Hello, I have checked some papers where it is mentioned that arid/ semi-arid climatic conditions is responsible for high fluoride concentration in groundwater. Can someone please explain me how it happens!
Dear Mondal,
The hydrodynamics of the aquifer system play an important role in the contact time between water and rocks. Thus, when the flow of groundwater is relatively slow, a marked increase in the levels of Total Dissolved Substance (TDS) and fluorine is observed. The role of climate is therefore very important here. In fact, the low rainfall in arid regions generally results in longer groundwater transfer times than in wetlands. The longer contact time water / rock then cause enrichment of water in mineral substances. The importance of the hydrodynamics of the system is also noticeable by the presence of a quantity of fluorine often stronger in captive aquifer than in the superficial aquifers. In arid regions, evaporation affecting surface water and soils, which can infiltrate, is also an important factor in increasing groundwater concentrations.
With my best regards
Prof. Bachir ACHOUR
Thank you so much Sir. I am more keen to understand the chemistry behind "evaporation affecting surface water and soils, which can infiltrate, is also an important factor in increasing groundwater concentrations". I saw in some papers stating that it helps calcite to precipitate and because of that fluoride concentration increases in groundwater but how exactly it happens is not covered there!
hi
the dissolution Fluorite mineral (CaF) increases the Ca concentration in groundwater. So that, the released Ca can react with carbonate in groundwater and shift the calcite equilibrium into precipitation.
all the best
The two main factors in controlling the fluoride concentration in a semi-arid area are transpiration factor which increases fluoride minerals and nearby aquifer influence which can play a role to decrease the concentration.
You can see detailed geochemistry of factors controlling the F concentration in a semi-arid area in the attached paper.
In sub-Saharan Africa, fluorine is not particularly abundant in arid or semi-arid zones. It is rather a consequence of aquifer lithology (sedimentary rocks with fluor apatite). In Senegal, fluorine is particularly abundant (making hundreds of boreholes unusable for drinking purpose) in the Maestrichtien sandy aquifer, both in the south of the country (where annual rainfall is 1000 mm/year) and in the north (400 mm/year).
Yves Travi devoted his doctoral thesis in 1988 to these aquifers contaminated by fluorine (Hydrogeochemistry and isotopic hydrology of fluoride aquifers in the Senegal basin: origin and conditions of transport of fluoride in groundwater. PhD thesis in geology (Paris Sud). He shows that, as Mohamad Alhamed quotes it, the maximum fluorine content of groundwater is strongly correlated to its calcium content (CaF - Fluorite solubility is the main controlling factor).
For this reason, aquifer waters in arid climates containing a lot of calcium (especially in evaporite sedimentary contexts) do not have a lot of fluorine in solution.
Dear Mr. Mondal,
You may find the answer (with chemical equation and explanation) in the below article.
Article Groundwater fluoride contamination, probable release, and co...
Regards,
Indrani Mukherjee
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