Conclusions and Recommendations

Although antibiotics have received increased study relative to other PPCPs in ecosystems, it remains clear that a greater understanding of environmental fate and effects is required. Specific areas of study should include the partitioning of antibiotics from water to soil, sediment, and other environmental compartments, the mechanistic

Figure 1 Examples of potential worse-case scenarios for studying watersheds impacted by antibiotics used in veterinary (North Bosque River dominated by confined animal feeding operations (CAFOs)) and human medicines (Trinity River dominated by wastewater effluent outfalls).

ecotoxicological effects (e.g., target-specific responses that can be related to ecologically relevant effects) of antibiotics on organisms residing in terrestrial and aquatic ecosystems, and the individual, mixture, and degradate effects of antibiotics on structural and functional response variables, particularly in stream ecosystems. Whereas ecological risk-assessment procedures for antibiotics and other PPCPs employ deterministic hazard quotients to characterize risk, probabilistic approaches are increasingly applied to other chemical stressors because variability associated with exposure and effect estimates and various uncertainties may be quantified. Increased information for occurrence, fate, and effects will support more definitive probabilistic assessments of antibiotics in the environment. Such assessments will ultimately reduce uncertainty associated with current application of default application factors, dilution scenarios, and potentially affected ecosystem components (e.g., microorganisms and higher trophic levels). Lotic mesocosms provide valuable model study systems for antibiotics because appropriately designed systems integrate physical, chemical, and biological factors within exposure/threshold response experiments using model systems that are the most likely to receive antibiotic contaminants.

Empirical models of contaminant transport and fate in streams have not been linked to physically based, high-fidelity hydrologic models to date. However, cutting-edge watershed models such as the Soil Water Assessment Tool (SWAT) that respond to changes in both external and internal processes that affect stream flow, turbidity, nutrients, etc., provide an approach with increased environmental realism and are recommended for antibiotics and other PPCPs. Coupling such state-of-the-art spatial analysis, loading estimates similar to those used in the Pharmaceutical Assessment and Transport Evaluation (PhATE) model, and SWAT watershed modeling techniques with an enhanced understanding of loading rates, fate pathways, and the magnitude, frequency, and duration of various exposure routes will lead to reduced uncertainty associated with predictions of emerging water quality issues (e.g., antibiotics, other PPCPs) and reduced incertitude associated with appropriate water resource management decision making.

Regions with intensive CAFO and municipal waste-water irrigation to terrestrial systems likely represent ideal study sites for terrestrial ecosystems. Similarly, potential worse-case scenarios such as CAFO-influenced and rapidly urbanizing, effluent-dominated streams may provide useful model systems for defining the occurrence, fate, and effects of antibiotics in aquatic systems and are likely candidates for retrospective risk assessments. Water resource management decisions associated with antibiotics in such watersheds, particularly those located in arid regions, will continue to receive attention from the scientific, regulatory, and industry sectors as an ecotoxicology paradigm shift continues from the dilution (e.g., the solution to pollution is dilution) to the boomerang paradigm (e.g., what one discards into the environment may come back to be harmful).

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