STORMWATER RUNOFF AS A PRIMARY PATHWAY FOR WIDE-SPREAD DISTRIBUTION OF CHLORINATED SOLVENT AT MIDWEST MANUFACTURING PLANT

A Midwest Manufacturing plant utilized a 2,000-gallon Trichloroethene (TCE) above-ground storage tank (AST) from approximately 1978-1994 for the purpose of degreasing manufactured parts. Due Diligence investigations conducted in 2011 identified a release of the chlorinated solvent to on-Site soil, bedrock, and groundwater.

Subsurface geology consists of about 20 feet of brown and grey mottled silty clay with trace sand resting atop weathered grey shale bedrock. Near surface soil are derived from the complete in situ weathering of ancestral shale bedrock, while the deep soil is saprolitic.

Vertical permeability testing of the soil and in situ permeability testing of the aquifer conducted in the saprolite soils calculated a groundwater flow rate of 1.2 x 10^-7 cm/sec or 0.13 feet per year.

The conceptual site model (CSM) developed for the project determined that TCE was de minimisly released to the subsurface from the fill port during filling operations which occurred over 16 years. While reported concentrations of TCE in source area groundwater approached 180,000 µg/L, TCE at the downgradient property boundary, approximately 140 feet west of the former AST, was 2,200 µg/L.

The presence of TCE at the property boundary was not consistent with the CSM, especially considering that groundwater contaminant migration from the source area would have traveled less than 10 feet since 1978. Preferential pathways were investigated to determine how contaminants migrated such a distance from the source area. No subsurface pathways were identified.

In preparation of installation of a permeable reactive barrier to treat downgradient impacts, remediation design characterization (RDC) sampling was conducted along the impacted downgradient property. Fourteen soil borings were advanced along the property line with vertical analytical soil characterization performed. Soil analytical results identified VOCs in the phreatic zone and unexpectantly, VOC impacts were identified in the vadose zone

Surface topography was evaluated relative to the downgradient vadose zone impacts and the source area. Historical photos showed fill area was upslope from the downgradient impacts and characterized by a gravel parking lot/drive. Based on the soil data and topographic survey, a new CSM was developed for the Site.

The new CSM determined that product losses during fill operations resulted in a release to the subsurface proximal to the source area and would be further transported during precipitation events through the high permeability gravel surface pack.