Modeling fate and transport of "Contaminants of Emerging Concern" (CECs): is the Soil Water Assessment Tool (SWAT) the appropriate model?

Background/Question/Methods

As scientific and regulatory communities realize the significant environmental impacts and ubiquity of “contaminants of emerging concern” (CECs), it is increasingly imperative to develop quantitative assessment tools to evaluate and predict the fate and transport of these anthropogenic chemicals. CECs are a diverse group of micropollutants including pharmaceuticals, hormones, personal care products, pesticides, nanomaterials, surfactants, flame retardants, and industrial chemicals. Point and non-point sources of CECs include wastewater effluent and stormwater discharges; manure and municipal biosolids applications; and surface run-off, overflows, or seepage from sewers, septic tanks, and concentrated animal feeding operation lagoons.

Tools developed to predict fate and transport of historically studied industrial chemicals are applicable to some CECs, but inappropriate for others and often lack a watershed-specific focus. Development of the Soil and Water Assessment Tool (SWAT) started inthe 1980s to model water quality and agricultural practices at the watershed scale, and has been continuously updated following software and mathematical advances.  Our team of scientists with expertise in process-based modeling, chemical engineering, agricultural and civil engineering, water quality, and aquatic toxicology, collected empirical data and developed conceptual approaches for modeling diverse CECs from various sources, including selected herbicides/pesticides, veterinary pharmaceuticals (chlortetracycline and tylosin from swine-manure amended fields), organic wastewater contaminants, and trace elements. We explored the potential for using SWAT as a platform to model the fate and transport of CECs in both urban- and agriculturally-impacted watersheds. 

Results/Conclusions

Relatively limited information is available regarding CEC environmental occurrence, fate, effects and risks posed to ecological and human health. The SWAT model is proven to reasonably simulate landscape and in-stream transport of sediments and nutrients across numerous environments. SWAT is also configured to simulate transport of some pesticides, including atrazine.  Simulations of selected organic wastewater contaminants and veterinary pharmaceuticals using SWAT’s pesticide module approximated empirical results for some CECs, but not others. 

Our findings are contributing to SWAT model development, including modification of parameters to better reflect point and non-point source inputs and revision of algorithms to improve surface transport, soil physical and chemical reactions, and biological uptake and transformation processes.  Because the reactive and transport properties of trace elements differ from other CECs, an additional module for modeling trace elements is also under development. The current and future diversity of physicochemical and biological properties represented by CECs and the broad adaptability of the SWAT model present both opportunities and challenges to be met by the research and regulatory communities.