Modeling sulfur cycling and sulfate reactive transport in an agricultural groundwater system

Many irrigated agricultural areas worldwide suffer from salinization of soil, groundwater, and nearby river systems. Increasing salinity concentrations are due principally to a high water table that results from excessive irrigation, canal seepage, and a lack of efficient drainage systems, and lead to decreased crop yield. High groundwater salinity loading to nearby river systems also impacts downstream areas, where saline river water is diverted for application on irrigated fields. This paper presents a physically-based, spatially-distributed groundwater reactive transport model that simulates the fate and transport of sulfate, the principal salt ion in many salt-affected watersheds, in an agricultural groundwater system. The model, developed from the UZF-RT3D model that simulates chemical species transport in variably-saturated subsurface systems, accounts for sulfur cycling (crop uptake, organic matter decomposition, mineralization/immobilization) in the soil-plant system, oxidation-reduction reactions, including the oxidation of residual Sulfur in marine shale, and also the effect of dissolved oxygen and nitrate on sulfate chemical reduction. The model is tested at the small scale (i.e. soil profile) and at the regional scale (500 km2) in the Lower Arkansas River Valley (LARV) in southeastern Colorado, an area acutely affected by salinization in the past few decades. Results demonstrate that although the major sulfate reactive transport processes are accounted for, the model consistently under-predicts measured soil and groundwater sulfate concentrations, pointing to the need for a comprehensive salinity module that accounts not only for advection, dispersion, sulfur cycling, and oxidation-reduction, but also salt ion equilibrium chemistry that includes the dissolution and precipitation of salt minerals in the soil-aquifer system. However, the model can be a useful tool to assess sulfate fate and transport in areas that are not dominated by salt mineral precipitation and dissolution.