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Estimating groundwater residence time using multiple regression model based on fluoride dissolution: an exploration of possibilities
Authors:Abdelrahman Mohamed Abdelgawad  Kunio Watanabe  Mebruk Mohammed  Shinji Takeuchi
Institution:(1) Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-Ku, Saitama 338-8570, Japan;(2) Geosphere Research Institute, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan;(3) Tono Geoscience Center, Japan Atomic Energy Agency, 1-64, Yamanouchi, Akeyo, Mizunami, Gifu 509-6132, Japan
Abstract:High levels of fluoride concentration were observed in deep groundwater of the Mizunami area in Central Japan. Fluoride occurs mainly due to the reaction between granitic basement rock and groundwater. Granites were collected, crushed to powder, and then allowed to react with purified water for 80 days. Water–rock interaction results showed that the major factor affecting fluoride concentration is the residence time of the groundwater. Coexisting ions have also some contribution toward fluoride concentration. The groundwater residence time in the Mizunami area was estimated by applying results of water–rock interaction to correspond with field data. A regression model relating fluoride concentration, residence time, and coexisting ions was developed. The parameters of the regression model were determined using the genetic algorithms technique. Residence time was estimated by extrapolating experimental data to correspond with filed data. Near the recharge area, residence times in the potential fluoride source rock varied between 1 and 2,000 years, whereas near the discharge area residence times were in excess of tens of thousands of years. The groundwater residence time was also estimated by the groundwater particle-tracking-flow model. The estimates of groundwater residence time based on geochemical regression model were often larger than estimates of groundwater residence time developed by particle-tracking analysis using a groundwater flow model. There were large uncertainties—on the order of 10–10,000 years—in the estimates based on geochemical data.
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