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A PRELIMINARY ASSESSMENT OF THE OCCURRENCE AND POSSIBLE SOURCES OF MTBE IN GROUND WATER OF THE UNITED STATES, 1993-94U.S. Geological Survey Open-File Report 95-456 By Paul J. Squillace, John S. Zogorski, William G. Wilber, and Curtis V. Price
Table of Contents
Relation of MTBE in shallow urban ground water to historical use
Occurrence of MTBE in deeper ground water
Possible sources of MTBE in ground water
Note: this document is also available as a PostScript file (421K, compressed)
About 109 million people live in counties where fuel oxygenates are used to meet either one or both of the requirements of the Clean Air Act Amendments of 1990 (fig. 1). MTBE also is used in many other undefined areas to enhance the octane of gasoline, but its use is not mandated in these areas. Currently, fuel oxygenates are added to more than 30 percent of the gasoline in the United States (U.S. Environmental Protection Agency, 1994) and by the year 2000 it is projected that fuel oxygenates will be added to 70 percent of the gasoline pool in the United States (Shelly and Fouhy, 1994).
At the request of U.S. Environmental Protection Agency (USEPA), the Office of Science and Technology Policy, Executive Office of the President, has initiated an interagency assessment of the scientific basis and efficacy of the reformulated fuel and oxygenated gasoline programs. This assessment is being conducted under coordination of the National Science and Technology Council's Committee on Environment and Natural Resources. The assessment will compare oxygenated fuels with conventional gasoline, where feasible, in the following areas: potential health impacts, fuel economy and engine performance, potential effects on ground-water and surface-water quality, air-quality benefits, and economic analysis. In the spring of 1996, a final draft report is scheduled to be submitted to the National Research Council for review and recommendations.
MTBE is the most commonly used fuel oxygenate because of its low cost, ease of production, and favorable transfer and blending characteristics (Ainsworth, 1992; Mormile and others, 1994; Shelly and Fouhy, 1994). It can be produced at the refinery, blends easily without phase separation in gasoline, and the gasoline blend can be transferred through existing pipelines. Feedstocks for MTBE include isobutylene and methanol. In 1994, 6.2 billion kg (kilograms) of MTBE were produced in the United States, and between 1984 and 1994 the amount of MTBE produced has increased by about 26 percent annually (Kirschner, 1995). MTBE is an ether with a structural formula CH3OC(CH3)3. It is a volatile, flammable, colorless liquid at room temperature and has a terpene-like odor. MTBE is miscible in gasoline and is soluble in water, alcohol, and other ethers. It has a relatively high vapor pressure (3.27 to 3.35 X 104 Pa at 25 deg C), a high water solubility (23.2 to 54.4 g/L (grams per liter) at 25 deg C), and a low octanol/water partition coefficient (log Kow 0.94 to 1.16), as tabulated by Mackay and others (1993) from numerous investigations. Reported Henry's Law Constant varies from 59 to 305 Pa m3/mol (cubic meters per mole) (Mackay and others, 1993) but is generally assumed to be less than 100 Pa m3/mol.
MTBE is on the Hazardous Air Pollutants List with 189 other chemicals to be regulated under the Air Toxics Program of the 1990 Clean Air Act Amendments. Considerable public attention has been focused on MTBE in the atmosphere. Health complaints related to MTBE in the atmosphere were reported in Fairbanks, Alaska, in November 1992 (Begley and Rotman, 1993) when residents reported headaches, dizziness, irritated eyes, burning of the nose and throat, coughing, disorientation, and nausea (Moolenaar and others, 1994) after MTBE had been added to gasoline. Similar health complaints also have been registered across the country including Anchorage, Alaska; Missoula, Montana; Milwaukee, Wisconsin; and cities in New Jersey (Begley and Rotman, 1993; Price, 1995). But the number of complaints in these areas varied. A few epidemiological studies have been conducted in response to public concerns related to the health complaints listed above (Mannino and Etzel, 1993; Moolenaar and others, 1994; White and others, 1995), and a few laboratory studies of healthy adults have been conducted (USEPA, 1994a). Controlled exposure to MTBE in laboratory air did not cause an increase in symptoms or in objective measures of irritation (USEPA, 1994a). Although both of these types of studies have led to a better understanding of human exposures to MTBE, they can neither confirm nor dismiss the existence of acute health effects from exposure to MTBE alone or in gasoline in at least some individuals (USEPA, 1994a).
MTBE is a potentially important ground-water contaminant because of its mobility and persistence (Garrett and Moreau, 1986), and because it is tentatively classified by the USEPA as a possible human carcinogen. In nonoxygenated gasoline, the monocyclic aromatic hydrocarbons, which include benzene, toluene, ethylbenzene, and the three xylenes, m-, o-, and p- (BTEX compounds), are the most soluble and most mobile components in gasoline. In oxygenated gasoline, MTBE is even more soluble and mobile than any of the BTEX compounds (Garrett and Moreau, 1986; Barker and others, 1990; Luhrs and Pyott, 1992; Odermatt, 1994). In fact, evidence indicates that MTBE moves as rapidly as a conservative tracer (Barker and others, 1990; Hubbard and others, 1994). MTBE persists in ground water under both aerobic and anaerobic conditions (Barker and others, 1990; Suflita and Mormile, 1993; Hubbard and others, 1994; Mormile and others, 1994; Yeh and Novak, 1994) because it resists physical, chemical, and microbial degradation. It is anticipated that the USEPA draft drinking-water lifetime health advisory for MTBE will be assigned a value of 20 or 200 mg/L (micrograms per liter); an advisory of 20 mg/L will be used if the tentative classification as a possible human carcinogen is accepted, otherwise the advisory will be 200 mg/L (USEPA, 1995). The health advisory is the maximum concentration in drinking water that is not expected to cause any adverse effects over a lifetime of exposure, within a specified margin of safety. The USEPA expects to issue the final health advisory in the fall of 1995. MTBE is on the USEPA's Drinking Water Priority List, which means it is a possible candidate for future regulation under the Safe Drinking Water Act. There are no current Federal regulations that require municipalities to test for MTBE in drinking water; consequently, there are few data on the occurrence of MTBE in ground water.
The USEPA has tentatively classified MTBE as a possible human carcinogen on the basis of studies that show MTBE is a carcinogen in animals (Burleigh-Flayer and others, 1992; Chun and others, 1992; Belpoggi and others, 1995); however, no epidemiological studies have been conducted to determine if MTBE is a human carcinogen. Documents describing the health effects from exposure to MTBE have been written by Environment Canada and Health Welfare Canada (1993), Prah and others (1995), Research Triangle Institute (1994), and the USEPA (1992a, 1994a). Rats and mice exposed to MTBE by inhalation or ingestion showed increased incidence of benign and malignant tumors, and lymphomas and leukemias (Burleigh-Flayer and others, 1992; Chun and others, 1992; Belpoggi and others, 1995). MTBE is absorbed rapidly and extensively from the respiratory and gastrointestinal tracts, and unchanged MTBE, and to a lesser amount the metabolite tertiary butanol (Savolainen and others, 1985) are the main respiratory excretion products (Research Triangle Institute, 1994). Tertiary butanol has been determined to be a carcinogen in laboratory animals (Cirvello and others, 1995). MTBE also is metabolized to formaldehyde, methanol, formic acid, carbon dioxide, 2-methyl-1,2-propanediol, and a-hydroxyisobutyric acid (Research Triangle Institute, 1994). MTBE and its metabolites show little tendency to distribute and accumulate in tissues, although MTBE and tertiary butanol may distribute to the brain at concentrations similar to blood concentrations (Research Triangle Institute, 1994). Most MTBE and tertiary-butanol are rapidly excreted (Research Triangle Institute, 1994), but there is evidence showing that a portion of these chemicals are deposited in deeper body stores and are slowly excreted over a longer period of time (Moolenaar and others, 1994; Prah and others, 1995). MTBE is not expected to bioaccumulate in surface-water aquatic organisms (Research Triangle Institute, 1994).
This report presents preliminary findings on the occurrence and possible sources of MTBE in ground water in selected areas of the United States sampled as part of the U.S. Geological Survey's National Water Quality Assessment (NAWQA) Program. This program is designed to describe current water-quality conditions and trends for 60 of the largest and most important river basins and aquifer systems nationwide (Leahy and Thompson, 1994). Investigations in these 60 areas, referred to as "Study Units," are the principal building blocks of the NAWQA program. Ground-water data from the first 20 Study Units are summarized in this report. The data include analyses from water samples collected from 193 shallow monitoring wells, 12 springs and 5 shallow water supply wells in urban areas; 549 shallow wells in agricultural areas; and 412 wells screened in deeper parts of 9 Study Unit areas representing regionally extensive aquifers. Samples from these wells were analyzed for 60 VOCs, 82 pesticides, up to 17 trace elements, 15 major inorganic compounds, 6 nutrients, and dissolved organic carbon (filtered, 0.45 mm (micrometer)).
The first step in the design of a Study-Unit Survey is the division of the ground-water resource into aquifer subunits, generally 3 to 5 per Study Unit; priority was given to those aquifer subunits where ground water is used for human consumption. The subunits serve as a first-order subdivision of the Study Unit into aquifer zones that are expected to be homogeneous in water-quality characteristics compared with the Study Unit as a whole. This subdivision is based mainly on identification of major hydrogeologic settings. For each aquifer subunit, the well-selection process and sampling strategy are designed to achieve a preliminary assessment from a combination of new and existing data. As a general planning guideline, an areal sampling density of at least one well per 100 km2 (square kilometer) is desired, and at least 20 wells are selected in each subunit. Perhaps the most significant factor that affects the utility of Study-Unit Surveys is that locations of existing wells are biased relative to the ground-water resource. Because existing drinking- water wells generally will not contain large concentrations of contaminants generally associated with point-source contamination, the Study-Unit Surveys will be biased against large concentrations of contaminants; however, the Study Unit Surveys will probably not be biased relative to smaller concentrations of contaminants generally associated with nonpoint-source contamination. Different type of wells (domestic, public water supply, or irrigation) may be biased in different ways. In general, each Study-Unit Survey uses as few different types of wells as needed to obtain adequate spatial (areally and with depth) distribution of ground-water samples from the aquifer system. Wells are selected for sampling using a grid-based random sampling approach (Scott, 1990).
Table 1. Components and attributes of the National Water-Quality Assessment ground-water sampling design -------------------------------------------------------------------------------------------------------------------------------------------------------- Study-Unit Survey Urban Land-Use Agricultural Land-Use Studies Studies -------------------------------------------------------------------------------------------------------------------------------------------------------- General objective To supplement existing data in To examine natural and human factors that affect the quality of providing a broad overview of shallow ambient ground water that underlie urban and agricultural the quality of "used" ground areas. water within each Study Unit. Spatial scale Ground-water resource in study Uppermost part of ground-water system in specified land-use unit. Spatial density of wells is settings. generally one well per 100 km2. Number of areas sampled 9 Study-Unit areas representing 8 urban areas 21 agricultural areas regionally extensive aquifers. Number or kind of wells 412 total 5 drinking water wells 549 total sampled 193 monitoring wells 12 springs in Atlanta __________________ 210 total --------------------------------------------------------------------------------------------------------------------------------------------------------Land-Use Studies are designed to assess the concentrations and distribution of water-quality constituents in recently recharged ambient ground water (generally less than 10 years old) associated with the most significant settings of land use and hydrogeologic conditions in each Study Unit. Two to four Land-Use Studies typically are completed in each Study Unit. The priority of potential Land-Use Studies is based on a combination of Study-Unit and National priorities. Factors considered in assigning priorities include importance of the land-use setting to quality of ground water withdrawn and used in the Study Unit, regional significance of the hydrogeologic setting, contamination potential of the targeted land use, and geographic correspondence to subunits concurrently sampled as part of the Study-Unit Survey. Wells selected for a Land-Use Study are randomly distributed throughout the occurrences of the land-use setting of interest within the Study Unit using a grid based random sampling approach (Scott, 1990). Generally, a minimum of 20 wells are sampled in each land-use setting. Some studies used existing wells, others drilled new wells, whereas still others used a combination of new and existing wells. Because of the difficulty in finding suitable wells within Atlanta, Georgia, some springs were sampled, and the results are included with data from the wells in this report. The wells sampled for the Land-Use Studies have a short (ideally less than 3 m (meters) in length) open interval located near the top of unconfined aquifer. Only wells located in recharge areas underlying or immediately downgradient from the land use of interest are selected. Ideally, observation wells or low-capacity existing wells are selected to avoid the complexities of determining contributing areas to heavily pumped wells. Many wells are installed by NAWQA to meet these criteria. Well selection and installation procedures are discussed in more detail by Lapham and others (in press).
Because there may be numerous point sources of contamination in an urban area, a randomly located well may be affected directly by nonpoint-, or point-source contamination, or both. MTBE may be used in gasoline to meet the requirements of the 1990 Clean Air Act Amendments in the urban areas studied (fig. 1). MTBE also may be used in premium gasoline in many of the urban areas studied. To avoid skewing the characterization of water quality toward defined contamination plumes in urban areas, existing wells that were installed to define the downgradient extent of a contamination plume were not considered for sampling. Furthermore, samples were collected from only nine wells that were drilled to define the quality of ambient ground water upgradient of point-source spills. This well-selection process essentially excludes from the study those parts of the aquifer where known point-source plumes are well defined; therefore, depending on the number of defined plumes within the study area, the water-quality data from the Urban Land-Use Studies may be skewed toward nondetectable or lower concentrations of contaminants. For example, if all the point-source contamination in an urban area were defined by observation wells and were sufficiently spread out so that the plumes did not overlap, then none of the wells selected for an Urban Land-Use Study in this area would be affected by, or would characterize, point-source spills. The water-quality data from this hypothetical Urban Land-Use Study would characterize only contaminants from nonpoint sources. In the Denver Urban Land-Use Study, six wells were sampled that were drilled to define the quality of the ambient ground water before it is affected by leakage from a regulated unit as defined by the USEPA (1992b). Samples from five of these wells had detectable concentrations of MTBE at 0.5, 9.6, 33, 800, and 23,000 mg/L. Two of these wells were upgradient of plumes that were not associated with gasoline sources (Breton Bruce, U.S. Geological Survey, written commun., 1995) although this does not mean that another undefined plume is the source of MTBE in these wells. Samples from these two wells had concentrations of MTBE of 33 and 800 mg/L. In the Atlanta Urban Land-Use Study, three wells were located upgradient of plumes, but no MTBE was detected in samples from these wells. None of the other six Urban Land-Use Studies used upgradient wells associated with point-source spills.
Sampling procedures are discussed in Koterba and others (in press) and are briefly summarized here. Submersible pumps were cleaned before a sample was collected. The normal decontamination procedure included an initial flush with nonphosphate detergent, a deionized VOC blank-water rinse, a methanol rinse, and a final deionized water rinse. Samples were collected in an environmental chamber that protected the samples from airborne contamination. Powderless latex gloves were worn whenever a sample was collected. Samples were preserved with hydrochloric acid to pH of 2 and chilled until analyzed by the U.S. Geological Survey's National Water Quality Laboratory in Arvada, Colorado. Study Units with Land-Use Studies collected 73 quality-control samples to assess possible contamination from sample collection and shipment. None of these samples had MTBE concentrations greater than the reporting level of 0.2 mg/L.
VOC analyses were performed using purge and trap capillary gas chromatography/mass spectrometry. The method of analysis is discussed in detail by Raese and others (in press) and Rose and Schroeder (1995) and is similar to USEPA Method 524.2, revision 3.0. The precision and accuracy of MTBE analysis in reagent water was demonstrated by 215 laboratory spiked sample analyses performed in 1994-95. The concentrations of MTBE in spiked samples ranged from 0.10 to 5.0 mg/L. The mean recovery for each concentration of MTBE analyzed ranged from 83 to 102 percent, and the relative standard deviation ranged from 2.0 to 13.2 percent. MTBE was not detected in any of 277 laboratory blank samples analyzed in 1993-95, and only one laboratory blank sample had a detectable concentration of toluene at 0.2 mg/L.
Figure 2. Concentration of MTBE versus total concentrations of BTEX compounds in shallow ground-water samples from Urban Land-Use Study areas, 1993-94.
MTBE was detected more frequently, and in larger concentrations, in shallow ambient ground water in urban areas compared to shallow ground water in agricultural areas. At a reporting level of 0.2 mg/L, MTBE was detected in 27 percent of 210 shallow urban wells and springs, and in only 1.3 percent of 549 shallow agricultural wells sampled. MTBE was detected in samples of shallow ground water in all eight urban Land-Use Studies areas but in only 3 of 21 Agricultural Land-Use Studies. In urban areas, MTBE was detected in shallow ground water in Denver, Colorado; New England (specifically urban areas within Connecticut, Massachusetts, and Vermont); Reno, Nevada; Albany, New York; Dallas/Fort Worth, Texas; Las Vegas, Nevada; Atlanta, Georgia; and Albuquerque, New Mexico (fig. 3). In agricultural areas, MTBE was detected in southern Colorado, New England, and eastern Pennsylvania. The maximum concentration of MTBE detected in shallow ground water in urban areas were over 100 mg/L (fig. 4) whereas the maximum concentration in shallow ground water in agricultural areas was 1.3 mg/L. The median value for urban areas lies below the reporting level of 0.2 mg/L; however, this median was estimated to be 0.02 mg/L using the maximum-likelihood estimator (Helsel and Hirsch, 1992).
Figure 3. Frequency of detection of MTBE in shallow ground water from Urban Land-Use Study areas, 1993-94.
Of the 210 shallow Urban Land-Use wells and springs sampled, 73 percent had concentrations less than the reporting level of 0.2 mg/L, 24 percent had concentrations of MTBE ranging from 0.2 to 20.0 mg/L, and 3 percent had concentrations exceeding 20.0 mg/L, which is the estimated lower limit of the USEPA (1995) draft drinking-water health advisory level (fig. 4). Five of the 210 shallow urban wells and springs sampled were used as a source of drinking water; however, none of the samples from the 5 drinking-water wells had detectable concentrations of MTBE. In seven of the eight urban areas studied, the sampled ground water was from the uppermost part of an aquifer used for drinking water or possibly was connected to an underlying aquifer that is used as a municipal water supply. In general, public-water supplies draw water from deeper parts of the ground-water system, and the Study-Unit Survey data may provide a better indication of the contaminates that presently exist in drinking-water supplies. None of the samples from the Agricultural Land-Use wells or Study-Unit Survey wells had concentrations of MTBE that exceeded 20.0 mg/L.
Figure 4. Concentrations of MTBE in shallow ground water from Urban Land-Use Study areas, 1993-94.
MTBE was detected most frequently in samples of shallow urban ground water in Denver, Colorado, and New England (fig. 3), but the reason for its frequent detection is not known. In Denver, samples from 79 percent of the shallow urban wells (23 of 29 wells) had detectable concentrations of MTBE, and in New England, samples from 37 percent of the wells (13 of 35 wells) had detectable concentrations of MTBE. The frequent detection of MTBE in these two areas may be related to the fact that the aquifer, and the overlying unsaturated zone, consists of very conductive sand and gravel and that the median depth to water was very shallow---4.3 m in Denver and 2.8 m in New England. However, the mean annual precipitation is about three times greater in New England than in Denver (U.S. Geological Survey, 1970), and therefore the ground-water recharge in these two areas may be substantially different.
The infrequent concurrent detection of MTBE with BTEX compounds suggest that point-source leaks are not the principal source of the MTBE detected in urban ground water, although the lack of association does not completely rule out point-source spills as a potential source. MTBE plumes originating from point-source gasoline spills would generally occupy a larger proportion of the subsurface compared to BTEX compounds, and concentrations of MTBE at the leading edge of the plume would be small but would be expected to increase in time. Therefore, if the small concentrations of MTBE detected in shallow urban ground water originated from gasoline spills, then generally one would expect the concentrations of MTBE and detections of BTEX compounds, to increase with time at these same wells. MTBE plumes will generally occupy a larger portion of the subsurface compared to BTEX compounds for three reasons: (1) MTBE is persistent in aerobic and anaerobic ground water (Barker and others, 1990; Suflita and Mormile, 1993; Hubbard and others, 1994; Mormile and others, 1994; Yeh and Novak, 1994); (2) MTBE can occur in large concentrations in gasoline as previously discussed; and (3) MTBE does not sorb to aquifer material and is more mobile in ground water than other BTEX compounds based on field data and physical and chemical properties (solubility, vapor pressure, Kow and Koc) (Garrett and Moreau, 1986; Barker and others, 1990; Luhrs and Pyott, 1992; Odermatt, 1994). In fact, evidence indicates that MTBE moves as rapidly as a conservative tracer (Barker and others, 1990; Hubbard and others, 1994) and that MTBE plumes are likely to undergo only dispersive attenuation.
Possible nonpoint sources of MTBE include atmospheric deposition and stormwater runoff. Once in the atmosphere, MTBE can partition into precipitation and be transported in stormwater runoff into streams or into shallow ground water with recharge from stormwater runoff or infiltration of precipitation. MTBE is released to the atmosphere from a variety of sources including industrial discharges, and mobile sources such as automobiles, and during refueling of automobiles. With the possible exception of industry, the amount of MTBE released to urban atmosphere from various sources is not well documented; therefore, the principal source of atmospheric MTBE can not be easily identified. Refueling at service stations is a source of MTBE to the atmosphere, but there have been only a few studies that measured concentrations in the atmosphere (Johnson and others, 1994; Anderson and others, 1995). In 1992, the release of MTBE from industry in the United States accounted for only 0.03 percent of the MTBE that was produced. According to USEPA's Toxic Release Inventory (1994d), about 94 percent of the MTBE released from industry was released to the atmosphere, 3.5 percent was discharged to surface water, and 2.5 percent was injected into wells. Recent evidence indicates that evaporative emissions from vehicles are far higher than had been thought (Stump and others, 1990; Calvert and others, 1993). Exhaust emissions of MTBE ranged from 0.9 to 81 mg/km (milligrams per kilometer) for various vehicles using gasoline with 11-16 percent MTBE by volume (Stump and others, 1990; Hoekman, 1992; Calvert and others, 1993). However, most mobile-source emissions are caused by a small percentage of the vehicles (Slinn and others, 1978); therefore, large errors may result when extrapolating emissions data for a few vehicles to the total vehicle population.
The transfer of MTBE from atmospheric gases to rainwater is dependent on the temperature and concentrations of MTBE in the air. Very little MTBE would be expected to partition to organic carbon or water on dust and other particulates in the atmosphere because of MTBE's high vapor pressure and the small amount of water that would be expected on particulates. Thus, the concentration of MTBE in precipitation can be predicted using a modified form of Henry's Law, assuming MTBE is in the gaseous phase and the concentrations in the atmosphere and in precipitation are in equilibrium. This equilibrium condition is supported by Slinn and others (1978) and Ligocki and others (1985). Slinn and others (1978) showed that a gas will reach equilibrium with falling raindrops within a few tens of meters of fall distance if the gas does not participate in chemical reactions within the droplet.
The concentration of a chemical in water can be calculated using the following equation presented by Schwarzenbach and others (1993):
Relatively few data are available on the concentrations of MTBE in the urban atmosphere. However, reported concentrations of MTBE in the atmosphere, including the air along roadsides, varied from less than 0.025 to 8.4 ppb (LaGrone, 1991; Kelly and others, 1993; Allen and Grande, 1995; Anderson and others, 1995). Concentrations were larger near point-source release areas. For example, the median concentrations of MTBE in the air at the perimeter of three refueling stations varied from 3 to 14 ppb (Johnson and others, 1994), and the largest concentration was 140 ppb (Johnson and others, 1994). The amount of MTBE released, wind direction, temperature, and distance from the source all would affect the concentrations of MTBE in the atmosphere at any one location. Because of dispersion, mixing, and MTBE's relatively short half-life in the atmosphere of 1 to 11 days (Howard and others, 1991) the concentrations of MTBE in the atmosphere and in precipitation would be expected to decrease with distance from the source.
Stormwater runoff may be another important nonpoint source of MTBE to ground water. The U.S. Geological Survey has collected stormwater-runoff samples in Dallas/Fort Worth, Texas; Denver/Lakewood, Colorado; Albuquerque, New Mexico; and Colorado Springs, Colorado, and elsewhere, to help municipalities meet the National Pollutant Discharge Elimination System (NPDES) monitoring requirements set by the USEPA. The above noted four cities currently are using fuel oxygenates in gasoline to meet requirements of the 1990 Clean Air Act Amendments. Stormwater samples were collected between February 1992 and May 1993, and during the summer, when MTBE is not used ubiquitously in gasoline. MTBE was detected in 14 percent of the 279 stormwater samples collected. Concentrations of MTBE ranged from the reporting level of 0.2 to 8.3 mg/L. When detected, the median concentration of MTBE in stormwater was 1.4 mg/L. Colorado Springs had the greatest number of detections of MTBE at 36 percent (15 of 42 samples) and Albuquerque had the fewest with 3 percent (1 of 30 samples). In Colorado Springs, MTBE was detected in samples collected during the winter only when MTBE was used as a fuel oxygenate in gasoline (von Guerard and Weiss, 1995).
Based on the USEPA's current understanding of MTBE's carcinogenicity, the concentrations in ground water reported in this study in most cases do not represent a risk to human health; samples from only 3 percent of the shallow wells and springs in urban areas had concentrations of MTBE that exceeded 20 mg/L, which is the estimated lower limit of the USEPA's draft drinking-water health advisory. Currently, MTBE is not a target analyte of monitoring programs of public water supplies and drinking water; however, MTBE can be analyzed by purge and trap, capillary column gas chromatography/mass spectrometry (Raese and others, in press), which is extensively used for VOC analysis by public-water utilities. Given the preliminary information discussed in this report, it may be advisable for urban water utilities to consider adding MTBE to their existing VOC analytical schedule.
How MTBE enters shallow urban ground water is not clear, and existing environmental monitoring programs may not be adequately designed to answer this question. Federal, State and local government agencies have made substantial investments to protect the quality of the environment, and the effectiveness of these efforts has been evaluated by assessing the resource that was focused for protection (for example, air, surface water, and ground water). The integration of monitoring programs would help to insure that efforts to protect or improve one component of the environment do not adversely affect another. Integrated environmental monitoring in a few major cities would provide an improved understanding of the source, transport, and fate of MTBE in ground water in urban areas.
Defining the source of MTBE in shallow ground water is essential to prevent further contamination, and to protect other vulnerable aquifers in the United States from contamination by MTBE or similar compounds. Questions related to the source of MTBE include: (1) Is the frequency of detection of MTBE in shallow ground water more related to its use or to aquifer vulnerability? (2) Is the source of MTBE in shallow ambient urban ground water primarily from nonpoint sources of contamination, such as precipitation and urban runoff? There may be areas of the country where the use of MTBE will not result in its infiltration to shallow or deeper ground water. However, before this can be determined, seasonal information is needed on how much MTBE is being used in major metropolitan areas. This information can be related to the frequency of detection of MTBE and to aquifer vulnerability. To define nonpoint sources of MTBE contamination, information is needed on the release of MTBE to the atmosphere from various activities. MTBE can be released to the atmosphere from a variety of sources including industrial stack and fugitive emissions, refueling at service stations, and mobile sources, such as automobiles. With the possible exception of industrial emissions, the amount of MTBE released to the urban atmosphere from these other sources is not well documented. Once MTBE is in the atmosphere, some can be returned with precipitation, but more research is needed to determine the concentrations of MTBE in precipitation and in surface runoff on a seasonal basis.
A better understanding of the transport of MTBE from land surface to shallow ground water, and from shallow to deeper aquifers would be used to protect public water supplies and in developing wellhead protection plans for public water supplies. Questions related to the transport of MTBE include: (1) Can MTBE in precipitation or stormwater runoff recharge the shallow ground water; if so, under what conditions and in what concentrations? (2) How quickly, and at what concentrations, can MTBE be transported from shallow to deeper ground water? (3) What is the maximum extent of a MTBE plume originating from a point source relative to the BTEX compounds? Depth to water, recharge rates, permeability of the unsaturated zone, and other hydrogeologic characteristics are likely to affect the transport of MTBE through the unsaturated zone. Because MTBE is mobile and persistent in ground water, it is reasonable to expect that it will move from shallow to deep ground water with time, but it is not known how quickly and at what concentrations. Knowledge on the maximum extent of a MTBE plume relative to BTEX compounds originating from a single gasoline contamination source will help determine if point-source contamination is responsible for the widespread detection of small concentrations of MTBE in the absence of BTEX compounds.
Additional study and data on the fate of MTBE are needed to determine if MTBE, or its degradation products, will accumulate in ground water over time. The accumulation of MTBE in ground water may not necessarily result in an increase in concentrations with time, but its detection would become more frequent. The degradation of some organic chemicals in aquifers can be very slow, with a half life of decades or longer, to breakdown to carbon dioxide and water. The degradation products of some organic chemicals can be toxic. Questions related to the fate of MTBE include: (1) What is the long-term fate of MTBE, and its degradation products, in ground water? (2) What is the half life of MTBE in ground water under aerobic and anaerobic conditions in various aquifers? There also may be degradation products of MTBE in the air, such as tert-butyl formate (Japar and others, 1991), which enter shallow ground water with recharge water. Investigation of these degradation products is necessary to a full understanding of the fate of MTBE.
In order to determine if MTBE concentrations are likely to rise above current levels and potentially rise to levels that pose a health threat, it is necessary to understand three things about the compound: (1) the pathways by which it enters the ground water, (2) the processes by which it is transported in ground water, and (3) the rates at which it degrades. Only when all three of these issues are reasonably well understood can meaningful projections be made of the potential for MTBE reaching dangerous levels over long periods of use. The U.S. Geological Survey is beginning to conduct research on aspects of all of these processes and is in close communication with other scientists studying these questions. In addition, the NAWQA program will continue to monitor some wells in all Study Units, providing a continuing empirical check on the changes in levels of MTBE in ground water. The U.S. Geological Survey will continue to report to the public, regulatory agencies, industry, and the scientific community on the results of its research and monitoring on this emerging water-quality issue.
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