In-Situ Bioremediation of Perchlorate in Soil

The discovery of perchlorate in soil and groundwater at a former road flare manufacturing facility in Santa Clara Valley, California has prompted an extensive program of soil and groundwater characterization, coupled with development, design and implementation of remediation activities on a highly accelerated schedule. Perchlorate concentrations in on-site groundwater have ranged up to a maximum of 550 μg/L in 2003. Perchlorate in on-site soil is predominantly limited to the former process area of the facility with a maximum concentration of 14 mg/kg. A combined soil and groundwater treatment system was initially designed for the site in September 2003, but was later revised to uncouple soil and groundwater treatment. The groundwater extraction and ion-exchange treatment system design was approved in October 2003 and the system was operational by the end of December 2003. The flow rate is anticipated to range seasonally from 75 to 200 gp, to maintain capture in a hydrogeologic environment that sees a seasonal watertable fluctuation of more than 20 feet. To address stakeholder and agency comments regarding the soil component of the combined system, a feasibility study (FS) was performed to address perchlorate remaining in vadose zone soils. The thickness of the vadose zone varies seasonally in response to watertable fluctuations, ranging between 16 to 36 feet. Approximately 40,000 yd³ of vadose zone soil is estimated to contain perchlorate above the site-specific soil screening level of 50 μg/kg generated following USEPA guidance. Based on detailed analyses of overall protection, effectiveness, implementability, community acceptance and cost, the agency and responsible parties agreed to a soil remedy that includes: (i) focused excavation and ex-situ biotreatment of soils containing perchlorate above 7.8 mg/kg (~ 800 yd³ of soil), and (ii) in-situ bioremediation of soils containing perchlorate below 7.8 mg/kg. The soil remedy leverages the infrastructure of the on-site groundwater containment system to facilitate in-situ remediation of the on-site soils. In-situ remediation will consist of amending a portion of the treated (i.e., perchlorate-free) effluent from the groundwater treatment system with a carbon-based electron donor, and applying the amended water to the ground surface above the targeted soil treatment area. As the electron donor infiltrates through the vadose zone soils, it will promote biodegradation of perchlorate. Laboratory biotreatability studies conducted coincident with development of the FS indicated that the addition of various electron donors (e.g., acetate, ethanol, oleate) promoted rapid biodegradation of perchlorate in the soils from concentrations of 1,000 ∞g/kg to less than 4 ∞g/kg within 13 days. The bacteria, which are naturally present in the soil, degrade the perchlorate to harmless chloride and oxygen. The in-situ remediation approach follows from work by Cox et al. (1999) and Borch et al. (2001) demonstrating treatment of perchlorate in vadose zone soils through ex situ composting and surface application and infiltration of electron donors, respectively. The groundwater extraction/treatment system, in addition to providing the treated infiltration water, ensures containment of perchlorate and its degradation products that are washed from the unsaturated soils. It is estimated that two years of continuous in-situ soil bioremediation may be sufficient to achieve the desired soil remediation objectives.