The spatial distribution and hydrogeological controls of fluoride in the confined and unconfined groundwater of Tuoketuo County, Hohhot, Inner Mongolia, China

Insufficient or excessive intake of fluoride can create health problems for humans. The objective of this study was to understand the occurrence of fluoride in groundwater as well as its spatiotemporal variation and sources in Tuoketuo County, Hohhot, Inner Mongolia, China. Sixty-seven water samples, from 34 phreatic and 33 confined sources, were collected from hand pumps, dug wells and tube wells. All samples were analyzed for pH, electric conductivity (EC), total dissolved solids (TDS), total hardness (TH), as well as content of Na+, K+, Ca2+, Mg2+, HCO3-, SO42-, Cl- and F-. The fluoride concentration in water samples varied from 0.40 to 7.20 (2.33 +/- 1.79) and 0.06 to 12.70 (1.65 +/- 2.38) mg/L in phreatic and confined water, respectively. An analysis of the hydrogeochemical characteristic of samples indicated that the high fluoride content of groundwater was caused by the alkaline environment with high levels of HCO3-, Na+, TDS, EC and with a Ca2+ deficiency. The highest concentration of fluoride (>5 mg/L) in phreatic water was located in the groundwater discharge area because this water was affected by the groundwater flow field. In confined aquifers, the concentration of fluoride was mainly controlled by lithology; a high fluoride content (>1.5 mg/L) was only observed in lacustrine platform zone in confined aquifers.