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extensive usage in veterinary (e.g., aquaculture, poultry) and human medicine. The quinolones along with most classes of antibiotics are highly water soluble with characteristically low octanol-water partitioning coefficients. Despite high water solubility, several antibiotic classes, notably tetracyclines, fluoroquinolones, and macrolides, are likely to strongly adsorb to soil, sediment, and organic material (including manure). Specifically, compounds that are zwit-terionic can adsorb to clay and organic matter through cation exchange and the degree of adsorption is pH dependent. Soil-to-water partitioning coefficients have been reported for tetracycline and oxytetracycline between 400 and 2000 (based on Kd values, l kg-1), fluoroquinolones have reported K¿ values of 250-6000 lkg- , and tylosin, a common macrolide used in both human and veterinary medicine, has reported K¿ values between 8 and 1301 kg- . Such high sorption to soils indicates that leaching through a soil column is unlikely and that contamination via runoff is most likely through movement attached to particles. Thus, these groups are less likely to contaminate surface and groundwater unless direct input from wastewater effluent or agricultural lagoons occurs. Other antibiotics with low Kd values (e.g., sulfamethazine) are more likely to be transported via water.

Once bound to sediment, soil, or manure, many antibiotics appear to be relatively persistent in the environment. An environmental half-life of greater than 30 days has been reported for chlortetracycline in chicken manure and soil; an oxytetracycline half-life has been reported as high as 150 days in marine sediment. Most fluoroquinolones have reported half-lives of greater than 30 days in sediment, while sulfa antibiotics including sulfadiazine, sulfamethoxazole, and sulfadimethoxine have reported half-lives in marine sediment of greater than 40 days, with some studies reporting no degradation after 180 days. In loam and clay-loam soils, a recent study reported that sarafloxacin degraded less than 1% after 65 days and ciprofloxacin has been reported to have minimal degradation after 40 days. Macrolides may be less persistent than other antibiotic classes with reported half-lives of less than 10 days.

Photodegradation is a major degradation pathway for several antibiotic groups including the fluoroquinolones and tetracyclines where half-lives may be only a few hours. Generally, photodegradation is expected to result in loss of antibacterial activity, although studies of comparative metabolite toxicity are only beginning to appear in the peer-reviewed literature. It is important to note that photodegradation is unlikely to occur once the compound is bound to sediment, prolonging the half-life of a number of antibiotics, particularly in highly turbid ecosystems. However, it is unknown whether antibiotics bound to organic matter and sediment remain biologically active. Thus, aqueous exposure of aquatic organisms to these compounds may be limited to effluent-dominated ecosystems receiving continuous loadings of antibiotics, leading to the potential 'pseudopersistent' scenarios described above. Sediment and organic material within effluent-dominated ecosystems may present continuous exposure at higher levels than measured in the overlaying water, highlighting the importance of characterizing exposure contributions via aqueous and dietary routes.

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