Geochemical Processes Controlling Fluoride Enrichment in Groundwater at the Western Part of Kumamoto Area, Japan

Hossain, Shahadat; Hosono, Takahiro; Yang, Heejun; Shimada, Jun

Geochemical Processes Controlling Fluoride Enrichment in Groundwater at the Western Part of Kumamoto Area, Japan

2016

Hossain, Shahadat | Hosono, Takahiro | Yang, Heejun | Shimada, Jun

This paper first time reports the geochemical processes that are controlling fluoride enrichment in the groundwater of western Kumamoto area, Japan. Fifty (50) groundwater samples were collected and analyzed for the study where fluoride (F⁻) concentration ranges from 0.1 to 1.57 mg/L. About 58 % of the shallow groundwater and 26 % of the deep groundwater samples contain fluoride concentration beyond the Japanese drinking water permissible limit (0.8 mg/L). High F⁻ is largely accumulated in the stagnant zone of the Kumamoto Plain area and associated with Na-HCO₃-type groundwater. High pH, high HCO₃, low Ca²⁺, and high Na⁺ are the major characteristics of high-F⁻ groundwater. Hydrolysis of F⁻-bearing minerals and desorption of F⁻ from hydrous metal oxides are considered to be the primary sources of fluoride in groundwater. A positive correlation between F⁻ and Na⁺/Ca²⁺ ratio (r ² = 0.53) indicates that major ion chemistry plays a significant role in fluoride mobilization. Weakly alkaline nature of groundwater with high pH (7.05–9.45) expedites the leaching process of exchangeable F⁻ from F⁻-bearing minerals as well as favors desorption of F⁻ from metal oxide surfaces. High HCO₃ ⁻ and high PO₄ ³⁻ in the groundwater facilitate desorption process as competing anions, while high Na⁺/Ca²⁺ ratio largely control this process by decreasing positive-charge density of the metal-oxide surfaces. High Na⁺/Ca²⁺ ratio is attributed due to the cation-exchange process, while high pH and HCO₃ ⁻ are the result of both silicate hydrolysis and microbial reduction processes. In addition, calcite and fluorite seem to have a control on groundwater fluoride geochemistry.

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