A Coupled Stream-Aquifer System Model to Simulate Nonpoint Source Pollutants across Irrigated Regions

Dissolved nutrients and trace elements in water systems have the potential to harm aquatic life, animals, and even humans at high concentrations. This study presents a model designed to characterize groundwater and surface water processes that affect nonpoint source flow and pollutant transport in irrigated agricultural areas over a regional scale. A flow model (MODFLOW-UZF), simulating groundwater and stream flow and interaction, is coupled with a transport model (RT3D-OTIS) to depict contaminant transport and chemical reactions. RT3D accounts for cycling and transport of species in the root zone, soil zone, and saturated zone of the subsurface, whereas OTIS accounts for species cycles and transport in the river network. The model is demonstrated in an application to a 500 km² region of Colorado’s Lower Arkansas River Valley. Aquifer and stream parameters are calibrated using a rich dataset to achieve model predictions of nitrate (NO₃) and selenium (Se) concentrations and loads to match observations in the river-aquifer system. Chemical redox reactions between Se and NO₃ are accounted for, with solute mass exchanged between the groundwater and river network on a daily basis according to flow rates predicted by MODFLOW-UZF. Simulation of baseline conditions reveals Se concentrations that are 4 to 5 times the regulatory standards and NO₃ concentrations that are near the interim standard. The aim of the model is to evaluate alternative land and water best management practices in comparison to baseline conditions to find ways to bring in-stream concentrations down toward compliance.