Formatted Title

An Innovative Approach to In Situ Soil Mixing Applied at a Chlorinated Solvent-Impacted Site in Batavia, New York

Background/Objectives

In situ chemical reduction using zero valent iron (ZVI) was selected to remediate soil and groundwater impacted with chlorinated solvents at a former heavy equipment manufacturing site in Batavia, New York. Volatile organic compound (VOC)-impacted soil, primarily consisting of chlorinated VOCs, was present in two areas referred to as Area of Concern (AOC) #1 and AOC #2. The most heavily impacted soil had total VOC concentrations greater than 500 parts per million. AOCs #1 and #2 were also each associated with a VOC groundwater plume that had migrated off site. Groundwater concentrations had decreased through natural anaerobic degradation prior to remediation but were still above the NYSDEC’s groundwater standards. Soil in an approximate 0.43-acre portion of AOC #1 and an approximate 0.15-acre portion of AOC #2 was remediated in situ using large-diameter auger soil mixing with the ZVI product Daramend®, manufactured by Evonik. The primary remedial goal was to decrease VOC concentrations in soil to mitigate potential continued contaminant loading to groundwater. Daramend® was selected for use as its proprietary organic amendment and is anticipated to enhance the natural anaerobic conditions present in the mixing areas, further contributing to VOC degradation.

As the mixing areas were adjacent to wetlands and a drainage ditch that empties into a wetland, and the water table is less than 5 ft. bgs, it was desirable to utilize a large-diameter auger method that did not require high volumes of drilling fluids to deliver and mix the Daramend®. An innovative method of auger soil mixing was developed by Geo-Solutions, Inc. (GSI) [remediation contractor] to reduce the volume of water with a two-step low-fluid process to reduce the risk of discharge of fluid containing VOCs from the mixing areas to the adjacent wetlands and ditch.

Approach/Activities

GSI developed grids consisting of 804 locations in the AOC #1 and AOC #2 mixing areas where Daramend® was introduced prior to mixing in either a slurry form, or dry. Daramend® was placed at each location to the specified treatment depths and then the tooling was extracted, leaving the Daramend® vertically distributed in the subsurface. The soil was then mixed using an 8-foot diameter mixing auger to achieve horizontal distribution and homogenization. Six hundred and sixteen (616) columns were mixed. Approximately 500,000 pounds of Daramend® were used to treat approximately 300,000 cubic feet of soil to a maximum depth of approximately 16 ft. bgs.

Results/Lessons Learned

The efficacy of the soil mixing in augmenting the natural attenuation of VOC concentrations occurring in groundwater will be demonstrated through groundwater monitoring. Two quarters of post-remedial groundwater sampling results will be available at the time of presentation. On a broader scale, the success in minimizing the risk of fluid discharge to the adjacent wetlands and ditch by minimizing water usage demonstrates that this new method of soil mixing may be appropriate at other sites with proximate natural features requiring protection. In the traditional wet-Kelly method of soil mixing, a reagent-water slurry is injected through the drill string to the auger. This traditional method typically requires an initial 2- to 3-foot excavation (“precut”) to contain the drilling fluids. For this project, using the traditional mixing method, it is estimated that approximately 400,000 to 700,000 gallons of water would have been introduced into the subsurface; up to 4,200 tons of soil would have required excavation (precut) and disposal in a landfill; and a post-mixing swell of approximately 3 feet may have occurred. Using this new method, only approximately 175,000 gallons of water were introduced into the subsurface, a precut was not required, and the maximum post-mixing swell was less than approximately 1.5 feet. As such, this new method not only reduced concerns relative to potential fluid discharges, but also incorporated sustainable use of water resources and landfill space and left the site in better condition for post-remedial management.