NAPL Risk in Sediment

Former Refinery


  • Confidential


  • Midwest

Key Elements

  • NAPL mobility in sediment
  • Former refinery
  • Regulatory negotiations
  • Remedial alternatives
  • NAPL characterization
  • Hydrogeologic evaluation
  • Geotechnical, biological, and toxicity investigations
  • Ebullition-facilitated NAPL flux
  • Sediment cap
  • Tier 3 ecological risk assessment

GEI performed technical evaluations of NAPL risk in sediment in former wastewater discharge lagoons and led the site-specific regulatory negotiations for a consent agreement.

The site is a former refinery adjacent to residential and industrial areas and a major US river, and the lagoons are affected by former refinery releases of metals and hydrocarbons. The lagoons are shallow, interconnected water bodies within the floodplain of the river that originated as borrow pits during levee construction in the 1950s. The NAPL present in sediment likely co-deposited with the sediment during refinery discharges. The preferred remedy by the regulatory agency was dredging everywhere NAPL was present, at a cost of approximately $40-60 million. GEI successfully led negotiations, including meeting with the agency, preparing documents
and work plans, and evaluating existing data to demonstrate that an alternate remedial approach was feasible. GEI then led field investigation efforts to evaluate remedial alternatives. To assess NAPL in sediment, GEI determined if the NAPL had the potential to migrate to the surface water body. One outcome of negotiations was that NAPL, as long as it was not migrating, could be left in place and capped.

To evaluate NAPL mobility in sediment, GEI collected multiple cores from ‘worstcase’ locations with NAPL (according to previous investigations) within the lagoons. The NAPL was evaluated in a laboratory setting according to a testing program that included UV light evaluation, water drive testing, and flex wall permeameter testing. Additionally, a hydrogeologic evaluation was performed in the lagoon sediments and underlying native clay using vibrating wire piezometers and temperature probes. GEI then evaluated vertical gradients and seepage rates. Additionally, GEI conducted geotechnical, biological, and toxicity investigations and evaluations to demonstrate that portions of the riverfront water bodies were of no more than minimal risk and did not require a remedy.

Upon performance of the field investigations, ebullition-facilitated NAPL flux was observed in the lagoons despite not having been observed previously. GEI evaluated the NAPL flux rate from ebullition and determined that the negotiated presumptive remedy approved by the agency, of a sand with added granulated activated carbon, would be insufficient. GEI is currently evaluating alternate cap materials for a protective sediment cap and alternate remedies such as engineered wetlands, and preparing for additional regulatory negotiations. A cap remedy will result in a cost savings to the client of approximately $15-20 million.


cap remedy cost savings

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