Aerobic bioremediation of 1,2 dichloroethane (1,2 DCA) and vinyl chloride (VC) was evaluated at field scale in a layered, silty and fine-sand anaerobic aquifer. Maximum concentrations of 1,2 DCA (2 g/L) and VC (0.75 g/L) in groundwater were within 25% and 70% of pure compound solubility, respectively. Aerobic conditions were induced by injecting air into sparging wells screened 20.5-21.5 m below ground (17-18 m below the water table). Using a cycle of 23 h of air injection followed by three days of no air injection, fifty days of air injection were accumulated over a 12 month period which included some longer periods of operational shutdown. Oxygen and volatile organic compound probes, and multilevel samplers were used to determine changes of the primary contaminants and the associated inorganic chemistry at multiple locations and depths. Air (oxygen) was distributed laterally up to 25 m from the sparge points, with oxygen partial pressures up to 0.7 atmospheres (28-35 mg/L in groundwater) near to the sparge points. The dissolved mass of 1,2 DCA and VC was reduced by greater than 99% over the 590 m(2) trial plot. Significantly, pH declined from nearly 11 to less than 9, and sulfate concentrations increased dramatically, suggesting the occurrence of mineral sulfide (e.g., pyrite) oxidation. Chloride and bicarbonate (aerobic biodegradation by-products) concentration increases were used to estimate that 300-1000 kg of chlorinated hydrocarbons were biodegraded, although the ratio of 1,2 DCA to VC that was biodegraded remained uncertain. The mass biodegraded was comparable but less than the 400-1400 kg of chlorinated compounds removed from the aqueous phase within a 10,000 m(3) volume of the aquifer. Due to the likely presence of non-aqueous phase liquid, the relative proportion of volatilisation compared to biodegradation could not be determined. The aerobic biodegradation rates were greater than those previously estimated from laboratory-based studies.