Optimal Fluoride Concentration in Drinking Water as a Function of Calcium Content

Abstract

Fluoride (F-) is known to have both beneficial and adverse effects on humans, depending on the total intake. Drinking water, usually obtained from groundwater, is the primary source of fluoride intake. Estonian aquifer systems differ from each other in bedding conditions, hydraulic parameters and chemical composition and fluorides are released into the groundwater mostly through water-rock interaction. The present study bases on long term groundwater monitoring data and was undertaken to assess the occurrence of fluoride values in abstracted drinking water over the whole of Estonia and their relation to groundwater’s calcium content. Fluoride concentration in 4404 water samples ranges from 0 to 6.71 mg/l. Values higher than 1.5 mg/l were detected in 8 % of samples, mostly in Silurian-Ordovician aquifer system, composed of limestones and dolomites. Low fluoride area in southern Estonia coincides with the outcrop of Devonian sedimentary rocks, where the major source of drinking water is terrigenous Middle-Devonian aquifer system. The occurrence of fluorides is correlated to variations in groundwater chemical type, which is the function of the proportional content of main cations and anions. The highest F- values prevail in wells, which produce the water with low Ca2+ content and vice versa. Thus, the content of calcium ions in groundwater has an important effect on fluoride concentration, insofar as Ca is an element that removes F from water through CaF2 formation and precipitation. Generally, in terrigenous aquifer systems consisting of sand- and siltstones the optimal fluoride content (0.7-1.2 mg/l) is achieved at the calcium concentration within 30-120 mg/l, in carbonaceous Silurian-Ordovician aquifer calcium content which provides the optimal fluoride amount in carbonaceous aquifer system is 150-340 mg/l.