Fluoride distribution in water and the governing factors of environment in arid north-west China

In arid areas of north-west China, fluorides occur predominantly as NaF and KF. They are fairly ubiquitous in both natural surface waters such as rivers, lakes and reservoirs, and in ground-water. Abundant fluoride sources, a relatively closed hydrological network, and the dry climate of the arid inland basin provide favorable conditions to the dissolution, migration and enrichment of fluoride. The formation of inland river systems has led to specific fluoride migration and distribution patterns. This has resulted in three main hydro-chemical zones: (1) leaching-runoff; (2) runoff-evaporation and (3) dissolution-evaporative enrichment zones. In Cretaceous and lower Pleistocene lake deposit depressions and on the margin of desert areas, F−is not only enriched in ground water compared to surface waters, but also shows a vertical zoning, i.e. the greater the depth the greater the F−content. F−enrichment and deficiency are closely related to the hydro-chemical characteristics of the body of water. Generally, low-SO2−4and low-Ca2+alkaline waters environments, e.g. Cl−•–HCO−3–Na+or HCO−3–Na+waters, are favorable to the dissolution and enrichment of F−.