The effect of fertilizer on chemical composition of groundwater was investigated from May 1991 to May 1994 in a farm land located West of Fukuoka city. It was ascertained that nitrate and dissolved oxygen concentrations in groundwater were greatly affected by the surface conditions of the farm land: paddy fields (April-August), vegetable fields (September-March), greenhouses (all seasons), fallow in farm land consolidation.
In the paddy fields, nitrate and dissolved oxygen concentrations decreased gradually from May, showing the minimum at the end of August. After the paddy fields were converted to vegetable fields, both concentrations began to increase, showing the maximum in March. This periodic variation of both concentrations is probably due to the change in the groundwater in view of the reduction or oxidizing conditions caused by the paddy field and vegetable field, respectively. In the land used for greenhouses, nitrate and dissolved oxygen concentrations in the groundwater were constant in high stage in all seasons. When the land was in consolidation, farming was stopped and the land was bare. During this period, since fertilizer was not used, the nitrate concentration was lower than that in the period the land had been used as vegetable field. On the other hand, the dissolved oxygen concentration was constant in high stage.