GZA’s remedial design at this former manufactured gas plant (MGP) needed to address coal-tar-impacted soils to depths of up to 35 feet below ground surface (bgs).


GZA first evaluated a remedial action involving excavation and disposal of tar-impacted soil to depths of up to 35 feet bgs and below groundwater in soils with high permeability. However, pump tests proved that this approach would require a significant dewatering and water treatment effort. As an alternate, GZA performed bench scale mix testing for an in-situ stabilization/solidification (ISS) approach. Due to Site access constraints, bucket mix ISS was then selected to treat 7,000 cubic yards (CY) of tar-impacted soils to depths 15-35 feet bgs. While bucket mixing is greater than the depths typically used, GZA was confident it would be successful with rigorous QA/QC verification. GZA’s construction management and remedial oversight included asbestos abatement, building demolition, and a recommendation to use uncoated crushed concrete from the building foundation as backfill for an underground gas holder after it was emptied and cleaned. The EPA’s Environmental Footprint Analysis (SEFA) quantified significant reduction of carbon dioxide equivalents when performing ISS as a source control option over an excavation, removal, and thermal desorption alternative.


GZA’s thorough review of alternative remedial approaches and effective implementation of ISS bucket mixing at up to 35-feet below grade, one of the deepest bucket mixing depths, produced a more favorable environmental construction footprint while satisfying the client’s regulatory obligations.