Ethanol in Groundwater at a Pacific Northwest Terminal

摘要

In March 1999, a 19,000-gallon release of neat ethanol occurred from an above ground storage tank at a Bulk Fuel Terminal located in the Northwest United States ("Pacific Northwest Terminal"). Ethanol is completely miscible in water and at high concentrations (>20,000 ppm) can enhance the solubilization of benzene, toluene, ethylbenzene, and the xylenes (BTEX) from nonaqueous phase liquid (NAPL). Ethanol can be degraded in both aerobic and anaerobic environments. There is some evidence that the presence of ethanol can inhibit BTEX biodegradation. The presence of ethanol can reduce interfacial tension, increasing the mobility of NAPL. Historical groundwater monitoring data were available from existing monitoring wells to delineate a preexisting dissolved hydrocarbon plume. In late March 1999 four existing monitoring wells were sampled and analyzed for BTEX, total petroleum hydrocarbons (TPH), and ethanol. In June and July of 1999, nine monitoring wells were installed to delineate ethanol and hydrocarbons in the subsurface. These wells were sampled eight times between June 1999 and July 2001. Since May 2000 the groundwater sampling protocol has included analytes for in situ bioremediation. In December 2000, nine additional monitoring wells were installed to define vertical hydraulic gradients. Eighteen new monitoring wells were installed over an 18-month period in response to the March 1999 ethanol release. These wells provide adequate delineation of the ethanol plume. The presence of ethanol has affected pre-existing petroleum hydrocarbon in both the NAPL and the dissolved phases. NAPL thickness in a near-source monitoring well has exceeded two feet. In one downgradient well, benzene concentrations increased by a factor of 15 five months after the release and have since remained about ten times above the pre-release level, suggesting evidence for cosolvent effects of ethanol and lower benzene intrinsic biodegradation rates. The presence of ethanol has created a strongly anaerobic groundwater system, demonstrated by low or nondetectable dissolved oxygen, depleted sulfate and nitrate, and elevated methane concentrations (as high as 30,000 ug/L). Dissolved methane concentrations have increased steadily over the last year. Declining ethanol concentrations in near-source monitoring wells and the lack of ethanol in downgradient monitoring wells provide evidence for ethanol biodegradation.