Fluoride in Campi Flegrei volcanic aquifer, south Italy: A comparison between water and rock composition

Campi Flegrei aquifer is situated in an area with a long history of volcanic activity. Although groundwater geochemistry has been studied since the late 20th century to characterize volcanic activity in the study area, limited information is available about fluoride geochemistry. To fill this gap, 44 groundwater samples were collected to determine concentrations of Ca2+, Mg2+, Na+, K+, Cl-, SO42- and F- using ion chromatography together with alkalinity, electric conductivity (EC), temperature and pH via portable devices. The results indicate a positively skewed distribution of F- concentration, ranging from 0.02 to 0.72 meq/l (0.26 meq/l on average). The alkaline waters generally contain more fluoride than acidic waters, being consistent with fluoride desorption tendency at pH>7. However, previous studies showed significant heterogeneity of the groundwater body due to site-specific geochemical processes. In the present investigation, the empirical cumulative distribution function of F-/Cl- molar ratio is used to determine a threshold value and categorize the groundwater samples in: Gr1 (F-/Cl- < 0.065), mostly characterized by chlorine-rich groundwaters with the highest values of Ca2+, Mg2+, Na+, K+, Cl-, SO42-, alkalinity, EC and temperature; and Gr2 (F-/Cl- > 0.065), mostly composed of bicarbonate-rich groundwaters with the highest F- and pH values. The F/Clratios of whole-rock and water follow the decreasing order of: ignimbrites (pre-40 ka) > Campanian Ignimbrite (~39.3 ka) > tephra (post-15 ka) > Gr2 > Quarto Plain groundwater > rainwater > Gr1 > seawater. Composition of rock, Quarto Plain groundwater, rainwater and seawater was obtained from literature. Quarto Plain groundwater, which is a shallow meteoric component influenced by water‒rock interaction, reveals F-/Cl- higher than rainwater, indicating that rock leaching increases F-/Cl-. The interaction is more significant in a subset of Gr2, mostly far from the area of intense hydrothermal activity, leading to elevated F-/Cl- in the other subset of Gr2 downstream. F-/Cl- of Gr2 is almost similar to post-15 ka rocks. On the other hand, low F-/Cl- in Gr1 might explain that different processes (e.g., water‒rock interaction, precipitation of F-bearing phases, F- volatility and the rising geothermal fluids) affect F- geochemistry. Hence, empirical cumulative distribution function of F-/Cl- molar ratio can be an effective tool in distinguishing the groundwater samples that their F- load directly relates to lithology and does not undergo significant changes. Careful interpretation of sulfate-rich groundwater geochemistry is required because low Cl- content, high temperature and, in some cases, low pH probably influence their classification. It is worth mentioning that the highest F- concentration was detected in Stufe di Nerone well (a member of Gr1) which might show geothermal contribution to F- content of groundwater.