air pollution
...quifers with time. MTBE enters the environment, and eventually the groundwater, mainly from leaking underground fuel tanks and associated piping, but also from incomplete combustion in internal combustion engines, spilling and evaporation during transportation and refueling, and watercraft exhaust. Atmospheric precipitation may be another potential source of MTBE in groundwater, because MTBE percolates easily through soil due to its small molecular size and solubility in water, allowing it to move rapidly into groundwater. The Environmental Protection Agency has classified MTBE as a possible human carcinogen, but no drinking-water regulation has been established for the compound. The Environmental Protection Agency has issued a drinking water advisory of 20- 40 micrograms per liter, based upon odor and taste thresholds, and to provide a large margin of safety from carcinogenic effects. Since February 1997, the California Department of Health Services has required public water suppliers to monitor their drinking water sources for MTBE. As of December 1997, about 23%of drinking water sources in California had been sampled for MTBE contamination. Of those sites tested, 33 or 1.3%, had detectable levels of MTBE. Of the contaminated sites tested, 36% had MTBE levels above the state's proposed drinking water standard. Some water systems only test every three years for volatile organic compounds, such as MTBE, so it will be the end of 2000 before all systems will have been tested. For MTBE, this frequency of impact to public drinking wells may not be a reliable indicator of future trends because it reflects a history of releases, including those involving gasoline formulations containing no or only low volumes of MTBE. It also appears that dissolved benzene plumes were of larger regulatory concern than MTBE in previous studies. Most studies have indicated that MTBE does not biodegrade easily under various environmental conditions. If a research investigation determines that a compound does not degrade, a half-life is not reported and the compound is classified as recalcitrant. MTBE is generally reported as recalcitrant, and there are no widely accepted estimates of the half-life. Investigators have reported that MTBE is recalcitrant in anaerobic laboratory studies including denitrifying conditions, sulfate-reducing conditions, methanogenic-reducing conditions, and anaerobic conditions in landfill-affected aquifer material, soils, and sludges. One 1995 result indicated there was no degradation of MTBE in an aerobic laboratory study after more than 100 days of incubation. Degradation of MTBE has been reported on occasion and this indicates that some microorganisms are able to degrade MTBE. Resent research has demonstrated that bacterial populations and certain pure bacterial strains, when isolated from biotreated sludges and other sources, have the ability to use MTBE as a sole carbon source. Engineers in the laboratory of Marc Deshusses, an assistant professor of chemical engineering at University of California at Riverside, are studying how microorganisms with an affinity for MTBE degrade the additive under various conditions. They found the biodegradation rate of MTBE in both laboratory flasks and bioreactors was greatly improved by adding trace amounts of peat humic substances, large organic molecules that can be extracted from peat. The substances seem to stimulate the microbes, but scientists do not yet know how. There are no studies of effects on humans of long term exposure to MTBE. Studies used to determine the hazards have been done with laboratory animals, which creates many limitations and uncertainty. Animal tests performed in 1997 were not conducted by exposing animals to MTBE in drinking water, but rather by introducing oil containing MTBE directly into their stomachs several times a week. The Environmental Protection Agency determined, although useful for identifying potential hazards, limitations of the reported studies do not allow confident estimates of the degree of risk MTBE may pose to humans from low-level drinking water contamination. In 1997, the Ca...