A Coupled Reactive Transport and Equilibrium Chemistry Model for Assessment of Salinity in Regional- Scale Groundwater Systems

Secondary salinization is a major dilemma in many irrigated regions. Increasing salt concentrations are due principally to dissolution from subsurface formations and evaporative concentration associated with a high water table and flows that result from excessive irrigation, canal seepage, and a lack of efficient drainage systems, leading to decreasing crop yield. High groundwater salinity loading to nearby river systems in turn impacts downstream areas, with saline river water diverted for application on irrigated fields. A three-dimensional coupled groundwater reactive transport and equilibrium chemistry model has been developed to simulate the fate and transport of salt ions in regional groundwater systems. The base model is UZF-RT3D, amended with a new Salinity Equilibrium Chemistry (SEC) module to create a coupled model that simulates the distribution of major salt ions (sulfate, calcium, magnesium, sodium, and chloride, carbonate, bicarbonate) due to advection, dispersion, source/sink mixing, redox reactions, precipitation-dissolution, complexation, and cation exchange. For application in agricultural areas, the model also accounts for crop uptake, soil organic matter decomposition, and mineralization/immobilization of carbon, nitrogen, and sulfur species. It is applied to a 500 km² agricultural area in Colorado’s Lower Arkansas River Valley, and tested against extensive data on salt ion groundwater concentrations in the soil zone and in the saturated zone of the aquifer, and groundwater salt loadings to the Arkansas River stream network. Results indicate that the model can be a useful tool in simulating salt ion fate and transport in highly-salinized aquifers, with the potential for application to other salt-affected regions worldwide.