Modeling of carbon dioxide transport and production in soil: 1. Model development

TLDR

CO₂ concentration in the unsaturated zone is critical for predicting vadose‑zone chemistry, groundwater recharge, and global carbon balance, and CO₂ can move through both liquid and gas phases. The study introduces SOILCO2, a predictive simulation model built on process‑oriented relationships. SOILCO2 couples one‑dimensional water flow, multiphase CO₂ transport, heat flow, and a production model driven by microbial and root respiration, solving the resulting PDEs with finite element and finite difference methods.

Abstract

Knowledge of the CO 2 concentration in the unsaturated zone is essential for prediction of solution chemistry in the vadose zone and groundwater recharge as well as for quantifying carbon source/sink terms as part of the global CO 2 mass balance. In this paper we present a predictive simulation model, SOILCO2, based on process‐oriented relationships. The model includes one‐dimensional water flow and multiphase transport of CO 2 utilizing the Richards and the convection‐dispersion equations, respectively, as well as heat flow and a CO 2 production model. The transport of CO 2 in the unsaturated zone can occur in both the liquid and gas phases. The gas transport equation accounts for production of CO 2 and uptake of CO 2 by plant roots associated with root water uptake. The CO 2 production model considers both microbial and root respiration which is dependent on water content, temperature, growth, salinity and plant and soil characteristics. Heat flow is included, since some gas transport parameters, partitioning coefficients and production parameters are strongly temperature dependent. The resulting set of partial differential equations is solved numerically using the finite element and finite difference methods.