Formatted Title

In Situ Thermal and Bioremediation of a CVOC Source Zone in South America: Challenges and Post-Treatment Findings

Background/Objectives

A combination of in situ thermal remediation and in situ bioremediation was selected for the remediation of a chlorinated solvent and NAPL contaminated site. The site is approximately 50,000 square feet and is located in a mixed commercial and residential area. Contaminants are distributed throughout the site in soil and groundwater to depths of 11 m below ground surface. Extensive plumes of dissolved PCE, TCE, and 1,1,2-TCA and their degradation products were characterized. Source zones were characterized by the presence of both LNAPL (C8-C10) and DNAPL (primarily TCE).  The subsurface consists of highly stratified silty-sandy clays with compact lenses of sandy units. The industrial activities at the site were terminated in the 2010s with the intention of future residential use. Therefore, maximum remediation of the contamination is sought in order to comply with local regulations.

Approach/Activities

A feasibility study was conducted to determine the most appropriate remediation strategy, taking into account sustainability criteria, including a quantitative estimate of the carbon footprint. In situ thermal desorption combined with in situ bioremediation was selected as the remediation approach. The two hot spot zones were treated by in situ thermal remediation using electrical resistance heating (ERH). The total treatment zone in the two hotspots compromised 8200 cubic meters. A total of 60 electrodes were installed to a depth of 12.9 m below ground surface (bgs) in the saturated and unsaturated zones to reach 80-100 degrees Celsius. Vapor extraction wells were co-located with the electrodes to minimize drilling waste. The extracted vapors were treated with both vapor phase carbon adsorbers and a catalytic oxidizer, where the VOCs were converted to CO₂, water (H₂O). Plume remediation was accomplished by injection of a substrate to enhance the in situ biodegradation of CVOCs, while partially raising the aquifer temperature to approximately 35-40 degrees Celsius to stimulate microbial to enhance microbial activity (heat enhanced plume attenuation [HEPA]).

Site conditions and project constraints presented unique technical challenges that required creative solutions. Due to the complex geology of the site and other constraints, an accurate estimate of the DNAPL phase was not possible. This required an adaptable design for vapor treatment. At times, a high-efficiency treatment method was required due to the extremely high mass recovery rate. An automated, remotely controlled system was designed to log all data and events during operation. Site-wide groundwater leveling and monitoring during the thermal remediation thermal remediation operation, was required which necessitated flexible heating/cooling periods.

Results/Lessons Learned

The presentation will focus on selected findings and challenges encountered during the implementation of the remediation activities. For example, very high VOC concentrations in the vapor phase (>3000 ppm) were detected within the first 10 days of operation, which required very rapid adaptation of the treatment system. Accumulation of corrosive compounds downstream of the catalytic oxidizer also presented an operational challenges. Rapid solutions to these technical challenges were developed and applied, resulting in safe operation of the system.