Effect of radial flow on deviations from local equilibrium during sorbing solute transport through homogeneous soils

Several laboratory and theoretical studies of sorbing solute transport through one‐dimensional homogeneous soil columns have recently addressed the validity of the local chemical equilibrium assumption (LEA).This paper extends previous theoretical results to the case of ideal radial flow in homogeneous aquifers. Sorption kinetics are assumed to follow a first‐order reversible rate law. Two sample problems are considered: one involves injection of a contaminant pulse into a diverging radial flow field; the other, extraction of polluted groundwater by a purge well in a converging radial flow field. An analytical time moment analysis is performed to derive formulas for solute breakthrough curve time moments. Comparison of time moment formulas for the kinetic and equilibrium models leads to the definition of criteria for LEA validity. These criteria explicitly show the effect of basic system parameters (e.g., pumping rate, dispersion coefficient, distribution coefficient, reaction rate coefficient) upon deviations from equilibrium behavior. For the case of converging radial flow, formulas are derived to calculate the impact of desorption kinetics upon the time required for aquifer decontamination. The results also show that significant nonequilibrium effects are confined to the vicinity close to the injection‐extraction wells.