MTBE in California's Public Drinking Water Wells: Have Past Predictions Come True?

Abstract

Extensive water quality monitoring data in California are now available for methyl tertiary butyl ether (MTBE) that span the past 16 years. The statewide database includes most of the active (and some inactive) public drinking water wells in the state, most of which have been repeatedly sampled for MTBE during 1995 to 2010. The cumulative detection frequency of MTBE in public drinking water wells in California at any concentration level is approximately 1.6% and 0.9% based on a one-detection criterion and two-detection criterion, respectively. The detection frequency of MTBE is approximately two to three times lower at concentrations above the state drinking water standards. Since 1996, the annual detection frequency of MTBE at any concentration level has ranged from 0.4 to 0.9% (one-detection criterion) and 0.2% to 0.7% (two-detection criterion). Despite repeated sampling, MTBE has been detected in only 1 or 2 years for most public drinking water wells that have ever been found to contain MTBE at any concentration level. The annual average detected concentration of MTBE has ranged from 5 to 16 μg/L (arithmetic mean) and 2 to 5.5 μg/L (median), excluding two outlier samples. MTBE detections are more prevalent in select geographic locations, rather than widespread across the entire state. The cumulative detection frequency for MTBE is approximately 1% for wells that belong to very small and small public drinking water systems (i.e., that serve between 25 and 3,300 people) and is approximately 2% for wells that belong to large and very large public drinking water systems (i.e., that serve >10,000 people). No detections of tertiary butyl alcohol (TBA) were found from 1996 to 2000, although few samples of TBA were collected during this time period. From 2001 to 2010, the cumulative detection frequency of TBA in public drinking water wells in California at any concentration level was approximately 1.4% and 0.2% based on a one-detection criterion and two-detection criterion, respectively. The cumulative detection frequency of tetrachloroethylene (PCE) in public drinking water wells in California is at least six times greater than that of MTBE and TBA, and unlike the trends observed for MTBE and TBA, PCE is often consistently detected in the same wells over time. The results of this analysis can be used to assess historical and current detections of MTBE in public drinking water wells in California. Also, if coupled with other chemical and site-specific information, the results can be used to assess potential future impacts of MTBE on drinking water supplies.

Keywords:

• methyl tertiary butyl ether (MTBE)
• drinking water
• groundwater
• tertiary butyl alcohol (TBA)
• tetrachloroethylene (PCE)
• risk assessment

Acknowledgment

This work was funded by the MTBE joint defense group, comprising oil and chemical companies that have historically sold, manufactured, delivered, and/or stored gasoline containing MTBE in the United States and who have been involved in MTBE-related litigation. The author, who currently serves as an expert witness in litigation matters related to MTBE, had sole responsibility for the writing and content of this article.

Notes

^ASampling dates vary for MTBE (4/10/95 to 10/26/10), TBA (5/1/96 to 10/12/10), and PCE (1/3/95 to 10/26/10); does not include full year data for 2010

^BExcludes water type S, M, W, and blank; status codes MW, AG, and misclassified/no longer used (PU, PN, PT, PR); samples collected prior to 1995 or unknown date; and false positive (F), invalid (I) or questionable (Q) finding modifier

^CNote that several sources are coded as having more than one status

^DAn active source which is sampled after any treatment (AT), not treated (AU), or sampled before any treatment (AR)

^ESample point within the distribution system after treatment (DT) or before treatment (DR)

^FCombined sources which are treated (CT), not treated (CU), raw (CR) or mixed (CM)

^GA source which is not in service for periods of one year or greater and which provides raw water which is sampled after treatment (IT), without any treatment (IU), or before any treatment (IR)

^HA source which is used less than 15 calendar days per year, with periods not to exceed five consecutive days and which provides raw water which is sampled after treatment (ST), without any treatment (SU), or before any treatment (SR)

^IA source which is no longer being used, with no intention of being used in the future, and which is not destroyed

^JA source which is filled and capped with no possiblity of being used in the future

^AIncludes wells with all types of water status (e.g., raw, treated, untreated, inactive, on-standby, abandoned, destroyed)

^AIncludes wells with the current water status of “raw” or “untreated” (AU, AR, DR, CU, CR, IU, IR, SU, SR)

^AIncludes wells with the current water status of “active or “distribution” or “combined” (AT, AU, AR, DT, DR, CT, CU, CR, CM)

^AU.S. EPA public drinking water system classification according to the number of people served

^BNot included in definition of public water system which provides water for human consumption to at least 15 service connections or serves an average of at least 25 people for at least 60 days a year

^CThe detection frequency for wells belonging to the largest drinking water systems (i.e., those serving >1,000,000 people) is 8% (i.e., 8/100).

^AIncludes wells with all types of water status (e.g., raw, treated, untreated, inactive, on-standby, abandoned, destroyed)

^AIncludes wells with all types of water status (e.g., raw, treated, untreated, inactive, on-standby, abandoned, destroyed)