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 to 21.5 m below ground (17 to 18 m below the water table). Using a cycle of 23 h of air injection followed by three days of no air injection, 50 days of air injection were accumulated over a 12-month period that included some longer periods of operational shutdown. Oxygen and VOC 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 to 35 mg/L in groundwater) near to the sparge points. The dissolved mass of 1,2-DCA and VC was reduced by >99% over the 590 m2 trial plot. The 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 byproducts) concentration increases were used to estimate biodegradation of 300 to 1,000 kg of chlorinated hydrocarbons, although the ratio of 1,2-DCA to VC that was biodegraded remained uncertain. The mass biodegraded was comparable to but less than the 400 to 1,400 kg of chlorinated compounds removed from the aqueous phase within a 10,000 m3 volume of the aquifer. Due to the likely presence of nonaqueous-phase liquid, the relative proportion of volatilization compared to biodegradation could not be determined. The aerobic biodegradation rates were greater than those previously estimated from laboratory-based studies.