Revisiting a Fractured Bedrock Conceptual Site Model to Support a Technical Impracticability Waiver Zone Amendment



Revisiting a Fractured Bedrock Conceptual Site Model to Support a Technical Impracticability Waiver Zone Amendment

Presenter:  Douglas C. Allen, P.G. (Haley & Aldrich, Inc.)
Authors:  Douglas C. Allen, P.G. (Haley & Aldrich, Inc.),  Patrick J. Renshaw (PPL Electric Utilities Corp.),  Christopher Langman, CHMM (UGI Utilities, Inc.)


Downhole bedrock geophysical logging and pressure pulse testing were used at a former MGP Superfund site to support a proposed adjustment to the Technical Impracticability (TI) Waiver Zone boundary. The TI Waiver Zone was previously established to address residual coal tar in fractured bedrock beneath the UGI Columbia Gas Plant Site, located along the Susquehanna River in Columbia, Pennsylvania. The bedrock investigations were completed to study persistent concentrations of MGP-related constituents in a recently decommissioned off-site pumping well, which had been operating for approximately 55 years. As stipulated in the MGP Site’s Record of Decision (ROD), cessation of bedrock groundwater pumping at the off-site pumping well and persistent concentrations of MGP-related compounds in groundwater collected from the well prompted a Focused Feasibility Study (FFS) to evaluate remedial options for dissolved phase compounds present outside the TI Waiver Zone boundary. To inform the FFS process, investigations were completed to update the conceptual site model (CSM) for bedrock described in the ROD. The investigations included discrete-depth groundwater sampling, downhole hydrophysical logging, and a bedrock pressure pulse test to determine whether the off-site pumping well was hydraulically connected to source material within the TI Waiver Zone, or if the persistent concentrations indicated a second, unknown source which would require further investigation and remediation. The supplemental bedrock investigation confirmed that the off-site pumping well is downgradient from residual coal tar in bedrock under ambient, non-pumping conditions. The FFS, approved by USEPA, concluded that the existing remedy for groundwater, Natural Gradient Flushing, was appropriate for MGP-impacted groundwater near the former pumping well. Haley & Aldrich recommended to USEPA that the horizontal and vertical TI Waiver Zone boundary should be adjusted, avoiding a second remedy on a third-party property while maintaining the TI Waiver Zone previously established for the Site.

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