The Environmental Fate and transport of Trenbolone Acetate Metabolites in Agro-Ecosystems
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Trenbolone acetate (TBA) is a potent synthetic androgen that is widely used to promote weight gain in beef cattle. A possible consequence of its use is endocrine disruption in aquatic vertebrates, which could occur if trace concentrations (>10 ng/L) reach receiving waters. While the ecotoxicity of TBA metabolites, including 17alpha-trenbolone (17a-TBOH), 17a-trenbolone, and trendione, have been reasonably well characterized, their fate and transport in the environment is poorly characterized. Therefore, lab, plot, and field scale experiments were used to quantify the mass of TBA metabolites that could transport from agro-ecosystems to receiving waters. Observed leaching of 17a-TBOH from manure during irrigation closely matched estimates made from a 1-D diffusional model, suggesting that leaching during irrigation was diffusion limited. When applied to rainfall events, the model drastically underestimated the leachable mass, likely because it didn't account for physical disturbances due to raindrops. Therefore, a power model based on rainfall intensity was fit to the data to describe leaching under different rainfall scenarios. Based on these models, 1,600-22,000 ng and 1,800-4,000 ng 17a-TBOH is expected to leach during 1-5 cm rainfall events and 6-12 hour irrigation events, respectively. To evaluate attenuation, 17a-TBOH was continuously applied to 3-5 m vegetative buffer strips, and ~80% removal was observed. While >97% removal was expected based on equilibrium partitioning, colloid facilitated transport and non-equilibrium conditions likely limited removal. Based on these data, concentrations in runoff from irrigated pastures were predicted to be <1 ng/L, but actual first flush concentrations were 11 ng/L. Runoff concentrations dropped from 11 ng/L to 2 ng/L after only 3 m of overland flow and exceeded 1 ng/L levels for <30 minutes. During rainfall events from small (6-10 m2) plots, average 17a-TBOH concentrations were 2 ng/L. Using mass balance estimates, runoff concentrations are expected to be <1 ng/L as long as stocking densities are <25 animals/hectare, which is the upper limit recommended for pastures. Given a likely no observed effects concentration of 1 ng/L, the ecological risk due to TBA use on well-managed pastures is low. When cattle excrete wastes directly into surface waters, however, several attenuation processes (e.g., sorption, transformation, infiltration) are ineffective, and concentrations are expected to increase 15-20 fold. Furthermore, while densities of 25 animals/hectare is the upper limit for pastures, animal feedlot densities can be much higher, and with higher implant dosages, concentrations as high as 100-250 ng/L are predicted in runoff from feedlots, indicating that the risk of transport on feedlots is substantially larger compared to more diffuse rangeland or irrigated pasture sources.