Degradation and transport of veterinary antibiotics and estrogens in a vegetated buffer strip system

Knowledge of veterinary antibiotic (VA) and estrogenic hormone persistence in soil is critical to understanding environmental risks associated with these potential contaminants. Understanding sorption and transport of the VAs sulfamethazine (SMZ) and lincomycin (LIN) and the estrogenic hormones estrone (E1) and 17[beta]-estradiol (E2[beta]) is important for assessing and mitigating the movement of these VAs and estrogens to surface or groundwater resources. Vegetative buffer strips (VBS) affect soil properties that enhance the removal of organic pollutants and thus may be a useful tool for mitigating VA and estrogen transport from agricultural lands. Native warm-season grasses used in VBS have shown to enhance soil quality by stimulation of microbial activity and production of DIBOA-Glu (DBG), a phytochemical capable of degrading soil contaminants, such as herbicides. Therefore, the objectives of this study were to: (1) evaluate SMZ and LIN degradation in soils in presence and absence of the phytochemical DBG; (2) evaluate the degradation of steroidal estrogens estrone (E1) and 17[beta]-estradiol (E2[beta]) in soils; (3) and investigate the effectiveness of different VBS vegetation and buffer widths on the transport of SMZ and LIN in runoff from soil amended with VA-spiked poultry litter using simulated rainfall events. Vegetative buffer strip treatments consisted of tall fescue (TF), switchgrass hedge and tall fescue (Hedge+TF), warm season native grasses (Native), and control (no vegetation). Buffer widths were 0, 2, 5, and 9 m. Degradation experiment results for SMZ and LIN in soil followed pseudo first-order kinetics and showed that soil half-lives (t0.5) for SMZ ranged from 17.8-30.1 d, with no significant differences in degradation kinetics between treatments with or without DBG. For LIN, results showed t0.5 ranged between 9.37-9.90 d, degradation kinetics were similar (no significant difference) between treatments with and without DBG. The results indicated that DBG driven hydrolysis did not play a significant role in degrading either target VA in the soil matrix. These results also showed that degradation of LIN was 2-3 times more rapid than SMZ in soil, suggesting that LIN may be a better choice for use as a broad-spectrum VA given its lower persistence in soil. Degradation experiment results for the estrogenic hormones E1 and E2[beta] in VBS soil showed that the half-life (t0.5) for E1 ranged between 4.71-6.08 d and the half-life for E2[beta] ranged between 5.59-6.03 d. Both estrogens followed pseudo first-order degradation kinetics. Results showed that E1 was present as a metabolite in the E2[beta] treated mesocosms, and conversely, E2[beta] was present as a metabolite in the E1 treated mesocosms. The transformation of E2[beta] to E1, and vice-versa, was rapid with high metabolite concentrations in all mesocosm treatments within 24 h. The interconversion of E2[beta] and E1 in soil results in greater overall persistence, leading to more sustained estrogenicity and greater environmental risk associated with land application of these two estrogenic compounds. Results from the VBS runoff experiment showed that the VBS buffer width impacted VA loads much more than vegetation treatment. The VBS treatments were very successful at achieving sediment load reductions compared to the bare ground control (reductions of 40 percent at 2 m, 70 percent at 5 m and 86 percent at 9 m), but the grass treatments had greater runoff compared to the control. This limited the ability of the VBS to reduce VA loads in runoff as increased infiltration was not a mechanism for VA retention in this study. However, VBS treatments significantly reduced total (dissolved-phase + sediment-bound) SMZ and LIN loads in surface runoff by 29 to 62 percent). Total and dissolved-phase VA loads were significantly reduced only by TF treatment while there was no significant vegetation effect for sediment-bound VAs. Buffer width was a more significant factor for total, dissolved-phase, and sediment-bound LIN loads, with significant load reductions for each increase in width, while total and dissolved-phase SMZ had similar load reductions at each width. Sediment-bound loads of both VAs were strongly affected by buffer width. Among grass treatments, TF was overall most effective at reducing total VA loads in runoff (55 percent for SMZ and LIN). The results showed that grass treatment had less effect on VA loads than buffer width, and the grass treatments tested here were less effective at reducing SMZ and LIN loads in surface runoff than has been demonstrated for other contaminants, such as sediment, nutrients, and herbicides. To effectively reduce these VA loads in runoff, especially dissolved-phase transport, would require lower source-to-buffer area ratios and more land out of production than for other agricultural contaminants.

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