Simulating salinity transport in High-Desert landscapes using APEX-MODFLOW-Salt

Abstract

Salt pollution in semi-arid landscapes is detrimental to food production, ecosystem biodiversity, soil health, and water supply. High-desert watersheds, due to natural salt deposits, are a key source of salinity in river basins. Although studies have aimed at simulating and estimating salt movement in these landscapes, approaches have relied on lumped, statistical procedures that treat salt as a conservative solute rather than a collection of reactive salt ions. These approaches are limited in quantifying the source, transport, and fate of salt under both historical and future conditions. This study presents a new physically-based spatially distributed salinity transport model for watershed systems of various spatial scales. The model combines the watershed model APEX, the groundwater model MODFLOW, the groundwater reactive transport model RT3D, and a new salinity module to simulate the fate and reactive transport of eight major ions (SO42-, Cl-, CO32–, HCO3–, Ca2+, Na+, Mg2+, K+) in soils, aquifers, and streams of a complex watershed system. Salt loading to streams occurs via surface runoff, erosion runoff, soil lateral flow, and groundwater discharge. The linked APEX-MODFLOW-Salt model is applied to the Price River Watershed (PRW) (4,886 km2), Utah, USA, a portion of the salinity-laden Upper Colorado River Basin, to estimate spatio-temporal loadings from the various hydrologic pathways and total salt yield in the river during 1995–2015. Results indicate that the vast majority (89%) of dissolved salt in the Price River and its tributaries is loaded by groundwater, and that more than half of the salt is in the form of sulfate (SO4). Erosion runoff due to rainfall events is responsible for only 1.5% of total salt yield in the PRW. Total annual dissolved salt loading from the PRW through its outlet is approximately 63,500 Mg. From the results, we conclude that managing salinity in the PRW may need to focus principally on groundwater and its sources.