Paul A. Blackwell

The sorption and transport of a sulphonamide antibiotic in soil systems

Veterinary medicines are administered to animals to treat disease and protect their health. After administration, the substances can be metabolised and a mixture of the parent compound and metabolites may be excreted in the urine and faeces. For animals on pasture, the excreta will be released directly to soil whereas for intensively reared animals, the main route of entry will be through slurry and manure spreading. Whilst the behaviour of other classes of substance (e.g. pesticides and nutrients) that are applied to soil is well understood, limited information is available on the transport and fate of veterinary medicines applied to soils. Laboratory and field studies were, therefore, performed to investigate the sorption behaviour of the sulfonamide antibiotic, sulfachloropyridazine, in soil and to assess the potential for sulfachloropyridazine to move from soil to surface waters and groundwaters. Sorption coefficients (K(D)) for the compound in soil and soil/slurry mixtures were low (ranging from 0.9 to 1.8 l kg(-1)) and indicated that the substance would be highly mobile. Field studies on a clay field supported these observations and demonstrated that, after application, the compound was rapidly transported to surface waters, concentrations of up to 590 microg l(-1) being observed in drainage waters. Leaching studies at a sandy site indicated that the substance had a low potential to leach to groundwaters, concentrations in the soil pore water being below or close to analytical detection limits. An assessment of currently available models for predicting concentrations of veterinary medicines entering surface waters indicated that for sulfachloropyridazine, the methods provide reasonable estimates, predicted concentrations being within a factor of two of the maximum measured concentrations. The approaches may not, however, be appropriate for use on highly hydrophobic substances or for predicting groundwater concentrations.

Fate of veterinary antibiotics in a macroporous tile drained clay soil

The environment may be exposed to veterinary medicines administered to livestock through the application of organic fertilizers to land. For other groups of substances that are applied to agricultural land (e.g., pesticides), preferential flow in underdrained clay soils has been identified as an extremely important mechanism by which pollution of surface waters can occur. This study, therefore, was performed to investigate the fate of three antibiotics from the sulfonamide, tetracycline, and macrolide groups. Pig slurry was applied to a field in arable production in two consecutive years and the fate of the compounds was monitored in the soil and drainage water. Both sulfachloropyridazine and oxytetracycline were detected in soil at concentrations up to 365 and 1691 microg/kg, respectively. Subsequently, peak concentrations of the two substances in drainflow were 613.2 and 36.1 microg/L, although mass losses to the receiving water were less than 0.5%. In contrast, tylosin was not detected at all. These findings could be explained by the persistence and sorption characteristics of the antibiotics, while preferential flow via desiccation cracks and worm channels to the tile drains was found to be the most important route for translocation of the chemicals. Thus, when the soil was disced prior to slurry application, losses were reduced significantly. It is evident that processes governing pesticide fate also apply to veterinary antibiotics.

Ultrasonic extraction of veterinary antibiotics from soils and pig slurry with SPE clean-up and LC-UV and fluorescence detection.

A simple and rapid analytical method is presented in which the three veterinary antibiotics oxytetracycline (OTC), sulfachloropyridazine (SCP) and tylosin (TYL) are simultaneously extracted and determined in four different soils. Extractions were carried out by a combination of ultrasonic agitation and vortex mixing using a mixture of methanol, EDTA and McIlvaine buffer at pH 7 as the extractant solution. The extracts were then cleaned-up by a tandem solid phase extraction (SPE) method using an Isolute SAX anion exchange cartridge to remove natural organic matter and an Oasis HLB polymeric cartridge to retain the study compounds. Analysis was by HPLC-UV with additional fluorescence detection for SCP. Recoveries were in the range 68-85% for SCP in all soil types, 58-75% for OTC in sandy soils, 27-51% for OTC in clay containing soils, 74-105% for TYL and 47-61% in a clay soil. OTC and SCP were also extracted from liquid pig manure using a mixture of EDTA and McIlvaine buffer at pH 7 with ultrasonic agitation and vortex mixing with SPE clean-up and HPLC-UV analysis. Recoveries were greater than 77% and 58% for OTC and SCP, respectively. Limits of detection were 18mugkg(-1) for OTC and SCP and 40mugkg(-1) for TYL in soils and 70mugL(-1) for OTC and 140mugL(-1) for SCP in pig slurry.

Characterisation of the abiotic degradation pathways of oxytetracyclines in soil interstitial water using LC-MS-MS.

The fate of oxytetracyclines (OTCs) in soil interstitial water was investigated and the structure of a number of degradation products elucidated in a time-related experiment. A previously developed separation method for LC-MS-MS able to base separate and quantify OTC and three of its epimers and degradation products was applied. Compounds detected were 4-epi-oxytetracycline (EOTC) (t(R)=3.0 min), OTC (t(R)=4.4 min), alpha-apo-oxytetracycline (alpha-apo-OTC) (t(R)=11.4 min) and beta-apo-oxytetracycline (beta-apo-OTC) (t(R)=18.4 min). Furthermore, we tentatively identified 4-epi-N-desmethyl-oxytetracycline (E-N-DM-OTC) (t(R)=3.0 min), N-desmethyl-oxytetracycline (N-DM-OTC) (t(R)=3.5), N-didesmethyl-oxytetracycline (N-DDM-OTC), 4-epi-N-didesmethyl-oxytetracycline (E-N-DDM-OTC) (t(R)=3.7 and 4.7 min) and 2-acetyl-2-decarboxamido-oxytetracycline (t(R)=8.7) in all samples. Most compounds were only present in trace concentrations (less than 2%) relative to the parent OTC. EOTC was on the other hand formed up to a ratio of 0.6 relative to parent OTC concentration. Only EOTC, E-N-DM-OTC, N-DM-OTC, N-DDM-OTC and E-N-DDM-OTC were formed during the time-related experiment. All other compounds were probably only present as impurities in the spiked OTC formulation as they declined in concentration from the start of the experiment. Half-lives (T(1/2), days) of the OTCs in soil interstitial water were in the order of 2 days (EOTC) to 270 days (beta-apo-OTC).

Effects of agricultural conditions on the leaching behaviour of veterinary antibiotics in soils

Antibiotics may be released to soils during the application of manure as fertiliser to land. The compounds may subsequently be transported to and contaminate groundwater and surface waters. This paper describes a series of lysimeter-based studies to explore the leaching behaviour of three veterinary antibiotics (sulfachloropyridazine, oxytetracycline and tylosin) under different conditions that could occur in the agricultural environment. The specific objectives were to: (1) explore the influence of slurry amendment and incorporation on leaching; (2) assess the effects of climate on leaching behaviour; and (3) evaluate the predictive capability of a leaching model used in the regulatory assessment of veterinary medicines. Sulfachloropyridazine was detected sporadically in leachate at concentrations up to 0.66 microg L(-1) under typical irrigation conditions and more frequently at concentrations up to 8.5 microg L(-1) under extreme irrigation conditions. Incorporation and timing of rainfall had no effect on leaching behaviour. Oxytetracycline and tylosin were not detected in any leachate samples. These differences in behaviour were explained by the sorption and persistence characteristics of the compounds. Comparison of the experimental measurements with simulations from the leaching model indicated that the model greatly underestimates the transport of antibiotics to groundwater which raises questions over the application of these models in the regulatory risk assessment process.