In-Situ Anaerobic Remediation of Perchlorate-Impacted Soils

ARCADIS is currently leading the development, evaluation, and selection of a final groundwater remedy for a 94-acre manufacturing facility in the western U.S. This facility has produced small explosives and vehicular safety products since 1957. As a result of historic operations, soil and groundwater at the site have been impacted by perchlorate. To date, remediation efforts have been focused on seven source areas and their associated groundwater plumes, some of which are commingled with volatile organic compounds (VOCs) and other contaminants. The complexity of the contaminant distribution and the need to avoid the disruption of previous site operations and future redevelopment has presented a significant remedial challenge.

Since November 2000, ARCADIS has designed, pilot-tested, and implemented a series of innovative interim corrective actions utilizing both in-situ chemical oxidation and anaerobic In-Situ Reactive Zone (IRZ) technology. The interim corrective actions were designed to reduce contaminant mass in source area soil and shallow groundwater to improve overall groundwater quality and reduce off-site migration while final remedial alternatives were evaluated. Given the success of the in-situ anaerobic bioremediation of perchlorate-impacted groundwater (up to 99-percent reduction of pre-treatment perchlorate concentrations ranging from 200,000 to 500,000 micrograms per liter [μg/L] after one year), ARCADIS proposed the evaluation of similar treatment of unsaturated soil to eliminate a continuing source of groundwater impacts.

In December 2003, ARCADIS initiated a field study to evaluate the in-situ anaerobic treatment of perchlorateimpacted vadose zone soils. The study focused on the uppermost 40 feet of soil in an area measuring 40 feet by 30 feet. The shallow overburden in this area is comprised primarily of silts containing varying percentages of sands and clays with thin and narrow lenses of interbedded poorly graded sands and lean clays. Baseline soil sampling in the treatment area identified perchlorate concentrations exceeding 1,000 micrograms per kilogram (μg/kg) between approximately 10 and 40 feet below ground surface (bgs), with the highest concentrations identified at approximately 30 feet bgs (up to 13,000 μg/kg). As part of the study, dilute solutions of corn syrup and ethanol were injected under high pressure throughout the study area, saturating the soil and delivering organic carbon to promote the microbial reduction of perchlorate. Post-injection performance monitoring included the collection of soil samples, gauging and sampling of vadose zone pore water using both pore water observation wells and lysimeters, and monitoring of groundwater quality below the study area. The results eight months after the vadose zone IRZ injection event confirm that 1) post-treatment perchlorate concentrations were reduced by 81 to 93 percent with 100-percent reductions at many locations, 2) the injection technique adequately distributed the substrate solution at required total organic carbon (TOC) concentrations through the targeted soil volume, 3) TOC was utilized as the solutions drained through the formation, and 4) the treatment did not impact groundwater underlying the field study area.

Based on the results of the field study, this treatment technique will be considered in the site feasibility study as a viable and cost-effective alternative to excavation and off-site disposal. Additionally, key monitoring parameters were evaluated. Based on a statistical analysis of soil sampling results, a UCL95-based cleanup goal of 530 μg/kg was achieved. Available data indicate an additional injection event could achieve a significantly lower cleanup goal, if required.

Current feasibility study evaluations indicate excavation and off-site disposal could cost up to $170 per cubic yard (cy) of in-place soil and may not be implementable given the depth of impacts in many source areas. By comparison, the in-situ field study treated 1,750 cy of soil in-place at a cost of approximately $133,000, yielding a unit cost of approximately $75 per cy with no disruption to facility operations or potential impacts to future redevelopment. Further cost analysis indicates that significantly lower unit costs (as low as $40 per cy) could be achieved for full-scale implementation. The technology was successfully applied to 40 feet bgs and is likely implementable to the deepest impact intervals at the site (approximately 100 feet bgs). It is anticipated that a full-scale application at the site may need to address at least 100,000 cy of soil; thus the potential savings from the vadose zone IRZ approach translates into millions of dollars. Overall, the results indicate the technology can significantly support stakeholder goals of achieving site cleanup goals, the lowest lifecycler remediation cost, negligible impacts to site operations, and successful site redevelopment.

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