The use of vegetation to remediate soil freshly contaminated by recalcitrant contaminants

The use of vegetation to remediate soil contaminated by recalcitrant hydrocarbons was tested under field conditions. Specifically, an evaluation was made of the effectiveness of deep rooting grasses, Johnsongrass and Canadian wild rye in the dissipation of TNT and PBB's in the soils freshly contaminated to an initial concentration of 10.17+/-1.35 for TNT and 9.87+/-1.23 mg/kg for PBB. The experiment used 72 (1.5m long and 0.1m diameter) column lysimeters with four treatments: Johnsongrass; wild rye grass; a rotation of Johnsongrass and wild rye grass; and unplanted fallow conditions. In the laboratory, immunoassay test procedures determined the TNT and PBB concentrations in the soil, leachate, herbage and root samples. The root characteristics such as total root length, rooting density, and root surface area were quantified to a depth of 1.5m. Changes in microbial biomass were assessed for both rhizosphere soil and the bulk soil during the 2-year study. The largest and most rapid loss in soil chemical concentration was for TNT, which decreased to less than 250 microg/kg, the detection limit, by 93 days after germination. The PBB was at or near the detection limit of 500 microg/kg by 185 days after germination. There was no perceptible difference in contaminant concentration in the soil between the vegetation treatments and/or with depth.