Formatted Title
Confronting ISTR's Ultimate Challenge: Pre-ZVI Altered Permeabilities and Floating LNAPLs/Sinking DNAPLs on Bedrock
Background/Objectives
A historically contaminated site adjacent to a Sanitary Landfill had experienced persistent challenges owing to a composite of contaminants, primarily DNAPL, which has sunk into weathered bedrock, and LNAPL which floats atop the groundwater table. The site's history reveals a usage span from the 1960s to 1979 for petroleum hydrocarbon and chlorinated solvent disposal. A prior treatment with zero-valent iron (ZVI) in 2008 has further complicated the scenario by altering the soil's permeability. Within this backdrop, the focused target treatment zone (TTZ) of 15,072 ft2 to an average depth of 49 ft to the top of the competent bedrock has become a spotlight of intricate remediation due to its high concentrations of VOCs and SVOCs, with their varied physical and chemical characteristics. This paper delves deep into the complexities of implementing in situ thermal remediation (ISTR) in such a challenging environment.
Approach/Activities
Thermal conductive heating (TCH) was employed, utilizing 127 strategically placed wells to ensure uniform and controlled heating of the TTZ. Some VOCs, with lower boiling points, were extracted early in the process, while heavier SVOCs, having higher boiling points, emerged later in the remediation timeline. This temporal distinction required meticulous planning and continuous monitoring to ensure optimal extraction and to avoid potential vapor breakthroughs. Tailoring the ISTR approach to efficiently heat DNAPL in the weathered bedrock and the floating LNAPL, while accommodating for their different dynamics and the diverse VOCs and SVOCs associated with each, was a unique challenge. The vapor treatment system, based on the cooling-compression-condensing (C3) methodology, was tailored to efficiently manage the wide-ranging concentrations of VOCs and SVOCs, achieving an impressive removal rate of over 99.9% on the vapor stream before releasing into atmosphere.
Furthermore, the incorporation of ZVI for pre-treatment presented complex implications. While ZVI utilization set a foundational approach for intervention, its resultant effects on permeability added complexity to the remediation procedure. In this endeavor, enhanced MPE/SVE wells were meticulously employed within zones exhibiting diminished permeability. Modifications to the ISTR strategy were imperative to guarantee comprehensive coverage and optimal effectiveness within the TTZ.
Results/Lessons Learned
As of the current phase, over 20,000 lbs of VOCs (including dissolved phase and NAPL) have been extracted, surpassing the upper limit of the initial contaminant mass estimation, underscoring the success of the tailored approach despite the complexities. This case serves as a testament to the potential of ISTR in treating zones with diverse contaminant profiles and altered permeabilities. It reinforces the importance of methodologies that are adaptable to unique challenges that are presented by each site and paves the way for future remediation strategies in similarly complex scenarios. Soil and groundwater confirmation sampling are scheduled for October 2023 to ascertain if remedial goals have been achieved.