Keywords:Minamisoma City, Fukushima Prefecture, groundwater flow, water quality, stable isotopes, residence time
After the Great East Japan Earthquake occurred, the investigation near the coast area in northern part of Fukushima prefecture has been carried out to clarify the groundwater flow and residence time in this area. The measuring of EC, pH, water temperature, ORP and sampling of groundwater, spring water and river water have been carried out several times at Shinchi town, Soma city, Minamisoma city, Namie town, Okuma town, Iitate village and Date city since September in 2012. The water quality of shallow groundwater and spring water show mainly the Ca-HCO3 type. On the other hand, the water quality of shallow spring water which was affected by the tsunami shows Na-(Cl+SO4) type. However, the quantity of inorganic ions (as EC value) is decrease gradually, so the influence of the tsunami decreases gradually. The water quality of the deep groundwater and spring water which are located near the coastal area show the Na-HCO3 type. It is estimated that the residence time of these deep groundwater and spring water relatively long. In some sites, the concentrations of iron and manganese in the water are high affected by the geology. As the result of CFCs and SF6 analysis, it is estimated that the residence time of shallow groundwater (Ca-HCO3 type) is about 10 years and that of deep groundwater and spring water (Na-HCO3 type) is about 60 to 70 years. This result is consistent with the 3H, δ18O and δD analysis. As a result of altitude effect, it is anticipated that the recharge area of deep groundwater which shows the Na-HCO3 type is about 300 m higher than that of shallow groundwater which shows the Ca-HCO3 type. These informations of groundwater flow are useful for considering the groundwater use with revival of the coastal area. We will continue the investigation of coastal area and make clear the groundwater flow system.