Concentrations and Controlling Factors of Nitrate in Groundwater under Agricultural Fields and Rural Settlements in Aichi Prefecture, Japan.

In order to evaluate the factors influencing nitrate concentration in groundwater under agricultural fields or rural settlements, properties of groundwater collected from 465 irrigation wells in Aichi prefecture, Japan was analyzed. Nitrate concentration levels depended on the land use, well depth, and oxidation-reduction potential (Eh) of groundwater.Nitrate concentration in shallow groundwater with greater than 250mV of Eh were related to the land use of the surrounding area. Groundwater under upland field showed higher nitrate concentration than those collected from the sites of other land use types. Groundwater under rural settlements also showed a noticeably high nitrate concentration. Groundwater under paddy fields and forests were low in nitrate concentration. Nitrous oxide concentration was also high in oxic groundwater and significantly correlated with nitrate concentration, r=0.781.The nitrate concentration levels of groundwater in which Eh value was≤250mV, did not noticeably increase regardless of land use type. Judging from the Eh level and the existence of denitrifying bacteria in most shallow groundwater, denitrification is considered to be an important factor for maintaining low nitrate concentration in anoxic groundwater.