THE URBAN ATMOSPHERE AS A NONPOINT-SOURCE FOR THE TRANSPORT OF MTBE AND OTHER VOLATILE ORGANIC-COMPOUNDS (VOCS) TO SHALLOW GROUNDWATER

Infiltration and dispersion (including molecular diffusion) can transp ort volatile organic compounds (VOCs) from urban air into shallow grou ndwater. The gasoline additive methyl-tert-butyl ether (MTBE) is of sp ecial interest because of its (1) current levels in some urban air, (2 ) strong partitioning from air into water, (3) resistance to degradati on, (4) use as an octane-booster since the 1970s, (5) rapidly increasi ng use in the 1990s to reduce CO and O-3 in urban air, and (6) its fre quent detection at low microgram per liter levels in shallow urban gro undwater in Denver, New England, and elsewhere. Numerical simulations were conducted using a 1-D model domain set in medium sand (depth to w ater table = 5 m) to provide a test of whether MTBE and other atmosphe ric VOCs could move to shallow groundwater within the 10-15 y time fra me over which MTBE has now 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 to permit significant VO C movement by diffusion into shallow groundwater. Case 2 considered a steady atmospheric source plus 36 cm/y of net infiltration; groundwate r at 2 m below the water table became nearly saturated with atmospheri c levels of VOC within 5 y. Case 3 was similar to case 2, but consider ed the source to be seasonal, being ''on'' for only 5 of 12 months eac h year, as with the use of MTBE during the winter fuel-oxygenate seaso n; groundwater at 2 m below the water table became equilibrated with 5 /12 of the ''source-on'' concentration within 5 y. Cases 4 and 5 added an evapotranspiration (ET) loss of 36 cm/y, resulting in no net recha rge. Case 4 took the ET from the surface, and case 5 took the ET from the capillary fringe at a depth of 3.5 m. Net VOC mass transfer to sha llow groundwater after 5 y was less for both cases 4 and 5 than for ca se 3. However, it was significantly greater for cases 4 and 5 than for 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 o n each downward infiltration event, and also on the ET-induced flow. T he ability of MTBE to reach groundwater in cases 2-5 is taken as evide nce of the potential importance of urban air as a non-point source for VOCs in shallow urban groundwater. Two subcases were run for both cas e 4 and case 5: subcase a (water and VOCs move with ET) and subcase b (water only moves with ET).