A multidisciplinary approach for groundwater quality assessments in complex hydrogeological settings

The groundwater quality assessments are challenging in complex hydrogeological settings and highly anthropized areas where geogenic and anthropogenic pollution may coexist. The objective of this study was to elucidate groundwater quality patterns beneath an inactive landfill in a coastal region of central Italy by integrating chemical-physical and geochemical parameters with isotopic and microbiological analyses. The groundwater under the landfill, predating the EU Landfill Directive (1999/31/EC), is under pump-and-treat remediation. The site features a complex stratigraphy of fluvio-palustrine sediments, eolian sands, and volcanic deposits of Pleistocene age, with a water table aquifer overlying Pliocene clays. Sampling was performed from 13 piezometers within the landfill and two surface water sites between March and July 2024. Laboratory analyses were conducted to measure the concentrations of major, minor, and trace cations and anions (with a specific focus on Fe, Mn, and As), dissolved organic carbon (DOC), and isotopes (δ18O, δ2H, δ13C, tritium and 87Sr/86Sr). Microbiological analysis were performed by flow cytometry (microbial cell abundance) and spectrofluorimetry (microbial respiration rates). Upgradient of the landfill, the aquifer exhibits oxidizing conditions, with low concentrations of metals and bicarbonates. Electrical conductivity (EC, μS/cm) is higher near the most upstream piezometers, where chloride concentrations exceed 800 mg/L. In the downgradient zone, high concentrations of Fe (4.2 mg/L) and Mn (1.1 mg/L) – occasionally exceeding the legal limits for groundwater – are associated with the strongly reducing conditions of the aquifer, driven by the presence of fluvio-palustrine deposits rich in peat, as identified through available borehole logs. The presence of As (1.3-15.4 μg/L) was likely due to interaction of groundwater with the volcanic deposits in the area. The leachate-tracer tritium showed generally lower activity (0.4-5.5 U.T.) than previous measurements, implying that historical contamination is currently declining. DOC concentration has a range from 0.5 to 7.4 mg/L, higher downgradient. Surface water sampled in two sections in the nearby river is highly oxygenated and rich in organic matter. Microbial cell abundance ranged from 10^4 – 10^5 cells/mL in most of groundwater samples, with higher values downgradient (10^6 cells/mL). Microbial respiration showed an inverse relationship with DOC exclusively in downgradient piezometers. These data indicated a highly specific hydrogeological and geolithological context, further complicated by anthropogenic activities throughout the region. As suggested by the Na/Cl ratio and the 87Sr/86Sr ratio, high chloride seems linked to mixing with fossil seawater, likely associated with a geological history marked by marine incursions following the end of the last glaciation (Würm). Elevated metal levels were connected to anoxic conditions promoted by the occurrence of fluvio-palustrine sediments, where heterotrophic microbial communities consume oxygen for organic matter degradation. Our findings highlight the critical need for tailored monitoring strategies that consider the unique hydrogeological and geolithological characteristics of the site, ensuring effective long-term management and protection of groundwater resources in similarly complex environmental settings.