A salinity reactive transport and equilibrium chemistry model for regional-scale agricultural groundwater systems

A three-dimensional coupled groundwater reactive transport and equilibrium chemistry model is presented that simulates the fate and transport of major salt ions in agricultural groundwater systems that span broad regions. The UZF-RT3D/SEC model amends the base model UZF-RT3D by coupling it with a new Salinity Equilibrium Chemistry (SEC) module to simulate the movement and transformation of major salt ions (calcium, magnesium, sodium, potassium, sulfate, chloride, bicarbonate, and carbonate) due to advection, dispersion, source/sink mixing, sulfur cycling, redox reactions, precipitation-dissolution, aqueous complexation, and cation exchange. The coupling procedure and the considered reactions make the model applicable for simulating regional-scale agricultural aquifer systems with data efficiency and an acceptable processing time. The model receives flow information from the MODFLOW-UZF flow model, which simulates variably-saturated groundwater flow in an efficient manner over regional scales. For use in agricultural areas, the model also accounts for crop uptake, soil organic matter decomposition, and mineralization/immobilization of carbon, nitrogen, and sulfur species. The model is applied to a salinity-impaired regional-scale (500 km(2)) agricultural area within the irrigated valley of the Arkansas River in southeastern Colorado. It is calibrated and tested against salt ion groundwater concentrations measured in monitoring wells, soil salinity measurements from throughout the region, and total salt loads discharging from the aquifer to the stream network of the Arkansas River. Results indicate that the model represents well the overall magnitude and spatiotemporal trends of soil salinity and groundwater salinity in the aquifer, along with salt loading to streams, and therefore can be used to investigate best management practices for salt remediation. This, along with findings indicating that the inclusion of equilibrium chemistry is vital for producing the correct magnitude of salt ion concentrations and mass loadings, render a tool useful for regional investigation of other highly-salinized aquifer systems.