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.

The use of box lysimeters with freshly contaminated soils to study the phytoremediation of recalcitrant organic contaminants.

The potential for phytoremediation of soil contaminated by trinitrotoluene (TNT) and 2,2',5,5'-tetrabromobiphenyl (PBB was used as a surrogate for PCBs) was examined in a 2-year study using box lysimeters under field conditions. The treatments were a warm season grass, Johnson grass, and a cool season grass, Canadian wildrye, and a rotation of Johnson grass and wildrye plus a fallow condition. The experiment was conducted using 12 large (1.50 m x 1.50 m x 0.75 m), in-ground, box lysimeters filled with a Weswood silt-loam soil freshly spiked with the TNT and PBB compounds to a concentration of 10 mg kg(-1) for each chemical. The lysimeters were sheltered to permit controlled applications of water. A total of five sampling rounds were conducted where soil, herbage, and leachate samples were collected for laboratory analysis. TNT and PBB concentrations were determined using the U.S. EPA approved immunoassay test procedures. In the soil, TNT concentrations dropped below the detection limit of 0.25 mg kg(-1) by day 92 and PBB concentrations dropped below the detection limit of 0.50 mg kg(-1) by day 184. There were no significant differences in chemical concentrations among any of the vegetated or fallow treatments at a significance level of alpha < 0.05. However, PBB soil concentrations rebounded above the 0.50 mg kg(-1) level by day 720 for all treatments. No detectable concentrations of TNT or PBB were found in any of the herbage samples or in the leachate.

Influence of salinity on bioremediation of oil in soil.

Spills from oil production and processing result in soils being contaminated with oil and salt. The effect of NaCl on degradation of oil in a sandy-clay loam and a clay loam soil was determined. Soils were treated with 50 g kg(-1) non-detergent motor oil (30 SAE). Salt treatments included NaCl amendments to adjust the soil solution electrical conductivities to 40, 120, and 200 dS m(-1). Soils were amended with nutrients and incubated at 25 degrees C. Oil degradation was estimated from the quantities of CO(2) evolved and from gravimetric determinations of remaining oil. Salt concentrations of 200 dS m(-1) in oil amended soils resulted in a decrease in oil mineralized by 44% for a clay loam and 20% for a sandy-clay loam soil. A salt concentration of 40 dS m(-1) reduced oil mineralization by about 10% in both soils. Oil mineralized in the oil amended clay-loam soil was 2-3 times greater than for comparable treatments of the sandy-clay loam soil. Amending the sandy-clay loam soil with 5% by weight of the clay-loam soil enhanced oil mineralization by 40%. Removal of salts from oil and salt contaminated soils before undertaking bioremediation may reduce the time required for bioremediation.