Study on groundwater flow system in a sedimentary rock area, 2; Case study for the Yoro River basin, Chiba prefecture

Sakai, Ryutaro; Munakata, Masahiro ; Kimura, Hideo

Japan Atomic Energy Agency has investigated a sedimentary rock area in the central part of Yoro river basin, in Chiba Prefecture for verification of analytical groundwater flow model. For the purpose of establishing a methodology for evaluation of regional groundwater flow system, investigations of flow rates, chemical compositions and isotope ratios for water samples collected from wells, rivers and springs were carried out. As a result, considerable amount of ground water which is NaCa-HCO type or evolved Ca-HCO type meteoric origin cultivated at the high permeable sand dominant layer preferentially flows toward its strike direction discharging at the downstream region of river. The evolved Ca-HCO type or Na-HCO type ground water with long residence time was inferred to form different flowpath toward the dipping direction of bedrock more than 100m at depth and to upwell to the downstream region of Urajiro River as a feeder of Yoro River through the low permeable mud layer.