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The Urban Atmosphere as a Non-Point Source for the Transport of MTBE and Other Volatile Organic Compounds (VOCs) to Shallow Groundwater

James F. Pankow,, Neil R.Thompson, Richard L. Johnson, Arthur L. Baehr, and John S. Zogorski

Department of Environmental Science and Engineering, Oregon Graduate Institute,
P.O. Box 91000, Portland, OR 97291-1000, USA
Phone (503) 690-1080, Telecopier (503) 690-1273, pankow@ese.ogi.edu

Infiltration and dispersion (including molecular diffusion) can transport volatile organic compounds (VOCs) from urban air into shallow groundwater. The gasoline additive methyl-tert-butyl ether (MTBE) is of special interest because of its: 1) relatively high levels in some urban air; 2) strong partitioning from air into water; 3) resistance to degradation; 4) use as an octane booster since the 1970s; 5) rapidly-increasing use in the 1990s to reduce urban CO and O3; and 6) frequent detection at low ug/L levels in shallow urban groundwater in Denver, New England, and elsewhere. Numerical simulations were conducted using 1-D model domain set in poorly-sorted medium sand (depth to water = 5 m) to test whether MTBE and other atmospheric VOCs could move to shallow groundwater within the type of 10-15 y time frame over which MTBE has been used in large amounts. Degradation and sorption were assumed negligible.

In Case 1 (no-infiltration, steady atmospheric source), 10 y was not long enough for significant VOC movement by diffusion into shallow groundwater. In Case 2, the addition of 36 cm of net infiltration provided movement, within 6 years, of water nearly saturated with atmospheric VOC levels to 2 m below the water table. In Case 3, consideration of a seasonal source that was "on" for 5 of the 12 months yielded groundwater at long times that was 5/12ths saturated with the source-on concentration. Cases 4 and 5 added an evapotranspiration (ET) loss of 36 cm/y, making for no net recharge. Case 4 took the ET from the surface, and Case 5 took the ET from the capillary fringe at 3.5 m. Net VOC mass transfer to shallow groundwater in both Cases 4 and 5 were less than in Case 3. However, it was significantly greater than in Case 1, even though Cases 1,4, and 5 were all no-net recharge cases. The mechanism responsible for this effect was the dispersion acting on each downward infiltration event, and also on the ET-induced flow. The ability of MTBE to reach groundwater in Cases 3-5 is taken as further evidence of the potential importance of urban air as a non-point source for VOCs in shallow urban groundwater.

Published:

1997, Environmental Science and Technology, v. 31, no. 10, p. 2821-2828.

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