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'A tool to identify numeric nutrient criteria with QUAL2Kw'
Andrew Hobson (a.ho@aggiemail.usu.edu), Utah State University; Bethany Neilson (bethany.neilson@usu.edu), Utah State University; Nicholas von Stackelberg (nvonstackelberg@utah.gov), Division of Water Quality, Utah Department of Environmental Quality

With increased pressure from EPA to preserve ambient water quality by prescribing in-stream nutrient standards, there is a need to develop modeling methods to aid development of numeric nutrient criteria. In support of this effort, a nutrient criteria tool is presented that interfaces with QUAL2Kw models to globally read, write, and save results of nutrient-loading scenarios. This tool is tested on seven models using a consistent data collection and model calibration strategy representing sites throughout Utah. By implementing the tool on these models, input concentrations of ammonium (NH4+) ranging from 0.5 to 32 µg/L and inorganic phosphorus (PO4-) ranging from 0.07 to 4.4 µg/L were found to exceed bottom algae thresholds of 150 mg Ch-a/m2. Conversely, NH4+ concentrations above 6,700 µg/L and PO4- above 940 µg/L exceeded dissolved oxygen thresholds of 5-6 mg/L. Some limitations of using mechanistic models in this manner were identified including model capabilities (e.g., steady-state versus dynamic), inclusion of appropriate processes, uncertainty in calibrated parameters, and site-specific conditions. Overall, it was found the nutrient criteria tool was helpful in efficiently identifying nutrient concentration scenarios that trigger a violation, the longitudinal locations where these violations occurred, and characterizing information regarding possible mechanisms behind such violations.