|
t
<br /> 22 percent of the 1 2 million petroleum underground sources including industrial discharges, and mobile
<br /> torage tanks at more than 500,000 sites in the United sources such as automobiles, and during refueling of
<br /> States have leaked as of July 1994 (USEPA, 1994b, automobiles With the possible exception of indust ,
<br /> 1994c) Although MTBE will volatilize from soils, it the amount of MTBE released to urban atmosphere
<br /> is also highly mobile in soil and can move into ground from various sources is not well documented,
<br /> water therefore, the principal source of atmospheric MTBE
<br /> can not be easily identified Refueling at service
<br /> The infrequent concurrent detection of MTBE with stations is a source of MTBE to the atmosphere, but
<br /> BTEX compounds suggest that point-source leaks are there have been only a few studies that measured
<br /> not the principal source of the MTBE detected in concentrations in the atmosphere (Johnson and others,
<br /> urban ground water, although the lack of association 1994, Anderson and others, 1995) In 1992, the
<br /> does not completely rule out pouit-source spills as a release of MTBE from industry in the United States
<br /> potential source MTBE plumes originating from accounted for only 0 03 percent of the MTBE that was
<br /> point-source gasoline spills would generally occupy a produced According to USEPA's Toxic Release
<br /> larger proportion of the subsurface compared to Inventory (19944), about 94 percent of the MTBE
<br /> BTEX compounds, and concentrations of MTBE at released from industry was released to the
<br /> the leading edge of the plume would be small but atmosphere, 3 5 percent was discharged to surface
<br /> would be expected to increase in tune Therefore, if water, and 2 5 percent was injected into wells Recent
<br /> the small concentrations of MTBE detected in shallow evidence indicates that evaporative emissions from
<br /> urban ground water originated from gasoline spills, vehicles are far higher than had been thought (Stump
<br /> then generally one would expect the concentrations of and others, 1990, Calvert and others, 1993) Exhaust
<br /> MTBE and detections of BTEX compounds, to emissions of MTBE ranged from 0 9 to 81 mg/km
<br /> increase with time at these same wells MTBE plumes (milligrams per kilometer) for various vehicles using
<br /> will generally occupy a larger portion of the gasoline with 11-16 percent MTBE by volume (Stump
<br /> subsurface compared to BTEX compounds for three and others, 1990, Hoekman, 1992, Calvert and others,
<br /> reasons (1)MTBE is persistent in aerobic and 1993) However, most mobile-source emissions are
<br /> anaerobic ground water (Barker and others, 1990, caused by a small percentage of the vehicles (Slinn
<br /> Suflita and Mormile, 1993, Hubbard and others, 1994, and others, 1978), therefore, large errors may result
<br /> Mormile and others, 1994, Yeh and Novak, 1994), when extrapolating emissions data for a few vehicles
<br /> (2) MTBE can occur in large concentrations in to the total vehicle population
<br /> gasoline as previously discussed, and (3)MTBE does
<br /> not sorb to aquifer material and is more mobile in The transfer of MTBE from atmospheric gases to
<br /> ground water than other BTEX compounds based on rainwater is dependent on the temperature and concen-
<br /> field data and physical and chemical properties trations of MTBE in the air Very little MTBE would
<br /> (solubility, vapor pressure, K,, and Ka) (Garrett and be expected to partition to organic carbon or water on
<br /> Moreau, 1986, Barker and others, 1990, Luhrs and dust and other particulates in the atmosphere because
<br /> Pyott, 1992. Odermatt, 1994) In fact, evidence of MTBE's high vapor pressure and the small amount
<br /> indicates that MTBE moves as rapidly as a conser- of water that would be expected on particulates Thus,
<br /> vative tracer (Barker and others, 1990, Hubbard and the concentration of MTBE in precipitation can be
<br /> others, 1994) and that MTBE plumes are likely to predicted using a modified form of Henry's Law,
<br /> undergo only dispersive attenuation assuming MTBE is in the gaseous phase and the
<br /> concentrations in the atmosphere and in precipitation
<br /> Possible nonpoint sources of MTBE include are in equilibrium This equilibrium condition is
<br /> atmospheric deposition and stormwater runoff Once supported by Sli nn and others (1978) and Ligocki and
<br /> in the atmosphere, MTBE can partition into precipi- others (1985) Slim and others (1978) showed that a
<br /> tation and be transported in stormwater runoff into gas will reach equilibrium with falling raindrops
<br /> streams or into shallow ground water with mcharge within a few tens of meters of fall distance if the gas
<br /> from stormwater runoff or infiltration of precipitation does not participate in chemical reactions within the
<br /> MTBE is released to the atmosphere from a variety of droplet.
<br /> 10 A Pref;urinary Assessment of the Occurrence and Possible Sources of MTBE In Ground Water
<br />
|