The Dozen Plain consists of the Saijo Plain and the Shuso Plain. Both plains are located on the coast of Seto Inland Sea with little rainfall and lack surface water resources, so groundwater has long been used. With almost 100% dependence on groundwater for domestic use, conservation and management of groundwater is an important policy issue for Saijo City. In both plains, infiltration from paddy fields plays an important role in recharging groundwater. Dependence on groundwater is high, and in the coastal plains, a rapid drop in groundwater and salinization occur during the irrigated low-rain season. In the Shuso Plain, areas with high concentrations of nitrate ions in the groundwater are distributed due to the use of fertilizers in orchards around the plains: 2021. Nitrogen-containing monovalent anion, one of the major sources of nitrogen nutrients absorbed by plants. Ammonia brought into the soil as mineralization of soil nitrogen or nitrogen fertilizer becomes nitrate ions by nitration under oxidative conditions, which lowers the soil pH. Clay and humus have the function of retaining cations, and soil can retain cations, but nitrate ions are anions and are not adsorbed, increasing the EC of the soil. Therefore, when there is a permeating water flow, it is easy to leach out to the lower layer. It can be denitrified in the leached lower reducing layer or dissolved in groundwater and become a source of groundwater pollution.

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