Transport and Transformation of Monensin in Dairy Waste Management Systems

Résumé du livre

The extensive use of veterinary antibiotics in animal husbandry has raised concerns regarding increased development of antibiotic resistant bacteria and toxicity in the environment. Monensin is a coccidiostat commonly used as a feed additive in cattle and poultry, which has variable toxicity among different species. At least 50% of the administered dose is excreted by cattle, and persists in waste storage lagoons which are often land-applied to crop fields for fertilization. The possibility of monensin transport to shallow groundwater after flood irrigation of a crop field was investigated through groundwater monitoring on a California dairy. Wells bordering an irrigated field were sampled over a six week period surrounding an irrigation event, along with multiple wells distributed throughout the dairy. Analysis of the electrical conductivity and chloride content of groundwater from the well downgradient of the irrigated field suggests that up to 7% of the well column consists of irrigation water. Despite a detection limit of 10 pg L−1, no monensin was detected in groundwater originating from the irrigated field. Monensin in the irrigation water ranged from 0.09 to 1.6 [mu]g L−1, and the method would have detected monensin if even 0.1% of the most diluted irrigation water had reached groundwater. These results suggest that monensin is mitigated by soil and not transported to groundwater following irrigation. In contrast, monensin was detected consistently in wells near the lagoon at concentrations from 0.9 to 48.8 ng L−1, suggesting that this aspect of the dairy waste management system is ineffective at preventing groundwater contamination. The effectiveness of different waste management treatments on the attenuation of monensin was investigated by small-scale lagoon microcosms. Although not all monensin bound to the manure was found to be water-extractable, monensin desorbed from manure rapidly. Manure was diluted and screened to 2 mm to replicate mechanical screening on a dairy. Aqueous monensin content of diluted manure slurries was not affected by screening of solids, and reached maximum concentration in water within a day of dilution. A comparison was made between aqueous monensin concentration in lagoon microcosms stored under anaerobic conditions for 36 d and microcosms receiving aeration. Aeration resulted in a large decrease in monensin half-life from100 d to just 14 d, suggesting that lagoon aeration could significantly reduce monensin concentrations and subsequent transport to groundwater. Sorption to soil and subsequent transformation is considered to be the dominant fate of monensin when lagoon water is used for irrigation. Since soils are heterogeneous and sorption will differ significantly between soil types, sorption of monensin to specific soil minerals was evaluated. Minerals such as manganese and iron (hydr)oxides are highly reactive and are known to abiotically transform organic chemicals after sorption, and these minerals were observed to bind and transform monensin. Monensin bound to birnessite ([delta]-MnO2) below pH 6, and to hematite ([alpha]-Fe2O3) and goethite ([alpha]-FeOOH) below pH 7.5. Abiotic transformation of monensin was observed after reaction with all minerals, with the following order of mineral reactivity based on surface area: hematite> goethite> birnessite. These results reveals abiotic transformation of monensin by redox active metal oxides as another plausible fate for monensin in the environment.