Stream Dynamics and Chemical Transformations Control the Environmental Fate of Silver and Zinc Oxide Nanoparticles in a Watershed-Scale Model Amy L. Dale, †,‡ Gregory V. Lowry, ‡ and Elizabeth A. Casman* ,† † Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ‡ Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States * S Supporting Information ABSTRACT: Mathematical models are needed to estimate environmental concentrations of engineered nanoparticles (NPs), which enter the environment upon the use and disposal of consumer goods and other products. We present a spatially resolved environmental fate model for the James River Basin, Virginia, that explores the influence of daily variation in streamflow, sediment transport, and stream loads from point and nonpoint sources on water column and sediment concentrations of zinc oxide (ZnO) and silver (Ag) NPs and their reaction byproducts over 20 simulation years. Spatial and temporal variability in sediment transport rates led to high NP transport such that less than 6% of NP-derived metals were retained in the river and sediments. Chemical transformations entirely eliminated ZnO NPs and doubled Zn mobility in the stream relative to Ag. Agricultural runoff accounted for 23% of total metal stream loads from NPs. Average NP-derived metal concentrations in the sediment varied spatially up to 9 orders of magnitude, highlighting the need for high-resolution models. Overall, our results suggest that “first generation” NP risk models have probably misrepresented NP fate in freshwater rivers due to low model resolutions and the simplification of NP chemistry and sediment transport. ■ INTRODUCTION In the absence of monitoring data, mathematical models are commonly used to predict the concentrations and speciation of chemical contaminants in the environment. The predicted environmental concentrations (PECs) can then be compared to laboratory-determined dose-response information to estimate risk. Several recent large-scale models 1-13 have estimated PECs and predicted the environmental fate of engineered nano- particles (NPs), which are now entering surface waters at low levels due to their use in products such as paints, sunscreens, textiles, and cosmetics. 14 Recent environmental fate models focus on NP-specific aspects of NP fate, or aspects that differentiate NPs from molecular contaminants (e.g., kinetic rather than equilibrium descriptors of NP heteroaggregation with soils and sedi- ments 5,7,8,11,15,16 ). Less attention has been paid to aspects of NP fate that are common to all contaminants, which are nonetheless important risk determinants. For example, most models have been solved at steady state 1,5,8 and/or been spatially unresolved, averaging concentrations over large regions (e.g., nations). 2,4,6-8 All have disregarded stream loads from surface runoff of NP-containing biosolids used as fertilizer or found them to be insignificant, 7,8 and none have considered spatiotemporal variability in sediment transport rates, described NP chemistry as a function of environmental conditions, or tracked NP reaction byproducts (metal ions, metal sulfides, etc.). We present results from a watershed-scale model designed to predict the fate of two NPs with different chemistries, silver (Ag) and zinc oxide (ZnO), at comparatively high spatial and temporal resolution and assess the impact of these simplifying assumptions on the utility of NP fate models for risk assessment. NPs and their reaction byproducts primarily enter surface waters via municipal wastewater. 14 During sewage treatment, most NP mass associates with the solid waste (biosolids), 6,17 so agricultural runoff following the application of treated biosolids to crops may contribute to total metal stream loads in regions where biosolids are land-applied. Runoff is an important vehicle for many pollutants found in biosolids. 18-20 The affinity of NPs and metal ions for ubiquitous natural particles such as soil, sediment, micro-organisms, and insoluble organic matter, ensures that solids transport in the environment controls metal transport. 21-25 Sediment deposition and scour in streams is highly variable. Deposition dominates in reservoirs and coastal plains, whereas scour dominates in mountainous Received: March 9, 2015 Revised: May 19, 2015 Accepted: May 27, 2015 Published: May 27, 2015 Article pubs.acs.org/est © 2015 American Chemical Society 7285 DOI: 10.1021/acs.est.5b01205 Environ. Sci. Technol. 2015, 49, 7285-7293