Retrospective assessments are performed for products once they enter the environment, or for site-specific locations believed to have experienced degraded ecological integrity.
However, studies of antibiotic impacts on ecosystem structure and fUnction are lacking, particularly for streams and terrestrial systems. It is plausible that ecosystems associated with intensive antibiotic use in agricultural practices or rapidly urbanizing regions may be particularly susceptible to antibiotic impacts based on relatively high introduction concentrations.
Of the various aquatic systems receiving point-source municipal discharges, effluent-dominated systems are most likely to experience water quality degradation by antibiotics. Instream flows of many historically ephemeral streams are perennially dominated by effluent discharges, especially in urbanizing areas located within arid and semiarid regions. For example, the majority of dischargers in the states of Texas, New Mexico, Oklahoma, Arkansas, and Louisiana are defined as effluent-dominated streams under low-flow conditions. Stream flows in the Trinity River basin below the Dallas/Ft. Worth metropolitan area are often >90% return flows from effluent discharges (Figure 1).
Watersheds influenced by confined animal feeding operations (CAFOs) or intensively reared animal farming (Figure 1) also represent potential study sites for antibiotics and other veterinary medicines. Efforts by researchers in the United States, Canada, and Europe are increasingly characterizing occurrence and fate of antibiotics in systems adjacent to CAFOs. However, occurrence, fate and transport, and effects in potential worse-case scenarios have not been investigated. As an example, the North Bosque River watershed located in central Texas, USA, contains a high density of dairy CAFOs in its headwater streams (Figure 1). Although this watershed is one of the most heavily studied in the United States for nutrients and pathogen loadings from dairy CAFOs to surface water, an understanding of antibiotic impacts on terrestrial and aquatic biota is not available.
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