking water.

Once it has been established that the site is in a vulnerable area, the distance to the nearest receptor must be determined. Receptors are defined as drinking water wells and surface water sources currently being used. Drinking water wells in San Diego County include public, private or irrigation wells.

Under current State guidelines, the distance to the receptor and the concentration of MTBE at the site are the determining criteria for classification of a site. The assumptions used for the State guidelines are very conservative and may result in a higher priority classification. When enough site-specific data is available, it may be possible to re-classify a site to a lower priority. The site-specific data would include: rate of movement of MTBE in the subsurface between the site and receptor; degradation rate of the MTBE at the site; and establishing the time needed for MTBE to reach the receptors. 

Resources will be directed to those sites that pose the greatest and most immediate threat to the receptors.  Sites which are determined to pose the greatest threat will be tracked more closely to assure a timely and effective investigation.

Prioritization of MTBE Cases

Lead agencies such as the San Diego County Department of Environmental Health are tasked with the responsibility to oversee the investigation and cleanup of sites where there have been or may have been releases of MTBE.

As lead agency, the Site Assessment and Mitigation Program (SAM) must examine cases that are located in areas most vulnerable to groundwater contamination and classify them based on concentration of MTBE contaminants and distance to receptors. The distance from the contaminant source to the receptor is used to initially estimate the level of risk posed to drinking water sources.

Requirements of the Responsible Party

As a responsible party, you are required to determine the distribution of MTBE in the subsurface and identify nearby receptors and the potential for migration of MTBE to receptors.

Investigation Priority Classification

One of our primary tasks is to assign an investigation priority classification to each site. Assignment of an investigation priority classification is based on the distance of MTBE contaminants to the nearest receptor.

Sites that are determined to pose the greatest threat will be given the highest priority. These sites must be tracked more closely to assure a more timely and effective investigation. Sites are classified with A being the highest priority and D the lowest in priority. DEH will conduct case review and send directive letters within 30 days of notification of an MTBE release.

The following is the initial investigation priority classification system established by the SWRCB in their guidelines:

Site ClassificationRegulatory Response Timing
Class A:Determine cleanup priority classification as soon as possible, not later than one year after notification or discovery of MTBE release.
Class B:Determine cleanup priority classification within two years after notification or discovery of MTBE release.
Class C:Determine cleanup priority classification within three years after notification or discovery of MTBE release.
Class D:Not located in an area that is most vulnerable to contamination and has concentrations of MTBE in groundwater over 5 ppb.
 Determine cleanup priority classification within five years after notification or discovery of MTBE release. 


If wells are suspected to be in the area but their exact locations are unknown, the site will be given a higher investigation priority classification rather then a lower classification until more information becomes available. The investigation priority classification may be decreased if it becomes apparent through an assessment investigation that the site does not pose a threat to nearby receptors.

Cleanup Priority Classification

Assignment of Cleanup priority occurs after sufficient data has been collected to estimate the travel time for the contaminant plume to reach a receptor but not more than the time specified in the Priority Classification. At that time in the process, the site is given a priority for remediation based upon the estimated plume travel time to the nearest down-gradient receptor, or other criteria determined by DEH.

Estimating the approximate travel time for a contaminant plume to reach a nearby receptor is a key part of the investigation. DEH will work with you and your consultant to evaluate travel time. This estimation will serve as a basis for the next step in the process, assigning a cleanup priority classification. The following is the cleanup priority classification:

ClassificationRegulatory Response Timing
Class 1:Groundwater MTBE plume travel time to nearest downgradient receptor: < 5 years
 Implement remedial action plan as soon as possible, not later than 1 year after determination of cleanup priority class.
Class 2:Groundwater MTBE plume travel time to nearest downgradient receptor: > 5 years and < 20 years
 Implement remedial action plan within 5 years after determination of cleanup priority class.
Class 3:Groundwater MTBE plume travel time to nearest downgradient receptor: > 20 years
 Direct cleanup resources to these sites after sites in classes 1 and 2 have been addressed.


The California Underground Storage Tank Program can provide more information regarding the guidelines entitled,  Final Draft Guidelines for Investigation and Cleanup of MTBE and Other Ether-Based Oxygenates State of California Draft Guidelines on MTBE.

Corrective Action
When DEH determines that a site requires remedial action, those actions must be taken expeditiously. In general, the type of response actions taken at MTBE release sites will be similar to the type of actions taken at traditional petroleum releases. The primary difference is that responses to MTBE will need to be swifter and more aggressive to reduce the spread of MTBE to a wider area.

Cleanup Fund
The Underground Storage Tank Cleanup Fund which is administered by the Division of Clean Water Program is tasked to respond in a timely manner to meet the time frames specified for higher priority MTBE cases.

If you have any questions, please contact Allen Patton at (916) 341-5250 at the State Water Resource Control Board.

Maximum Contaminant Levels

Maximum Contaminant Levels (MCLs)

Methyl-tertiary-butyl-ether (MTBE)
Primary MCL13 µg/l
Secondary MCL5 µg/l