The fate of atrazine and metolachlor, applied as a mixture, in soils taken from pesticide-contaminated sites (Alpha and Bravo) was determined in laboratory studies. Mineralization of metolachlor (less than 1%) in the Bravo sites was minimal after a 30-d. Degradation of metolachlor was greater in the Kochia scoparia rhizosphere soil. The amount of mineralization of atrazine at the Alpha site was as high as 62% of the applied 14C. A mass balance of the 14C applied revealed significant differences (p < 0.05) between the two soil types in soil bound residues, 14CO2, and the extractable organic fraction. The half-life for atrazine was significantly less in the rhizosphere soil (p < 0.05) with a half-life of 50 d compared to 193 d in the nonvegetated soils. A 14C-most-probable-number procedure was also carried out on soils from the pesticide-contaminated sites to determine specific atrazine and metolachlor degraders;To determine the influence of the presence of plants on the degradation of aged herbicide contaminants, vegetated and nonvegetated soil studies were conducted. The percent of applied 14C-atrazine remaining as atrazine in soil which was nonvegetated, or planted with Brassica napus or Kochia scoparia was 9.3, 6.5, and 4.2%, respectively. Combustion of plants revealed that 11% of the applied radioactivity was taken up in Kochia scoparia, while less than 1% was taken up in Brassica napus plants;A screening study was conducted to determine if soils obtained from several pesticide-contaminated sites had enhanced degradative capability to degrade four freshly applied herbicides. Atrazine mineralization was extensive in four of the soils. Minimal mineralization of pendimethalin, alachlor, and metolachlor was seen in this study. Atrazine, metolachlor, and trifluralin were applied in all possible combinations at a rate of 200 [mu]g/g each to soils exhibiting enhanced atrazine degradation in order to determine if higher concentrations and mixtures would inhibit degradation. Only the atrazine was radiolabeled. Rapid mineralization of atrazine occurred in all soil treatments, with 60% to 80% mineralization after 9 weeks;Deethylatrazine degradation was enhanced in soils which had received long-term exposure to atrazine compared with soils that had not received long-term atrazine exposure.