Tracing veterinary antibiotics in the subsurface e A long-term field experiment with spiked manure*

The purpose of this long-term experiment was on gaining more insights into the environmental
behaviour of veterinary antibiotics in the subsurface after application with manure. Therefore, manure
spiked with a bromide tracer and eight antibiotics (enrofloxacin, lincomycin, sulfadiazine, sulfamethazine,
tetracycline, tiamulin, tilmicosin and tylosin) in concentrations of milligrams per litre were applied
at an experimental field site. Their pathway was tracked by continuous extraction of soil pore water at
different depths and systematic sampling of groundwater for a period of two years. Seven target compounds
were detected in soil pore water of which four leached into groundwater. Concentrations of the
detected target compounds were, with few exceptions, in the range of nanograms per litre. It was
concluded that a large fraction of the investigated antibiotics sorbed or degraded already within the first
meter of the soil. Further, it was inferred from the data that long and warm dry periods cause attenuation
of the target compounds through increased degradation or sorption occurring in the soil. In addition, the
comprehensive data-set allowed to estimate a retardation factor between 1.1 and 2.0 for sulfamethazine
in a Plaggic Anthrosol soil, and to classify the individual compounds by environmental relevance based
on transport behaviour and persistence. According to the distribution of resistant genes in the environment,
sulfamethazine was found to be the most mobile and persistent substance.