Formatted Title

Lessons Learned from Two Phases of TCH Treatment at a Former MGP Site

Background/Objectives

Thermal remediation was selected to treat and remove free phase non-aqueous phase liquid (NAPL) and reduce benzene and naphthalene to below industrial/commercial standards at a former manufactured gas plant (MGP) in Alton, Illinois (Site). The Site is located immediately adjacent to active commercial and residential properties with portions of the source area extending below a sidewalk and an operating business. Subsurface conditions within the treatment area consisted of former MGP structures, including pipes and tanks containing NAPL, void spaces, a fill layer consisting of coal, brick, cinders, and other debris mixed with native silts and clays. Additionally, a 10-foot diameter, active sewer line constructed in the 1840s ran through the middle of the treatment zone. Its installation replaced a former creek bed and paved the way for construction of the MGP in 1855. Protection of the sewer from heat and NAPL intrusion served as a critical design constraint.

Approach/Activities

A source area investigation was conducted using high resolution site characterization. This included hydraulic profiling tool results paired with slug and pumping tests to characterize the hydrostatic units and a series of TarGOST® profile borings laid out on a 20-ft grid and corroborated with soil analytical results to define the extent of creosote-based NAPL. Results were input into a 3D visualization model used to select the target treatment zone (TTZ). Thermal conduction heating (TCH) was implemented to achieve a temperature of 100°C within the TTZ that extended from ground surface to the top of bedrock at 30 to 45 ft below ground surface. The site was divided into two phases spanning from 2018 to 2021. Phase I served as the pilot phase and encompassed the former Gas Holder Area (10,800 cubic yards [cy]), and Phase II included the remaining source area (24,700 cy). A phased approach was selected to better understand mass loading (and above ground treatment sizing), the level of treatment that could be achieved at 100°C, and impacts to the sewer, adjacent infrastructure, and the surrounding community.

Results/Lessons Learned

During Phase I, 32,000 lbs of mass was removed, predominantly in the vapor phase, despite initial estimates that a substantial amount of mass would be recovered as NAPL. Soil confirmation results showed treatment objectives could be achieved at 100°C, that NAPL demonstrated significant weathering post treatment, and that extended operations (up to 690 days) did not result in improved mass recovery after diminishing returns. The following lessons learned were incorporated into Phase II: 1) to enhance NAPL recovery, heating was slowed to maintain a longer NAPL recovery window between 60°C and 80°C. 2) specialty high-temperature rated positive-displacement pumps were deployed in MPE wells, 3) upgrades to thicker-walled heater casings to address corrosion and heater failure 4) additional extraction was added in the form of co-located screens and horizontal vapor extraction wells, particularly below and above the sewer line, 5) to protect the sewer, temperature monitoring and dousing points were installed to closely monitor and if needed, percolate water around the sewer for cooling purposes, and 6) operational duration of Phase II was tied to achieving diminishing returns (and optimizing mass removal) which resulted in a much shorter operational duration and reduction in costs. Despite a phased approach, new challenges arrived during Phase II. Rapid increases in subsurface temperatures (>400°C) were observed in several locations near the water table and persisted after shutting down heaters. These observations indicated smoldering of combustible material (e.g., coal tar, wood chips) which created voids beneath and adjacent to the thermal TTZ and cover. Strategies for managing attendant smoldering included close monitoring of temperature, installation of dousing points to protect the sewer, and repair of the insulated cover near a smoldering area and collapsed basement. Development of a 1D numerical model estimated up to 125,000 lbs of NAPL was destroyed in situ through smoldering, in addition to the 5,900 lbs recovered above ground during Phase II. Results, lessons learned, and strategies for managing smoldering will be presented.