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

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• 作者：James F. Pankow ; Neil R. Thomson ; Richard L. Johnson ; Arthur L. Baehr ; and John S. Zogorski
• 刊名：Environmental Science & Technology
• 出版年：1997
• 出版时间：October 1997
• 年：1997
• 卷：31
• 期：10
• 页码：2821 - 2828
• 全文大小：335K
• 年卷期：v.31,no.10(October 1997)
• ISSN：1520-5851

文摘

Infiltration and dispersion (including molecular diffusion)can transport volatile organic compounds (VOCs) from urbanair into shallow groundwater. The gasoline additivemethyl-tert-butyl ether (MTBE) is of special interest because ofits(1) current levels in some urban air, (2) strongpartitioningfrom air into water, (3) resistance to degradation, (4)useas an octane-booster since the 1970s, (5) rapidlyincreasinguse in the 1990s to reduce CO and O₃ in urban air,and(6) its frequent detection at low microgram per literlevelsin shallow urban groundwater in Denver, New England,and elsewhere. Numerical simulations were conductedusinga 1-D model domain set in medium sand (depth to watertable = 5 m) to provide a test of whether MTBE and otheratmospheric VOCs could move to shallow groundwaterwithin the 10-15 y time frame over which MTBE has nowbeen used in large amounts. Degradation and sorptionwere assumed negligible. In case 1 (no infiltration,steadyatmospheric source), 10 y was not long enough to permitsignificant VOC movement by diffusion into shallow groundwater. Case 2 considered a steady atmospheric sourceplus 36 cm/y of net infiltration; groundwater at 2 m belowthewater table became nearly saturated with atmosphericlevels of VOC within 5 y. Case 3 was similar to case 2,butconsidered the source to be seasonal, being "on" foronly5 of 12 months each year, as with the use of MTBEduring the winter fuel-oxygenate season; groundwater at2 m below the water table became equilibrated with⁵/₁₂of the "source-on" concentration within 5 y. Cases 4and5 added an evapotranspiration (ET) loss of 36 cm/y,resultingin no net recharge. Case 4 took the ET from the surface,andcase 5 took the ET from the capillary fringe at a depthof 3.5 m. Net VOC mass transfer to shallowgroundwaterafter 5 y was less for both cases 4 and 5 than for case3. However, it was significantly greater for cases 4 and5than for case 1, even though cases 1, 4, and 5 were all no-net recharge cases. The mechanism responsible for thiseffect was the dispersion acting on each downwardinfiltration event, and also on the ET-induced flow.Theability of MTBE to reach groundwater in cases 2-5 istakenas evidence of the potential importance of urban air asa non-point source for VOCs in shallow urban groundwater.Two subcases were run for both case 4 and case 5:subcase a (water and VOCs move with ET) and subcase b(water only moves with ET).