One of the greatest challenges remaining for remediating chlorinated volatile organic compounds (CVOC) is the treatment and/or control of large, dilute plumes. Current approaches to address this challenge are typically long-term and have high capital, operation, and maintenance costs. Cometabolism is showing significant promise in this area because organisms grow aerobically on a supplied substrate (e.g., propane or methane) rather than the trace chemical, allowing good degradation kinetics, minimal impacts to aquifer geochemistry, and the ability to achieve part-per-trillion chemical concentrations. The key objective of this project was to demonstrate effective in situ cometabolic treatment of a large, dilute CVOC plume using an approach that is both environmentally sustainable and cost effective.

This project entailed cometabolic biosparging using a line of sparge wells installed perpendicular to groundwater flow across the width of a large, dilute CVOC plume downgradient of Building 324 at the former Myrtle Beach Air Force Base in South Carolina. The 210-foot-wide groundwater plume, with cis-1,2-dichloroethene (cis-DCE) and vinyl chloride concentrations in excess of federal maximum contaminant levels (MCL), was successfully treated as it flowed through a biologically active zone (i.e., biobarrier) created by sparging oxygen, an alkane gaseous substrate (propane), and a gaseous nutrient (ammonia). The biosparging system, process controls, and system monitoring equipment were powered by an off-the-grid solar energy system. Oxygen, propane, and ammonia were stored on site in cylinders, and configured to provide the appropriate delivery pressures and flows.

An extensive monitoring network, consisting of 27 monitoring wells, six vapor probes, and four dedicated dissolved oxygen probes were installed, and construction of the biosparging system was completed in mid-July of 2019. Startup of the biosparging system occurred in late July, with oxygen-only sparging cycles being performed for several weeks to establish aerobic conditions within the aquifer. Propane and ammonia sparging cycles began in late September 2019, and continued until September 2020. Upon completion of system optimization, significant decreases in both cis-DCE and vinyl chloride groundwater concentrations were observed at the monitoring wells located within and downgradient of the biobarrier, with concentrations in most of the downgradient wells consistently measuring below MCL.

The data from this field test clearly show that propane, ammonia, and oxygen biosparging can be an effective approach to reduce and maintain concentrations of CVOC, such as cis-DCE and vinyl chloride, below relevant MCL. The off-the-grid solar-powered biosparging system proved to be highly reliable, simple to operate and maintain, and economical for dilute plume treatment. For many large, dilute plume applications, this type of biosparging system is expected to be significantly less expensive to install and operate than a conventional pump and treat system or other in situ approaches, such as a zero-valent iron barrier for groundwater treatment. (Project Completion - 2022)