Irrigation with concentrated animal feeding operation (CAFO) wastewater on croplands has been identified as a major source discharging steroid hormones into the environment. To assess the potential risks on this irrigation practice, the degradation kinetics and mechanisms of 17 alpha-estradiol-3-sulfate were systematically investigated in aqueous solutions blended with dairy wastewater. Dissipation of the conjugated estrogen was dominated by biodegradation under both aerobic and anaerobic conditions. The half-lives for the biodegradation of 17 alpha-estradiol-3-sulfate under aerobic and anaerobic conditions from 15 to 45 degrees C varied from 1.70 to 415 d and 22.5 to 724 d, respectively. Under the same incubation conditions, anaerobic degradation rates of 17 alpha-estradiol-3-sulfate were significantly less than aerobic degradation rates, suggesting that this hormone contaminant may accumulate in anaerobic or anoxic environments. Three degradation products were characterized under both aerobic and anaerobic conditions at 25 degrees C, with estrone-3-sulfate and 17 alpha-estradiol identified as primary metabolites and estrone identified as a secondary metabolite. However, the major degradation mechanisms under aerobic and anaerobic conditions were distinctly different. For aerobic degradation, oxidation at position C17 of the 17 alpha-estradiol-3-sulfate ring was a major degradation mechanism. In contrast, deconjugation of the 17 alpha-estradiol-3-sulfate thio-ester bond at position C3 was a major process initiating degradation under anaerobic conditions. (C) 2013 Elsevier B.V. All rights reserved.