Enhanced Anaerobic Bioremediation of Chlorinated Solvents

Problem

Previous facility operations resulted in the release of the chlorinated solvent perchloroethylene (PCE) from above ground storage tanks. Over a 30-year period groundwater impacts have begun to breach the property line. Hydrostratigraphy is defined as 14-feet of Wicomico Formation consisting of fine grained clayey sands overlying the Cooper Formation, a 66-foot thick aquitard composed of sandy clays. PCE levels in the source area exceed 415,000 ug/L, with daughter product levels of; TCE (47,000 ug/L), cis-1,2-DCE (35,800 ug/L), vinyl chloride (1,460 ug/L).

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Solution

Pelorus performed a feasibility assessment of enhanced anaerobic bioremediation and bioaugmentation for remediation of the chlorinated volatile organic compounds (CVOC) distributed throughout this site. Laboratory microcosm tests were performed on site sediment and groundwater to determine the rates of natural attenuation as well as the rate and extent of treatment attainable with specific nutrient formulations and a Pelorus culture of dehalogenating microorganisms. The Pelorus culture is capable of transforming chlorinated ethenes to ethene. A series of 30 microcosms were established using 160 ml serum bottles sealed with Teflon lined butyl rubber septum. Two different site sediment samples were subjected to testing; a high CVOC source area sediment, and a low concentration (800 ug/L CVOC) sample from the property line area. Microcosm testing was performed in triplicate with the 4 treatments being compared to a set of sterile controls.

After 10 months of laboratory testing, it was observed that certain nutrient additions resulted in rapid disappearances (> 90%) of PCE over a period 60 days. Transient accumulations of cis-1,2-DCE were observed. However, additional nutrient supplementation resulted in the cis-1,2-DCE being completely dechlorinated to ethene with low levels of vinyl chloride (55 ug/L) observed. An anaerobic bacterial enrichment culture is being isolated from the most active microcosms and is currently being maintained and studied for its optimum growth and dechlorination activity. If a need arises, the culture may be expanded in a large scale fermenter and used to inoculate (i.e., bioaugment) the site to achieve higher removal efficiencies.

In situ mixing is the single most critical element for achieving efficient in situ bioremediation. Due to the tight clay formations at this site, it would be difficult to add and distribute a nutrient formulation to stimulate anaerobic activity over a wide area of the site. A process utilizing hydraulic fracturing technology to emplace porous ceramic proppants was employed to install a nutrient distribution system consisting of 88 fractures throughout the site. The fractures were arranged in 22 "BioNets" each of which contained 4 fractures spaced on three foot vertical centers. Fracture diameters ranged between 40 and 60 feet depending on depth. Three months after installation and addition of electron donor nutrients, the range over which PCE levels have been reduced is 57% to 99%. Vinyl chloride levels have increased in some locations from 10 ug/L to 2,600 ug/L indicating biodechlorination is the principal removal mechanism. Nutrient additions will be ongoing for 18 months, followed by characterization of soil within the treatment zone for process performance evaluation.

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