Effect of sulfate on nitrate transport in volcanic ash soils sampled from the A and the B horizons

The contamination of groundwater by nitrate has become a worldwide problem. About 50% of the non-rice-producing crop areas are covered with volcanic ash soils in Japan. Therefore, it is important to analyze the nitrate movement in such soils for the conservation of groundwater quality. We investigated the nitrate transport in volcanic ash soils sampled from the A and the B horizons under competitive conditions with sulfate. Miscible displacement experiments were carried out using a soil column 13-cm long and 5-cm in inside diameter. Nitrate was eluted before 1 pore volume of discharge from the A horizon soil column due to anion exclusion. On the other hand, its movement was retarded in the B horizon soil column because of electrostatic adsorption. When sulfate was adsorbed on the soil before the percolation of the nitrate solution, nitrate percolation became faster than that in the absence of sulfate adsorption. When the nitrate solution contained sulfate, nitrate moved much more faster. These faster nitrate elutions were due to the competition of adsorption with sulfate. Some sulfate was strongly adsorbed on the soil and it was not easily exchanged with nitrate in the B horizon soil. It was gradually desorbed and discharged from the soil column during the percolation of the nitrate solution. Sulfate was slightly adsorbed on the soil of the A horizon, although no positive charges were observed. The nitrate for 10 mol_c m⁻³ solution flowed faster than that for the 1 mol_c m⁻³ solution in the B horizon soil, because the solid-liquid distribution ratio for the 10 mol_c m⁻³ solution was smaller than that for the 1 mol_c m⁻³ solution. Because sulfate increased the nitrate flow rate in the B horizon soil, application of sulfate to the soil is not recommended from the viewpoint of nitrate contamination.