Case Study: Manipulating Groundwater Redox Conditions for MTBE Remediation

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A pilot study was conducted to investigate the potential to foster anaerobic methyl tert butyl ether (MTBE) biodegradation. The pilot test site, located on Long Island, New York, is characterized as a glacial out-wash aquifer with a high groundwater velocity over 2 ft/day. The MTBE plume is typical of similar plumes that have quickly migrated in this glacial out-wash aquifer. It is over one mile long but less than two hundred feet across at its widest point. MTBE concentrations are variable and ranged up to 100 ppm.

The biodegradation of MTBE under anaerobic conditions has received recent attention by EPA/NRMRL (EPA-R-00-600) and other research groups. In some venues, fostering naturally anaerobic conditions to methanogenic levels could be a valuable tool in managing MTBE contaminated. This project was initiated to test our ability to do this by creating reductive conditions in the aquifer. A reducing zone was generated by the injection of a slow release carbon source, HRC ®, in a set of direct driven points placed in a transect across half of the plume (100 ft.) so that there was a treated and untreated region. The barrier configuration was deemed most appropriate for the hydraulic conditions at the site.

Through monitoring of ferrous iron, sulfate, methane and hydrogen concentrations, it was found that, in approximately five months, the HRC had driven the aquifer from iron reducing conditions into sulfate reducing conditions from the point of injection to more than three hundred feet downgradient. . Extensive sampling, from a network of monitoring wells, which also included well clusters for vertical delineation, did not exhibit conclusive evidence of MTBE biodegradation. Reasons for this include 1)highly variable MTBE distributions in this dynamic plume and 2) inability to achieve methanogenic conditions about halfway into the lifetime of the HRC injection. Reapplication of HRC is under consideration to attempt to drive the aquifer into methanogenic conditions.

Another metric for ascertaining MTBE degradation is the presence of daughter products such as tert butyl alcohol (TBA). Because TBA is a co-contaminant in gasoline and its quantification can be unreliable, there can be obvious limitations to using this parameter. An improvement on the quantification of TBA was made in order to alleviate some of the uncertainty. The co-contaminant problem was approached by assessing the distribution of TBA/MTBE mass ratio relative to groundwater flow and geochemical conditions. Some evidence exists for the occurrence of TBA above background levels, possibly due to localized zones of methanogenesis. An attempt to resolve this issue will be made if broader methanogenic conditions appear in relation to the ongoing action of HRC.

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