Modeling Solute Transport in Volcanic Ash Soils with Cation Exchange and Anion Retardation

Groundwater under agricultural lands is often contaminated by nitrate. In southern Japan, aquifers are covered by volcanic ash soils that can leach nitrate to the groundwater. In this study, column experiments using two volcanic ash soils (Kuroboku and Akahoya) were carried out. A mixed solution of KNO₃ and K₂SO₄ was used in the leaching experiments. Based on the experiments, a reactive transport model was developed using the Constrained Interpolation Profile method for ion transport and used to calculate chemical equilibrium for the cation exchange reactions. Anion adsorption was modeled by retardation in the numerical model. The developed simulation model results were compared to results obtained by the reactive transport model PHAST. The developed model was shown to quite well reproduce general characteristics of the experimental results. Also, the developed model results agreed well with results from PHAST. Slight discrepancy between observed and calculated breakthrough curves was probably caused by ignoring the kinetic reaction in the model calculations.     

Soil heterogeneity effects on acid flushing of lead-contaminated soil

A compact model for evaluation of acid flushing of heavy-metal-contaminated soil in a small-scale on-site treatment plant is proposed. The model assumes that the soil was re-packed in a container after excavation resulting in a soil structure with heterogeneous and random physical and chemical properties. To evaluate the effects of heterogeneity on the efficiency of contaminant removal by acid flushing, a numerical analysis of lead transport in the heterogeneous soil medium was performed. The model examines cation exchange and surface complexation reactions involving three cations (Ca, Pb, and H) and one anion (Cl) in both dissolved and exchangeable forms, two Pb surface complexes (SOPbCl and SOPbOH), and one Cl surface complex (SOH₂Cl). The transport of these species during flushing with acid in a synthetically generated two-dimensional heterogeneous soil was simulated in the model. Results indicated that the flushing fluid preferentially followed pathways with large permeability. The heterogeneous cation exchange capacity (CEC) distribution and surface complexation sites had a significant effect on the transport of dissolved species. Because the CEC was set to a relatively low value, Pb was adsorbed mainly as surface complexes (SOPbCl and SOPbOH). Simulation results suggest that blocks of low hydraulic conductivity located in the upper part of the model domain greatly impede solute transport. Ponding conditions did not significantly affect the efficiency of decontamination. The model and its results are useful in the design of small-scale treatment plants for acid flushing.     

Saltwater intrusion in coastal aquifer ??? comparison between the CIP and MOC simulation technique

Proper management and evaluation of environmental effects for coastal groundwater aquifers require accurate estimation of salt and fresh water movement. For this we compare two numerical methods to study saltwater intrusion. The two numerical methods are the CIP (constrained interpolation profile) and the MOC (method of characteristics). The two numerical methods are used to solve the salt transport equation. The two methods are applied and compared to saltwater intrusion experiments for confined and unconfined aquifer case studies. It is found that for both cases, the CIP-method is superior in terms of calculation time while keeping an acceptable numerical accuracy.