Applying groundwater dating and isotopic analysis to a saline-intruded coastal aquifer

This study investigates the complex hydrogeological dynamics of the Muravera coastal plain in southeastern Sardinia, Italy, a region grappling with substantial salinity intrusion. By integrating groundwater dating techniques with a comprehensive multi-isotope analysis, and further correlating these findings with traditional hydrogeological monitoring, the research successfully delineates distinct recharge locations and flow paths within the impacted aquifer. A robust suite of isotopic tracers was employed, encompassing tritium (³H), helium (He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), and noble gases, augmented by strontium and boron isotope analyses. Age tracer and noble gas concentrations were computed for the freshwater component of each sample by estimating the corresponding concentrations in the saltwater component. Noble gas-derived recharge parameters were then calculated using the PANGA code (Jung & Aeschbach, 2018), and the mean age of the freshwater fraction was determined using the TracerLPM software assuming a bimodal mixing model. The study delineated four distinct groundwater recharge sources within the aquifer system (Flumendosa River, Flumini Uri River, modern seawater and premodern seawater), and it provides a more comprehensive understanding of the complex hydrogeological dynamics of the aquifer. Critically, it identified a previously unmonitored and uncharacterized surface water body (Flumini Uri River) as a significant recharge source. Furthermore, the premodern seawater component exhibits a high terrigenic helium-4 concentration exceeding 4 x 10⁻⁷ ccSTP/g, typical for groundwater >1000 years old. The tracer data thus suggests widespread mixing of ancient seawater and modern freshwater across much the aquifer. This information can be used to improve water management strategies and ensure the sustainability of the resource. This study demonstrates the considerable value of our integrated isotopic and hydrogeologic approach for characterizing and effectively managing vulnerable groundwater systems.