Formatted Title
Stabilization of Toxaphene in Groundwater Using PlumeStop™
Background/Objectives
Remediation of toxaphene contaminated soil and groundwater was undertaken at the Cokeville Meadows National Wildlife Refuge (CMNWR) in southwestern Wyoming related to the former presence of a cattle dip tank at the site. Following two separate remedial excavations to remove toxaphene contaminated soil for offsite disposal, a project to stabilize the remaining toxaphene in place using PlumeStop™ (Regenesis, Inc.) was designed and implemented. The goal of this project was to bind remaining toxaphene to soil particles thereby reducing groundwater concentrations below the EPA cleanup level of 3 µg/L and preventing further expansion and migration of the toxaphene plume.
Approach/Activities
Kane Environmental worked with Regenesis Remediation Services (RRS) to design and implement a series of PlumeStop injections. PlumeStop combines colloidal activated carbon with a protective coating which degrades over time allowing the carbon to adsorb soluble constituents and bind to stationary soil particles. PlumeStop application design was tailored to site-specific parameters including toxaphene concentrations remaining in soil and groundwater, vertical and areal extent of the contaminated zone, groundwater water quality (nutrients and organic carbon content), as quality of the local river water to be used onsite to dilute PlumeStop prior to injections. RRS completed their design based on data provided by Kane Environmental.
In October 2020, under Kane Environmental direction, RRS completed PlumeStop injection at 27 locations within the approximately 1,600 square foot treatment aera. Injections were completed between 15 feet and 2 feet below ground surface. A total of 5,200 pounds of PlumeStop was injected in a total of approximately 19,500 gallons of solution.
Results/Lessons Learned
Kane Environmental resumed quarterly groundwater monitoring starting in January 2021. Initial results showed a marked increase in toxaphene concentration in the original source area. Downgradient and cross-gradient wells remained unimpacted by toxaphene. Subsequent quarterly sampling events showed decline in toxaphene concentrations in the source area, to the point of non-detection by the third quarterly event. Regulatory compliance was achieved following four quarters of groundwater compliance in the source zone, after six quarters following PlumeStop injections.
Based on communications with RRS during the design and implementation process, the toxaphene concentration in groundwater in the source area was expected to decline immediately following injections, and should have been measurably lower in the first quarterly sampling event. Following the significant increase in toxaphene concentration in the first quarterly sample, RRS informed Kane Environmental that the cold temperature of the Wyoming winter likely slowed the rate at which the protective coating on the PlumeStop colloidal particles “destabilized”, thereby slowing the rate at which toxaphene was bound and removed from solution in groundwater. RRS explained the increase in toxaphene concentration as “back diffusion”, or desorption of toxaphene from soil to groundwater brought about by the presence of the PlumeStop in the formation. In addition, the presence of PlumeStop in the groundwater sample may have contributed to the increase in toxaphene concentration in the laboratory sample.
While the effect of cold temperatures was not anticipated by Kane Environmental or RRS, accounting for significant uncertainty following the first post-injection monitoring event, the explanation provided by RRS regarding the updated timeline for achieving groundwater compliance was correct, and compliance was achieved within 9 months after injections. Application of PlumeStop was an effective remedy to remove soluble toxaphene from groundwater and achieve regulatory closure.