Spatial Distribution of Ions in Groundwater under Agricultural Land

This study investigated the characteristics of shallow groundwater quality under agricultural areas dominated by rice paddy fields, vegetable fields, and a residential zone on an alluvial basin by examining spatial distributions of Cl–, Na+, NO-3, and SO(2-)4 concentrations. Seasonal changes in spatial distribution of ion concentrations during one cultivation period and the causes of the changes were discussed. Changes in spatial distribution of Cl– and Na+ concentrations of the groundwater were mainly affected by seasonal changes in precipitation concentrations. NO-3 concentration was strongly influenced by fertilization of vegetable fields and a following heavy rainfall. Elevated NO-3 concentration was less likely to occur under paddy fields than under vegetable fields. It was suggested that paddy fields might mitigate NO-3 contamination of shallow groundwater possibly because of denitrification in paddy soil in an anaerobic state during ponding. As for SO(2-) 4, it was acknowledged that there could be a specific point source near a particular observation well.