Theme: 06. Petroleum and Heavy Hydrocarbon Site Strategies

Topic Area: 6b. In Situ Remediation of Petroleum Hydrocarbons

Formatted Title:

In Situ Solidification to Limit Uncertainties in LNAPL/Grossly Contaminated Media Remediation

Background/Objectives:

The scope and cost of light non-aqueous phase liquid (LNAPL)/grossly contaminated media (GCM) remediations tend to be inherently uncertain until after mobilization, presenting a challenge in accurately forecasting the approach, duration, level of effort, and cost required to achieve remedial objectives. In situ solidification (ISS) as a remedial approach under the appropriate circumstances can limit these uncertainties.

Approach/Activities:

Where conventional excavation, transport, and disposal (ETD) is employed, unforeseen circumstances related to excessive dewatering, slope stabilization or shoring, and incomplete delineation of the source area in pre-design phases can lead to change orders and budget overruns. At the Olean, New York Site (Site) ISS has been successfully used to limit uncertainty in remedial scope, duration, and level of effort. However, not all sites are suitable for this application. The Olean Site had a combination of a known deep source that would have required dewatering and shoring for ETD and a GCM source of which delineation was largely based on visual observations with vertical and horizontal extents pre-characterized. ISS was selected as a remedy because it eliminated the need for dewatering, could also be used as slope stabilization for limited shallow excavations, could benefit from the controlled use of swell in place of backfill to replace disposed materials, and, based upon pre-characterization results, enabled remedial end points that were satisfactory to regulators.

Results/Lessons Learned:

A treatment admixture of 6% Portland cement and 2% slag, by weight was selected following bench-scale testing. This mix was selected for its ability to balance the final monolith’s strength – providing increased durability, while minimizing permeability – thereby reducing the potential for leaching of treated waste, while balancing swell with the estimated disposal to minimize import costs to return the site to final grade and reduce overall project duration. Initial estimates to complete the scope via ETD ranged from $19 million to $28 million. The ISS scope has been broken into two phases, initially estimated to cost (combined) between $12 million to $15.5 million. The focused estimated cost range for ISS compared to ETD was preferable to all project stakeholders. Phase 1 was completed in 2022 for $7.5 million, Phase 2 was completed in 2023 for $5.5 million, keeping the project within the forecasted range. ISS has had such success that a Phase 3 to treat a previously out-of-scope plume for an estimated $2.5 million has been approved by regulators and the client for 2024. This has been made possible through several benefits ISS offered over ETD. ISS is completed from above the water table but allows treatment below the water table to depths of 90 feet or more. This eliminated shoring and dewatering, and related uncertainties regarding both volume of water generated and water pre-treatment requirements. The Site has required no dewatering to date. Not only does ISS eliminate the need for shoring to treat deep material but ISS, in both design and installation, is very similar to soil-mix walls, which was used to our advantage in eliminating the need for shoring to allow safe access to areas where Site constraints limited the available space for sloping. ISS causes post-treatment swell on-Site, therefore, some pre-trenching to accommodate anticipated swell is preferred. The pre-trenching and anticipated swell were balanced with the estimated disposal of approximately 40,000 tons of GCM to result in a net import of under 500 cubic yard to return the Site to preexisting grade. Elimination of such a significant import volume yielded dramatic benefitted the project’s financials and schedule. At the Site ISS modified to meet soil-mix retaining wall specifications has allowed 10- to 12-foot deep pre-trenching to facilitate complete treatment up to the property boundaries without the added mobilization or installation cost of shoring. Due to ISS’s related ability to pre-define remedial extent boundaries through pre-characterization, Phase I was completed adding less than 5% more treatment volume than what was anticipated during design, and none of the added treatment volume was due to observed LNAPL/GCM. Upon completion, the project is estimated to involve 100,000 tons of excavated soil, 50,000 cubic yards of impacted material stabilization through ISM, and a 9.35 acre engineered clean soil cover. Further evaluation and analysis of ISS’s benefits related to eliminating dewatering, being employed as a soil retention structure, optimizing swell as an alternative to backfill, and eliminating variable remedial end points will be presented. Also, a discussion of additional considerations related to ISS use, particularly focused on the added QA/QC and requirements for environmental stewardship post-treatment as it is neither a removal nor destruction remedy will be provided.

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Brian Robinson

Biography