ASSESSMENT AND REMEDIATION OF A MTBE/BTEX PLUME IN A GLACIAL LACUSTRINE AQUIFER IN NW MONTANA

Gasoline from a leaking underground storage tank located in Ronan, Montana contaminated the soil and groundwater with methyl tertiary butyl ether (MTBE), benzene, toluene, ethylbenzene, xylenes (BTEX), and other compounds. Complete remediation of the site has been difficult due to the presence of fine-grained glacial lacustrine sand, silt and clay sediments present beneath the site, and because the residual light non-aqueous phase liquid (LNAPL) contaminant plume extended beneath Highway 93, a primary north-south traffic corridor in NW Montana.

Petroleum release sites similar to Ronan exist throughout the northern United States in fine-grained glacial lacustrine sequences. These sites typically require much more rigorous and thorough hydrogeologic assessment at the beginning of the project. Significant source areas may be easily missed due to complex thinly bedded stratigraphy and the long, narrow plume geometries common to many MTBE sites. The Ronan site was recently characterized using cone penetrometer borings and a membrane interface probe (MIP). Results indicate much more complex stratigraphy present than that previously identified in earlier phases of site assessment. This approach, attempted at the beginning of the project, may have expedited the collection of data and completion of a more accurate site conceptual model.

In addition, the use of traditional technologies such as soil vapor extraction (SVE) and air sparging have been limited in reducing site-wide contaminant concentrations in soil and groundwater present at the Ronan site. Results of an innovative remediation approach using electrical resistance heating (ERH) indicate the lifespan of remediation required to treat fine-grained petroleum-contaminated sediments may be greatly reduced and thus, over the life of the project, be competitive with traditional technologies such as SVE and air sparging that may require a substantially longer period of time to achieve regulatory cleanup requirements.

Natural attenuation of MTBE/BTEX concentrations in the hyporheic zone may represent a viable control mechanism for the remaining dissolved plume near Spring Creek. Future source term remediation may bring the site to more rapid closure.