Estimating groundwater residence time using multiple regression model based on fluoride dissolution: an exploration of possibilities |
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Authors: | Abdelrahman Mohamed Abdelgawad Kunio Watanabe Mebruk Mohammed Shinji Takeuchi |
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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 |
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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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