FLOW AND WATER QUALITY OF UNCONFINED GROUNDWATER IN TAKAMATSU PLAIN

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The present study aimed to understand the distribution of flow and quality of unconfined groundwater in Takamatsu plain and to detect the source of nitrate nitrogen in groundwater in the left bank area of Koto River, where high nitrate concentration has been measured all the year round. Major results in this study are summarized as follows.1.Measurement of unconfined groundwater quality at 65 stations and the cluster analysis of the Stiff diagrams demonstrated that the groundwater quality can be classified into four categories, one of which shows anthropogenic effects.2.Intensive field measurement in the target area that locates on the alluvial fan of Koto River, the information on the land use and the fertilizers used along with the analysis of major anions and cations attributed the nitrate contamination in the area to the fertilizers.3.The nitrogen isotope ratio at several locations showed higher value than that reported in the past studies. The isotope-ratio analysis taking account of the effect of ammonia volatilization was then applied to indicate that both organic and inorganic fertilizers are responsible for the high concentration of nitrate nitrogen, which agreed well with the information of the fertilizers.4.Two dimensional numerical simulation of confined groundwater, which accounts for the interaction between confined and unconfined groundwater, showed that the confined aquifer receives a lot of unconfined groundwater in the target area and hence that the improvement of fertilization is necessary to prevent nitrate contamination in the confined groundwater.