Bioremediation of Soil Containing Toxaphene and DDT
The application of DARAMEND® served to reductively dechlorinate and subsequently mineralize chlorinated herbicides and pesticides present in soil. This project was the third in a series of three pilot-scale projects aimed at validating DARAMEND® bioremediation for use on various chlorinated organic pesticides.
Treatment of about 250 tons of soil impacted with various chlorinated pesticides including Toxaphene and DDT was conducted in a covered in situ treatment cell. Toxaphene, the principle target contaminant, is known to be extremely recalcitrant and is generally considered to resist biodegradation. Due to the in situ nature of this application, soil contaminant concentrations were highly variable and spatially distributed. For example, Toxaphene concentrations ranged from 1,600 mg/kg closest to the former production facility, to 12 mg/kg at the furthest point. The higher pesticide concentrations presented a particularly significant challenge for effective remediation.
The treatment process involved the application of DARAMEND® to a 60 cm thick layer of soil within a treatment area. To prevent unwanted precipitation from impacting treatment, the treatment area was covered with a polyethylene - clad greenhouse. Soil undergoing treatment was subjected to several anaerobic/aerobic treatment cycles. Anaerobic conditions were generated by the application of DARAMEND® organic amendments and zero valent powdered iron. Soil was then irrigated to increase the soil moisture content to 90% of the soil water holding capacity to minimize the diffusion of oxygen into the soil. Aerobic conditions were generated through the repeated introduction of oxygen into the soil via mechanical tillage. Eight separate treatment cycles were required to achieve the remedial goal in some areas of the treatment cell (total DARAMEND loading was <3.5% by weight).
The mean concentrations of the key target contaminants, Toxaphene and DDT, were reduced from 239 mg/kg and 89.7 mg/kg to 5.1 mg/kg and 16.5 mg/kg, respectively. This corresponded to 98% and 82% reductions in contaminant concentrations.
In heavily impacted areas of the treatment cell, the Toxaphene concentration was reduced from 1,600 mg/kg to 11 mg/kg after eight treatment cycles. The application of four additional treatment cycles in this area resulted in a further reduction of Toxaphene concentration to 5.1 mg/kg, which corresponds to a 99.7% reduction.
Estimates to apply this technology full scale at a similar site are approximately at $95/ton, including the treatment cell cover, and about $75/ton if the cover was not required.
Treatment of this material required application of eight treatment cycles. Each treatment cycle required approximately one month, for a total active treatment period of approximately eight months. Following a thorough analysis of data from the site, an additional four treatment cycles (for a total of 12 cycles) were applied to heavily impacted areas of the treatment cell.