Representatives from a large number of antibiotic classes including the macrolides, quinolones, sulfonamides, and tetracyclines have been reported in WWTP effluents, surface waters, sediments, and soils, indicating potential exposure via multiple routes for aquatic and terrestrial organisms. In most cases, and particularly within WWTP effluents, these compounds typically occur as mixtures in the environment. While no standardized analytical techniques are available for antibiotics in various matrices, liquid chromatography and gas chromatography tandem mass spectrometry (lC-MS/MS and GC-MS/MS) approaches are most commonly used to achieve low-level detection limits and high qualitative identification. Antibiotics are routinely detected in WWTP effluents and surface waters in the ngl~ range, with the highest levels detected in surface waters at 15 mgl1 for sulfadi-methoxine, a sulfonamide.

In addition to contaminating surface water, antibiotics from WWTP effluents may also contaminate related sediment and soil. For instance, in a recent study from the United States Geological Survey (USGS), the average concentrations of trimethoprin and erythromycin in WWTP effluents were relatively low, 111 ngl-1 and below detection limits, respectively. However, both compounds were also detected in downstream sediments, 1.22 and 5.47 mg kg-1, soils treated with municipal wastewater, 25.7 and 2660 mgkg-1, and in biosolid-derived components, 11.8 and 5 mgkg-1, respectively.

As discussed further below, the relatively low lipo-philicities of many antibiotics suggest that uptake into organisms is limited. However, previous studies identified oxytetracycline residues in fish following aquaculture application, and an even more recent study detected trimethoprim and erythromycin in tissues of fish residing in an effluent-dominated stream, corresponding with the USGS report of relatively high levels of these compounds in sediments downstream from a WWTP discharge. Thus, the occurrence of antibiotics and their potential to affect terrestrial and aquatic organisms via various exposure routes requires further investigation.

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