Sorption and degradation of steroid hormones in soils during transport: column studies and model evaluation

Natural and synthetic analogues of steroid hormones and their metabolites have emerged as contaminants of concern. Characterizing sorption and degradation processes is essential to assess the environmental distribution, persistence, and ecological significance of steroid hormones in terrestrial and aquatic systems. We examined the fate and transport of testosterone and 17beta-estradiol by conducting a series of fast-flow-velocity transport experiments under pulse-type and flow-interruption boundary conditions in columns packed with a surface soil, freshwater sediment, and two sands. Flow-interruption experiments provided independent estimates of degradation coefficients for the parent hormones and their metabolites, while pulse-input type experiments were used to identify transport mechanisms for hormones by employing forward modeling approaches. Estimated degradation rate coefficients (k) for the hormones from flow-interruption experiments ranged from 0.003 to 0.015 h(-1) for testosterone and from 0.0003 to 0.075 h(-1) for estradiol, similar to those observed in batch studies. Degradation rate coefficients for the two primary metabolites were 1-2 orders of magnitude larger than those for the parent chemicals. Estimated k values decreased with column life as a result of nutrient depletion. Large sorption by soils of the parent and metabolites (log Koc approximately 2.77-3.69) did not appear to hinder degradation; k values were an order of magnitude smaller than the estimated sorption mass-transfer constants. Differences in hormone breakthrough curves from a single-pulse displacement and those predicted using independently estimated parameters suggest that modeling hormone degradation as a simple first-order kinetic process may be sufficient, but not accurate.