Theme: 02. Assessing Remediation Effectiveness

Topic Area: 02b. Remedy Implementation: Assessing Performance and Costs

Formatted Title:

In Situ Solidification Approaches at MGP Sites to Reduce Costs/Uncertainties and Address Challenging Site Conditions

Background/Objectives:

The remediation of former manufactured gas plant (MGP) sites is an established field. These sites typically have impacts that include difficult-to-treat non-aqueous phase liquids (NAPL), in the form of coal tar oils, and recalcitrant polycyclic aromatic hydrocarbons (PAHs). In addition, subsurface conditions are often challenging with highly variable fill materials (e.g., debris, ash, cinders, etc.) and relic subsurface structures. Due to the difficult contamination and subsurface conditions, remediation approaches often involve some combination of excavation and off-site disposal, physical containment systems (e.g., barrier walls, surface caps), or hydraulic containment. These remediation approaches are effective and will continue to have a place in the remediation of these sites. However, these remedial technologies can be costly and may involve intensive operation, maintenance, and monitoring obligations that can last for years, decades, or more. Some common innovative in situ approaches, such as biological or chemical treatment, have limited applicability at these sites and are generally relegated to being used as a polishing step (i.e., after removal, treatment, and/or containment of source zones) or at the fringes of a plume. In recent years, in situ solidification (ISS), which involves the in-place mixing of cementitious grouts with impacted soil and groundwater to create a low-permeability monolith, has transitioned from an innovative remedial measure to one that has demonstrated itself as being effective in remediating NAPL and PAHs at these sites, and doing so at substantial cost savings or cost avoidance compared to more conventional approaches.

Approach/Activities:

This paper will discuss the application of ISS using excavator bucket mixing techniques at four former MGP sites located in the Northeast and summarize the design/implementation details, results, cost savings/avoidance, and lessons learned. Following several years of bench-scale testing and field testing, ISS was successfully implemented as a primary remedial measure at these four sites. At three of the four sites, ISS was implemented as a source zone remediation measure where a total of over 46,000 cubic yards of impacted soil was solidified to depths ranging down to 35 feet below ground surface. To contain swell from ISS operations and accommodate a 4-foot-thick site cover over the solidified monolith, which provides protection from the freeze-thaw cycle, a total of approximately 23,000 cubic yards of soil or solidified material was excavated from these areas as either pre- or post-ISS excavation activities. At the fourth site, ISS was implemented to construct subsurface perimeter walls, referred to as soil mix walls (SMW), around excavations and utilized as excavation sidewall support and groundwater control. A total of 8,000 cubic yards of ISS soil mixing was conducted to construct approximately 1,800 linear feet of SMW to facilitate over 32,000 cubic yards of excavation.

Results/Lessons Learned:

As a source zone remedial measure, ISS effectively immobilized NAPL impacts, reduced groundwater interaction with source material, and reduced overall project costs when compared to more conventional remediation approaches. Numerous performance samples were tested for unconfined compressive strength (UCS) and hydraulic conductivity (K), which consistently demonstrated the achievement of performance standards despite the highly heterogeneous subsurface conditions. Cores through the solidified mass also demonstrated the thoroughness of mixing and encapsulation of source materials (i.e., NAPL and NAPL-impacted soils). Although excavation and off-site disposal remains a component of any ISS remedy, as it is necessary to remove swell and provide room for surface cover construction, compared to its use as the primary remedial technology, ISS offered substantial, multi-million-dollar costs savings/cost avoidance at these sites. As a perimeter subsurface wall, ISS reduced the excavation quantity (no benching or sloping and no excavation expansion), limited excavation dewatering requirements, and provided protection to nearby sensitive features (utilities, railroad, roadways, buildings).

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Keith Bogatch