Fate of dissolved toluene during steady infiltration through unsaturated soil: II. Biotransformation under nutrient-limited conditions

Biotransformation rates for dissolved toluene in unsaturated sandy soil were determined in dynamic column infiltration experiments. Transformation rates under N-limited conditions and in the presence of sufficient oxygen were 8 to 35 mg (kg d)^-1 and appeared to follow zero order kinetics. Toluene-degrading microoganisms were demonstrated to increase significantly in both activity and numbers with exposure to toluene. Relatively low CO₂ production to oxygen consumption ratios were observed during these experiments suggesting incomplete toluene mineralization. Over the ranges tested, water flux (2080 cm d^-1) and toluene concentration (4-46 mg L^-1) appeared to have a secondary control on rate relative to nutrient and/or oxygen limitations. Under conditions where the oxygen concentration was near zero due to removal by toluene degradation in the soil columns, the transformation rate was sufficiently low to be insignificant relative to column residence time.