Reactive transport column experiment in volcanic ash soil and numerical modelling with anion and cation exchange reactions

Groundwater contamination by nitrate from agricultural fields is a problem shared by many parts of the world. Kagoshima prefecture, Japan, is an important agricultural district experiencing this problem. In this study, laboratory column experiments were performed and a numerical model was developed to improve understanding of nitrate transport process through volcanic ash soils overlying the aquifer in this region. The volcanic ash soil (Kuroboku) was packed into the column that was assembled by stacking rings of an equal height. A solution of different salts, representing a soil solution after fertilizer application was added from the top of the column by constant head boundary conditions. The inflow was stopped when the dispersion front reached about the middle depth. Cation and anion concentrations in the solid and the liquid phases were analysed by ion chromatography. Competitive sorption due to cation and anion exchange was observed. Concentrations of anion components in liquid phase exceeded the initial concentration of the mixed solution due to desorption from the solid phase. Transport processes in the column experiments were analysed with the aid of reactive transport simulations incorporating cation and anion exchange reactions. Vertical profiles of the Kuroboku-experiment were reproduced by the developed model. Our model qualitatively reproduced the anion concentration exceedence due to desorption, anion retardation and cation exchange.