MTBE in California Drinking Water: An Analysis of Patterns and Trends

Over the past decade, there has been much publicity surrounding the impact of Methyl tert -butyl ether (MTBE) on drinking water supplies in the United States. In California, the presence of MTBE in groundwater and drinking water has led to a ban on the future use of MTBE in gasoline. Other states, such as those in the northeast, are also seeking ways to reduce or eliminate the use of MTBE due to perceived threats to the environment and public health. Despite claims about the incidence of MTBE in drinking water, no comprehensive characterization has been conducted on the available drinking water monitoring data. This paper provides a detailed analysis of the MTBE drinking water data compiled by the California Department of Health Services (CDHS) from 1995 to 2000. We find that MTBE was detected in about 1.3% of all drinking water samples, 2.5% of drinking water sources, and 3.7% of drinking water systems in California over this 6-year period. Our analysis reveals that many drinking water sources are not sampled routinely for MTBE, and in those sources that appear to be affected by MTBE, the compound is not consistently detected. The majority of MTBE detections are also concentrated in several geographic areas, which contain about 9–21% of the total California population. Average detected MTBE concentrations have decreased significantly since 1995 and 1996, ranging from 5 to 15 ppb over the last 3 years depending on the outcome of interest. Of the samples in which MTBE was present above the analytical detection limit, the concentrations in approximately 73% of drinking water samples and 86% of drinking water sources and systems were below the State's primary health-based standard of 13 ppb. Our findings suggest that, although some drinking water supplies in California have been affected by MTBE, the majority of drinking water sources and systems either have not been affected at all or contain MTBE at concentrations below levels that are likely to be of health concern.