An Appraisal of Natural Fluorine Contamination of Paleogroundwater in Tozeur Oases, Southern Tunisia, with Emphasis on the Anthropogenic Impact - 2020

Hydrogeology, Geology, & Environmental Issues / Field Methods & Procedures

Tarki and 2 co-authors, reported in 2020 that in southern Tunisia, environmental threat related to fluorine contamination of the “Complex Terminal (CT)" main aquifer paleogroundwater in one of the most important and strongly vulnerable oases was assessed. Emphasis was placed to hydrogeological and hydrochemical processes controlling the occurrence and the transfer of fluorine to paleogroundwater.

The authors demonstrated that the fluorine is released from the Carbonate fluorapatite (CFA) deposit underlying the contaminated aquifer. The liberation of F is ensured by its exchange with hydroxyl and/or by the CFA dissolution through the passage of hot waters ascending from the deep “Continental Intercalaire (CI)" aquifer. The transfer of F-rich waters to the CT aquifer is occurred mainly through deep faults, aquitard windows and through leakage behind casing. This transfer is due to the potentiometric contrast between the two superimposed aquifers, which is enhanced by anthropogenic processes i.e. the overpumping of the CT groundwater and the CI wellbore integrity failure.

The authors concluded that integrated hydrogeological, hydrochemical and mineralogical analyzes can be employed to characterize F contamination of the
CT groundwater and to refine understanding of the associated hydrogeochemical processes. The most important findings and conclusions can be summarized as follows:

• The source of fluorine in the CT groundwater is the CFA mineral which makes part of the substratum the aquifer,
  
  
• The mechanism of the F release from the CFA is governed by several factors i.e. the chemistry of CT and CI groundwaters, the high temperature of ascending waters, the geothermal character of the CI waters and the chemistry of the CFA mineral,
  
  
• The liberation of F is controlled by its exchange with hydroxyl and/ or by the CFA dissolution through the passage of hot water ascending from the deep Continental Intercalaire (CI) aquifer,
  
  
• The transfer of F-rich waters to the overlain CT aquifer by the upward leakage of deep geothermal groundwaters is mainly through deep faults, aquitard windows and through leakage behind casing, and
  
  
• This transfer of F-rich waters is engendered by the potentiometric contrast between the two superimposed aquifers, which is enhanced by anthropogenic processes i.e. the overpumping of the CT groundwater and the CI wellbore integrity failure.