Formatted Title
Variability in Fill Material and an Abundance of Underground Utilities Complicates the CSM and Remedial Approach at a MGP Site on Lake Superior
Background/Objectives
At an historical manufactured gas plant (MGP) remediation site on Lake Superior, a variety of fill materials were placed over glacial sediment deposits. Varying depths and composition of fill material created challenges in characterizing flow and transport of groundwater impacted by benzene and PAHs. Deposits of wood chips, one of the fill materials, created a sink for MGP waste liquid. Cement and lime dust deposits affected groundwater pH. Further complicating site conditions, an abundance of underground utilities complicated groundwater flow and transport.
Approach/Activities
Many soil borings were advanced and logged over iterative site investigations. Cross sections were developed from these borings and hydrostratigraphic and lithologic units were defined. The lithologic units were incorporated into a three-dimensional (3-D) visualization of the site. The 3-D model was used to develop a conceptual site model (CSM) to characterize the groundwater flow and transport, and to estimate the current location of the plume.
Results/Lessons Learned
Despite an abundance of site characterization data, during remedial action that included the excavation of specific areas of the site and the installation of a biosparge remediation system, certain discrepancies were discovered between the CSM and the observed conditions. The primary source area of MGP waste was found to be offset from what was originally determined from historical mapping. The location and size of the underground utilities was not as mapped. The thickness of fill, type of fill materials, and the depth to the native clay-rich till varied significantley across the site. These site conditions had a significant effect on soil concentrations not fully explained by the CSM. The CSM did not represent small pockets of impacted fill material that are isolated from the main source area. The displacement of impacted material may have been the result of relocation from the source area to other areas during installation of underground utilities. The design of the biosparge remediation system and the remedial excavations were adjusted during construction to account for the observed differences in subsurface conditions.