TWO CONCEPTUAL MODELS OF THE SOURCE AND TRANSPORT OF VINYL CHLORIDE IN BEDROCK AQUIFERS AT THE CITY OF EDINA MUNICIPAL WELL

Vinyl chloride contamination detected in the City of Edina Municipal Well Number 7 (ED-7) triggered an environmental investigation aimed at identifying the contaminant source(s). Borehole geophysics, geochemistry and tritium analysis of discrete ground-water samples collected from the Prairie du Chien / Jordan aquifer at ED-7, and ground-water sampling of different aquifers were used to evaluate the contaminant source and transport mechanisms. A highly fractured zone within the Shakopee formation was identified to feed much of the water entering ED-7. The tritium analysis indicated the presence of older, not contaminated water above the fractured zone and younger, contaminated water within and below that zone within the Prairie du Chien / Jordan aquifer system.

VOC analysis indicated that the likely source of chlorinated solvents is present in the area two miles to the north of ED-7 in the City of St. Louis Park. The highest concentrations of chlorinated chemicals, along with their degradation compounds, were detected in the Platteville wells. The land use survey confirmed the potential presence of multiple sources of chlorinated solvents.

One conceptual model suggests that contaminants migrated through multi-aquifer wells and through the buried bedrock valley present near the source area. These pathways would allow the VOC to migrate to the St. Peter sandstone and further to the Prairie du Chien / Jordan aquifer. However, very minimal contamination found in the St. Peter wells near the source areas, measured hydraulic gradients and the preliminary results from a groundwater flow model suggest that contaminants detected in ED-7 may be coming from different direction. Shallow aquifer contamination with chlorinated solvents was identified at several industrial/commercial sites to the west of ED-7. These sites are close to the buried bedrock valley where Drift formations rest directly upon the Prairie du Chien formation.. Further verification of the two proposed competing conceptual models is important for making future remedial decisions.