Localized pollution of veterinary antibiotics in watersheds receiving treated effluents from swine farms.

Swine excrement is discharged into surface waters mainly as effluent in Asian countries. As swine production consumes more antibiotics and less water than humans, a mismatch of the size of swine farms and that of the rivers receiving their effluent could create severe pollution by antibiotics. However, little is known about the occurrence of antibiotics in such rivers. We therefore monitored seven veterinary drugs, six human drugs (including a metabolite), three drugs for both use (including a metabolite), and major water qualities at 30 sites in Japanese watersheds where swine outnumber humans and where their excrement is largely treated on-site by aerobic biological wastewater processes. The compositions of veterinary drugs differed substantially among sites, unlike human drugs, indicating various patterns of use among swine farms. Median concentrations at the 30 sites were <1 ng/L for seven out of the ten drugs used in livestock, whereas maximum concentrations were >1000 ng/L for three and 100-1000 ng/L for four of them, giving median-maximum among the sites of >3 log for two and 2-3 log for six of them. The spatial distribution ranges of concentrations of veterinary drugs were wider than those of human drugs (mostly <1.5 log) and other analytes (mostly <1 log), despite the correlation between those of total veterinary drugs and nitrogen, attributable to fewer swine farms than households, the intensive animal husbandry, and the various drug-use patterns among the farms. The range of maximum concentrations of veterinary drugs in the watersheds was comparable to those reported in other Asian watersheds with less strict management of swine excrement, attributable to their slow decay in conventional wastewater treatment on swine farms. Thus, attention should be paid to hot-spot pollution of antibiotics on large Asian swine farms adjacent to streams with limited dilution capacity.