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The concentration of a chemical in water can be too
<br /> calculated using the following equation presented by
<br /> chwarzenbach and others (1993) z so
<br /> 0
<br /> C� _ °K {t) ;520
<br /> KH a
<br /> where � 10
<br /> ate. 5 $v SU ti
<br /> C., = concentration of the chemical in water,in Z
<br /> Ui
<br /> moles per liter,
<br /> CQ = concentration of the chemical in the 2
<br /> atmosphere,in moles per liter, 0
<br /> R = gas constant of 8 21 x 10-2 atm L/(mol°K) 0 1
<br /> (atmosphere-liters per mole degrees 05
<br /> kelvin)K, F
<br /> T = temperature,in degrees kelvin,and U 02
<br /> KH = Henry's Law Constant,in atmosphere- z
<br /> liters per mole U 01
<br /> 01 1 10 100 1 000
<br /> Robbins and others(1993) showed that the temper- CONCENTRATION OF MTBE IN ATMOSPHERE(ppb)
<br /> ature dependence of KH for MTBE can be estimated Figure 5 Concentrations of MTBE in atmosphere and pre-
<br /> using the following equation for temperatures ranging cipitation at various temperatures
<br /> from 298 18 to 323 18°K, or 25 to 50°C
<br /> reported concentrations of MTBE in the atmosphere,
<br /> Kx W 1000rxp�184-7666)T (2) including the air along roadsides, varied from less
<br /> where than 0 025 to 8 4 ppb (LaGrone, 1991, Kelly and
<br /> others, 1993, Allen and Grande, 1995, Anderson and
<br /> T = temperature,in degrees kelvin others, 1995) Concentrations were larger near point-
<br /> source release areas For example,the median concen-
<br /> Equation 2 shows that Henry's Law constant trations of MTBE in the air at the perimeter of three
<br /> increases by a factor of about 2 for every 10-degree refueling stations vaned from 3 to 14 ppb (Johnson
<br /> increase in temperature Thus, assuming a constant and others, 1994), and the largest concentration was
<br /> concentration of MTBE in the atmosphere, concentra- 140 ppb Qohnson and others, 1994) The amount of
<br /> tions of MTBE in precipitation would be significantly MTBE released, wind direction, temperature, and
<br /> greater during winter months than in warmer summer distance from the source all would affect the concen-
<br /> months Figure 5 shows this relationship assuming the trations of MTBE in the atmosphere at any one
<br /> relation established by Robins and others (1993)holds location Because of dispersion, mixing, and WBE's
<br /> for temperatures below 25°C This figure shows that relatively short half-life in the atmosphere of 1 to l 1
<br /> assuming a concentration of 2 ppb (parts per billion) days (Howard and others, 1991)the concentrations of
<br /> MTBE in the atmosphere, the concentration of MTBE MTBE In the atmosphere and in precipitation would
<br /> in precipitation would increase almost tenfold, from be expected to decrease with distance from the source
<br /> about 0 4 to 3 pg/L, if the temperature decreases from
<br /> 20 to 0°C (fig 5) Therefore, the lower the temper- Stormwater runoff may be another important
<br /> attire, the greater the concentration of MTBE in nonpoint source of MTBE to ground water The U S
<br /> precipitation given a constant concentration in the Geological Survey has collected stormwater-runoff
<br /> atmosphere This relation is important because MTBE samples in Dallas/Fort Worth, Texas,
<br /> is used in the winter to control the concentrations of Denver/Lakewood, Colorado, Albuquerque, New
<br /> carbon monoxide in the atmosphere Mexico, and Colorado Springs, Colorado, and
<br /> elsewhere, to help municipalities meet the National
<br /> Relatively few data are available on the concentra- Pollutant Discharge Elimination System (NPDES)
<br /> tions of MTBE in the urban atmosphere However, monitoring requirements set by the USEPA The
<br /> Study Results 11
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