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UNSATCHEM: Unsaturated Water and Solute Transport Model with Equilibrium and Kinetic Chemistry

D. L. Suarez^* and J. imnek

USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507-4617

* Corresponding author ( dsuarez{at}ussl.ars.usda.gov).

ABSTRACT

Numerous models have been developed for predicting major ion chemistry in the soil zone and in recharge to groundwater. Soils that contain CaCO₃ are prevalent in arid and semiarid regions, as well as in humid and temperate regions that have been glaciated or contain carbonate bedrock. Under these conditions, carbonate-solution reactions and ion exchange are the dominant chemical processes. In this model we couple one-dimensional unsaturated water and solute transport with a major ion chemistry routine and plant water uptake. The model has several unique features, including expressions relating reductions in hydraulic conductivity to chemical factors, prediction of CO₂ partial pressure in the root zone based on a CO₂ production-multiphase transport submodel, kinetic expressions for silicate weathering, calcite precipitation-dissolution, and dolomite dissolution, representation of B adsorption using the constant capacitance model, a new method for predicting cation-exchange selectivity, the option to use Pitzer ion interaction expressions for high ionic strength, and a plant growth submodel that includes water, salinity, and O₂ stress. The chemical submodel considers equilibrium ion exchange, as well as various equilibrium and kinetic expressions for precipitation and dissolution of soil minerals, including gypsum, Mg carbonates, and sepiolite. The use of a predictive submodel for CO₂ production and transport allows for the calculation of CO₂ concentrations with depth and time. This enables us to avoid the assumption of constant CO₂ distribution or constant pH required by previous models. Use of kinetic expressions for carbonate chemistry allows a more realistic simulation of soil and groundwater solution composition as well as simulations of carbonate redistribution and climatic change with time.

Received for publication August 2, 1996.

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