Find the culprit

In Forensic Geology, Environmental Forensics is devoted to ascertaining contaminants in the soil/subsoil, surface waters, and groundwaters. In such cases, forensic geologists usually accomplish activities concerning geological, hydrogeological, geochemical, and geophysical research to individuate the source of the contaminant substance and discover if this latter depends on anthropogenic or natural origins.

Preliminary chemical data on groundwaters from some areas of the Peloritani Mountains showed anomalous high contents of fluoride F^-, a halogen element in the atmosphere, sea, fresh waters, and minerals. Natural sources of fluoride in the groundwaters are volcanic gas, the sea, and minerals. Fluoride is an essential component in around 300 minerals, among which the most diffused are fluorite and fluorapatite. A significant chemical feature of the ions of fluoride is that they have the same charge as the hydroxyl group OH^- and present an ionic radius very similar to OH^-. These chemical characteristics make it possible that F^- may readily substitute the hydroxyl group in minerals such as micas, X₂Y₄-₆Z₈O₂₀(OH,F)₄.

Most of the collected groundwaters in the present research were hosted in aquifers formed by Variscan high-grade metamorphic rocks provided with fracture permeability and in aquifers made up of Tertiary to Quaternary siliciclastic deposits with porosity permeability. These aquifers have a silicate composition and are rich in biotite. Among micas, biotite is the most diffused mafic mineral in the high-medium grade metamorphic rocks (augen gneiss, gneiss, mica schists) of the Peloritani Mountains. This mineral is also widespread in weathered monomineralic lithoclasts of siliciclastic deposits (Miocene, middle to upper Pleistocene, Holocene to Actual clastic deposits) deriving from dismantling the chain's metamorphic rocks.

Previous studies on biotite from acid plutonic rocks of India demonstrated that fluoride contents might reach a concentration up to 7 wt%. Biotite mica may be likely responsible for the natural fluoride contamination of some of the studied groundwaters, in some cases also commercially exploited in the past.

WHO suggests that the F^- concentration in the drinking waters must range between 700 and 1500 µg/l depending on the different climatic zones. Concentrations over 1500 µg/l in Italian drinking waters are prohibited and dangerous for public health.

The present research carried out in the Peloritani Mountains is devoted to: i) defining the actual geographical extent of the identified F^- anomaly; ii) studying the water/rock interactions to ascertain if the leaching of F^- from biotite mica, present in the high-grade metamorphic rocks and siliciclastic deposits, may be the natural phenomenon responsible for the ascertained fluoride contamination; iii) search for possible other causes of F^- contamination as the salt wedge intrusion in the groundwaters of the coastal areas.