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#          MTBE and other volatile organic compounds—New findings and         implications on the quality of source waters used for drinking-water         supplies      

##          How frequently are VOCs detected in ground water?      

         Volatile organic compounds (VOCs), which are widely used in the         manufacture of many products including refrigerants, plastics,         adhesives, paints, and petroleum products, have been detected in about         one-third of the wells sampled by the National Water-Quality         Assessment (NAWQA) Program of the U.S. Geological Survey         (USGS).1 Chloroform and other         trihalomethanes, the most commonly detected compounds, were found in         about 9 percent of the sampled wells. Solvents, particularly         chlorinated solvents, were found in about 8 percent of sampled wells.         VOCs predominantly occur in urban areas, often in mixtures;         specifically, 2 or more compounds were found in about one quarter of         the sampled urban wells.      

         A commonly detected VOC is methyl *tert*-butyl ether (MTBE), which is a         gasoline oxygenate designed to add oxygen to decrease vehicular carbon         monoxide emissions and ozone levels in the atmosphere. MTBE has the         highest production volume of all fuel oxygenates.      

-             Trihalomethanes, solvents, and gasoline oxygenates are the most            commonly detected volatile organic compounds in ambient ground            water.         

##          How often is MTBE detected in ground water?      

         MTBE was detected in about 5 percent of ground-water samples collected         by NAWQA across the Nation. The concentrations typically were low,         well below the U.S. Environmental Protection Agency (EPA)         drinking-water consumer advisory concentration of 20 to 40 micrograms         per liter, which is based on taste and odor thresholds. In fact, less         than 1 percent of samples included in the USGS studies exceeded the         EPA consumer advisory concentration of 20 micrograms per liter. Still         uncertain, however, are possible human health effects. EPA has         tentatively classified MTBE as a possible human carcinogen, but         because of insufficient toxicity studies has not instituted a         drinking-water health advisory or standard. EPA is using NAWQA         findings, along with other investigations and research, to investigate         possible drinking-water and human-exposure issues.      

##          Where is MTBE most likely found?      

         NAWQA findings indicate that MTBE is most frequently detected in         ground water underlying urban areas in comparison to agricultural and         mixed land-use settings. MTBE was detected in about 14 percent of         wells sampled in urban areas. In addition, incidence of MTBE increases         in high-use areas. Most of MTBE’s high use occurs in         Reformulated Gasoline (RFG) areas, where gasoline contains 11-percent         MTBE by volume. MTBE has been detected about 4 to 6 times more         frequently in the high-use areas than elsewhere.      

-             The detection frequency of MTBE in ambient ground water is highest            in urban land use.         

##          Is MTBE in drinking water?      

         In addition to its studies of ambient ground water, USGS is conducting         focused studies to assess MTBE concentrations associated with         drinking-water supplies. In cooperation with EPA’s Office of         Ground Water and Drinking Water, USGS examined data on finished         drinking water from selected communities in 12 states in the Northeast         and Mid-Atlantic Regions. The data show that MTBE was detected in 9         percent of the community water systems. Consistent with USGS studies         of ambient ground water, concentrations were low; less than 1 percent         exceeded the EPA consumer advisory concentration.      

         USGS presently is conducting a special study of source waters used for         community drinking-water systems. This 4-year study, currently in its         last year, is being performed in cooperation with the Metropolitan         Water District of Southern California, the Oregon Graduate Institute         of Science and Environmental Engineering, and the American Water Works         Association Research Foundation (AWWARF), and includes about 1,000         water utilities across the Nation. Findings are consistent with         previous NAWQA studies. Specifically, MTBE has been detected in low         concentrations, and less than 1 percent of the samples exceed the EPA         consumer advisory concentration. This study, which includes samples of         both surface-water and ground-water sources for drinking water, shows         that MTBE was detected more frequently in surface-water samples (14         percent) than in ground-water samples (5 percent). This finding can be         explained, in part, by the inclusion of many samples from large rivers         and reservoirs that are associated with substantial use of         recreational watercraft. Older models of watercraft motors are known         to release a fraction of non-combusted gasoline to water.      

         Increased occurrence of MTBE in large community water systems is also         reported in this study. Specifically, MTBE was detected in about 4         percent of community water systems serving less than 10,000 people,         and in nearly 15 percent of systems serving greater than 50,000         people. Wells providing drinking water for the larger community water         supplies often are co-located in urbanized settings, which are         associated with a higher incidence of MTBE. In addition, rates and         amounts of pumping likely play a role because increased pumping rates         draw water from extended areas, which thereby increases the likelihood         of intercepting a source of MTBE.      

##          Significance of findings      

-             MTBE was detected in ground water and drinking water at low            concentrations, typically at lower concentrations than the EPA            drinking-water consumer advisory concentration for taste and smell.            Less than 1 percent of the samples exceeded the concentration in            this advisory. Toxicity studies are not complete, however, and            human health issues, including non-carcinogenic and carcinogenic            effects, have not been clearly outlined. As a result, there is a            continued need to monitor and assess the presence of MTBE as the            Nation moves forward with the development of a health advisory and            water-resource strategies that reflect the occurrence and            distribution of MTBE.         
-             The distribution of MTBE contamination is not uniform. For example,            the frequency of MTBE detection increases with its use, which is            typically associated with urban and populated areas. MTBE has been            detected in about 1 out of 5 wells in MTBE high-use areas. These            findings suggest that continued routine monitoring of this chemical            is needed for drinking-water supplies, especially in the MTBE            high-use areas.         
-             Some areas are more vulnerable to MTBE contamination than others.            For example, the frequency of detecting MTBE increases with larger            community water systems, specifically those serving greater than            50,000 people. The difference in vulnerability is, in part,            attributed to natural features, land use, and human activities,            such as pumping and watercraft usage. This type of information is            essential in the prioritization and development of source-water and            drinking-water protection strategies, and in optimal and            cost-effective delivery of water.         

##          Future directions and needs      

-  Sources:  Both point and nonpoint sources contribute to            MTBE contamination. Improvements in management of this chemical            will depend on increased knowledge of the different sources. Key            questions include: What are the relative contributions of point            sources, such as leaking tanks, compared to nonpoint sources, such            as those related to stormwater runoff, watercraft, vehicular use,            and the atmosphere? Do existing pollution abatement programs            adequately address all of the important sources of MTBE to ground            water and surface water?         
-  Aquifer vulnerability and transport:  Because of the            complex nature and slow rate of ground-water flow, and the high            solubility and mobility of MTBE, the future distribution patterns            of this relatively new chemical, representing only about 10 to 15            years of use, is unknown. A greater understanding is needed of its            transport in different aquifers and environmental settings under            various pumping scenarios before its presence in future            drinking-water supplies can be fully anticipated. Differences in            natural factors, such as soils, geology, hydrology, and water            chemistry, can result in very different degrees of aquifer            vulnerability to contamination and in different rates at which            improved management can lead to lower incidence of contamination.            Key questions include: What factors control the transport of MTBE            as it moves from its sources, such as an underground tank, to            shallow ground water and to underlying deeper aquifers? What            affects the degree and timing of its occurrence in drinking-water            supplies?         

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### Footnotes

1 Detection does not necessarily         translate to risk. The NAWQA Program measures chemicals at very low         concentrations, often 10 to100 times lower than EPA standards and         health advisories. For example, the detection frequency for MTBE is         based on an assessment level of 0.2 micrograms per liter, which is 100         times lower than EPA’s drinking-water consumer advisory.         Low-concentration sampling is used to detect and evaluate contaminants         of concern, to track contaminant concentrations over time, and to         determine natural and human factors related to chemical occurrence.      

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*By John S. Zogorski, Michael J. Moran, and Pixie A. Hamilton*

         USGS Fact Sheet 105-01      

         October, 2001      

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         Contacts for additional information or questions:      

         Dr. John Zogorski, U.S. Geological Survey          National Water-Quality Assessment Program          1608 Mountain View Road          Rapid City, SD 57702          (605) 355-4560, ext. 214          (605) 355-4523 (fax)          jszogors@usgs.gov      

         Technical information supporting this fact sheet, as well as access to         other NAWQA publications, data, and maps, are provided on the Internet         at:      

http://wwwsd.cr.usgs.gov/nawqa/vocns/

         Information on the objectives and scope of the NAWQA Program is         provided on the Internet at:      

http://water.usgs.gov/nawqa

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