Fluoride in Groundwater Worldwide

Various publications and web-sites were consulted to obtain information about the occurrence of fluoride in groundwater. An overview of regions with groundwater containing fluoride in excess of 1.5 mg/l is given in Table 2 below. Table 2 contains also information on the fluoride source rock and the climate.

The Meig's classification adapted by Milich (1997) was used to characterize main climatic types according to precipitation/potential evaporation ratio (P/PE):

• hyper arid (P/PE<0.03)
• arid (P/PE<0.20)
• semi arid (P/PE<0.50),
• dry subhumid (P/PE<0.65),
• moist subhumid (P/PE<0.75)
• humid (P/PE>0.75).

Published country-based information about fluoride was combined with information regarding the geology and climate to assess the probability of occurrence of excessive fluoride concentrations. Four probability classes are distinguished:

I. High-probability:

• Geological formation with a fluoride contaminated groundwater area, situated in a hyper-arid or arid zone.

II. Medium-probability:

• Geological formation with a fluoride contaminated groundwater area, situated in a semi-arid or dry-subhumid zone.
• Geological formation or area, which has the characteristics of a potential fluoride rich environment, and is either a) located in a known fluoride problem country (according to IGRAC and/or Unicef) or b) adjoined to a fluoride problem country with the same geological formation crossing the border. In the second case no further evidence of fluoride contaminated groundwater is found in the literature so far. The climate is hyper-arid, arid or semi-arid in both cases.

III. Low-probability:

• Geological formation with a fluoride contaminated groundwater area, which continues in a moist subhumid or humid climate.
• Geological formation or area, which has the characteristics of a potential fluoride rich environment, and is either a) located in a known fluoride problem country (according to IGRAC and/or Unicef) or b) adjoined to a fluoride problem country with the same geological formation crossing the border. In the second case no further evidence of fluoride contaminated groundwater is found in the literature so far. The climate is dry-subhumid, moist- subhumid or humid in both cases.

IV. Assumed-probability:

• Geological formation or area, which has the characteristics of a potential fluoride rich environment, but is not located in a known fluoride problem country (according to IGRAC and/or Unicef), and no further evidence of fluoride contaminated groundwater is found in the literature so far. The climate is hyper-arid, arid, semi-arid or dry-subhumid.

The above categories were used in preparation of continental maps showing the probability of occurrence of excessive fluoride concentrations. A fifth class "not confirmed" was added. This class refers to those countries which were indicated by IGRAC and/or UNICEF as having problem with fluoride in groundwater but which have no specific information on fluoride source and its distribution. The probability classes and corresponding colours appearing on the maps are summarized in Table 1 below.

Table 1 Probability classes used on the continental maps

The geological world map (CGMW/UNESCO, 2000) was used to delineate the extent of endogenous (plutonic and metamorphic) rocks and extrusive volcanic rocks, which are assumed to be the main source of fluoride.

Information on spatial extent of climatic zones for individual continents was obtained from Milich (1997).

The six continental maps are shown here.

Table 2

Legend

*** No direct references. These areas are interpreted as a potential fluoride rich environment, based on their climate and geology, and often also based on neighboring fluoride-contaminated countries with comparable climate and geology.