In early 2001, the US Environmental Protection Agency is expected to adopt a secondary maximum contaminant level (SMCL) for methyl tertiary butyl ether (MTBE) in drinking water. This article presents the first and only consumer study to determine the odor threshold of MTBE in drinking water. A protocol, based on the American Society for Testing and Materials method E679–91, was augmented to address concerns raised by interested stakeholders. The study, which was conducted according to the final odor threshold protocol, used a panel of 57 consumers and yielded an odor threshold for MTBE in drinking water of 15 μg/L. The 15 μg/L threshold is the geometric mean of the individual thresholds for each of the 57 consumers. This consumer panel threshold is consistent with the trained panel thresholds reported from five other taste and/or odor studies, which ranged from 13.5 to 45.5 μg/L. Consequently, the authors recommend using the methodology presented in this article as the scientific basis for establishing the federal SMCL for MTBE and other organic chemicals in drinking water.
Recent legislation in several states has called for the removal of methyl tert-butyl ether ͑MTBE͒ from gasoline. In order to comply with Federal Clean Air Act requirements for carbon monoxide and ozone attainment, ethanol is being considered as a replacement for MTBE. The objective of this study is to evaluate the potential impact of ethanol on benzene plume lengths in subsurface environments following accidental spills of ethanol-blended gasoline. Two types of studies were conducted here. First, laboratory studies were performed using a pure culture indigenous to a gasoline-contaminated aquifer to evaluate the effect of ethanol on the rate of benzene biodegradation under aerobic conditions. Results from microbial studies showed that the biodegradation of 25 mg/L benzene was severely inhibited in the presence of 25 mg/L ethanol. While the enzymes responsible for benzene biodegradation by the culture were inducible, ethanol degradation appeared to be constitutive. Second, a two-dimensional model was developed to quantify the impact of ethanol on benzene plume lengths using weighted-average aerobic and anaerobic biodegradation rates for benzene in the presence and absence of ethanol. Model simulations indicated that benzene plume lengths are likely to increase by 16-34% in the presence of ethanol.
The addition of methyl tert-butyl ether (MTBE) to gasoline has resulted in public uncertainty regarding the continued reliance on biological processes for gasoline remediation. Despite this concern, researchers have shown that MTBE can be effectively degraded in the laboratory under aerobic conditions using pure and mixed cultures with half-lives ranging from 0.04 to 29 days. Ex-situ aerobic fixed-film and aerobic suspended growth bioreactor studies have demonstrated decreases in MTBE concentrations of 83% and 96% with hydraulic residence times of 0.3 hrs and 3 days, respectively. In microcosm and field studies, aerobic biodegradation half-lives range from 2 to 693 days. These half-lives have been shown to decrease with increasing dissolved oxygen concentrations and, in some cases, with the addition of exogenous MTBE-degraders. MTBE concentrations have also been observed to decrease under anaerobic conditions; however, these rates are not as well defined. Several detailed field case studies describing the use of ex-situ reactors, natural attenuation, and bioaugmentation are presented in this paper and demonstrate the potential for successful remediation of MTBE-contaminated aquifers. In conclusion, a substantial amount of literature is available which demonstrates that the in-situ biodegradation of MTBE is contingent on achieving aerobic conditions in the contaminated aquifer.
The discovery in California of methyl tertiary-butyl ether (MTBE) in surface waters used for recreational boating has raised concerns over the potential impact on drinking water quality. Concentrations of MTBE above the California secondary maximum contaminant level of 5 ppb have been reported. Here we present a model to predict the fate of MTBE in surface waters as a function of wind speed, water temperature, epilimnion depth, and lake surface area. The model was validated with MTBE concentration data from Lake Perris in southern California and Calero Reservoir in northern California. When applied to typical lake conditions in California [i.e., epilimnion depth <11 m (<35 ft) and water temperature >15ЊC], the maximum half-life for MTBE is <40 days for quiescent conditions, and as low as 6 days if the average wind speed is >4.5 m/s (10 mi/h). The model can be used for management of recreational boating based on a target MTBE concentration in the reservoir.
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