Use of minor and trace elements for identification of a coastal aquifer flowpath

Minor and trace element geochemistry, along with strontium isotopes in groundwater, was investigated in this study with two primary objectives: (I) to delineate groundwater flow paths within a coastal aquifer system and (II) to evaluate impact of former mining activities on groundwater quality. The study area is located in the Flumendosa Plain, on the southeastern coast of Sardinia. The plain area is primarily composed of Holocene alluvial deposits related to the recent depositional dynamic of the Flumendosa River. Pleistocene alluvial fan deposits, characterized by lower permeability, are widely exposed along the margin of the plain, particularly to the south, and in the piedmont areas of the main tributary valleys (e.g., Riu Pibilia, Flumini Uri). The surrounding hilly terrain is mainly composed of low-grade metamorphic rocks belonging to the Paleozoic basement of Sardinia. These rocks include sedimentary and volcano-sedimentary sequences that were metamorphosed to greenschist facies, dating from the Middle Cambrian to the lower Carboniferous (Carmignani et al., 2001). The alluvial aquifer is currently affected by overexploitation and significant seawater intrusion. Moreover, several former mining sites are located in the hills surrounding the plain, where mine workings and waste deposits interact with surface and groundwater flowing towards the plain (Cidu et al., 2008). A comprehensive set of major, minor, and trace elements was analysed in both groundwater and surface water samples and combined with strontium isotopes only in groundwater. Geochemical data were corrected for salinity contributions resulting from seawater mixing. The integration of these data with strontium isotope ratios enlightened the relative contributions of the Flumendosa and Flumini Uri to the aquifer. Furthermore, groundwater analyses revealed concentrations exceeding regulatory limits for Fe, Mn, Sb, and F, along with elevated levels of Cu, Co, Zn, Pb, Ni, and U. These findings exhibit a correlation with the substantial mineralisation present within the Palaeozoic basement. This correlation is further supported by the spatial distribution of these elevated concentrations, predominantly observed in groundwater samples located near bedrock exposures or watercourses receiving mine drainage.