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+, NO3-, and SO42- 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. NO3- concentration was strongly influenced by fertilization of vegetable fields and a following heavy rainfall. Elevated NO3- concentration was less likely to occur under paddy fields than under vegetable fields. It was suggested that paddy fields might mitigate NO3- contamination of shallow groundwater possibly because of denitrification in paddy soil in an anaerobic state during ponding. As for SO42-, it was acknowledged that there could be a specific point source near a particular observation well.