Groundwater mixing assessment in the Pontina Plain (Central Italy): preliminary results of the CARG survey performed for the Terracina geological map

Assessing water resource quality, and ground-and-surface water interactions related to land use and climate changes is essential to groundwater assessment. In Central Italy, springs fed by carbonate aquifers are commonly the major contributors of drinking and irrigation water supply. In the Pontina Plain, they encompass mixing processes that we aim to understand. In the frame of the new geological map survey of Terracina, the groundwater flowpaths will be retrieved from a hydrogeochemical monitoring of about 20 springs. In particular, we aim at defining the hydrogeochemical processes that control the groundwater chemistry and to determine the suitability of springs and groundwater for irrigation and drinking purposes based on the water quality indexes. Additionally, in order to investigate possible temporal variations in hydrogeochemical composition, historical data were collected too as they may be related also with ongoing tectonics. Chemical-physical parameters (pH, Electrical Conductivity, and Temperature) were measured on field, and groundwater samples to analyze major, trace and isotope content (2H and 18O of H2O) were sampled. The Piper diagram revealed that groundwater shows a hydrogeochemical evolution from the Calcium-Bicarbonate facies to the Sodium-Chloride water type. Geochemical modeling and saturation index computation of springs and wells confirm an interaction with carbonate rocks. Most of the springs and well water samples are saturated with respect to calcite and dolomite, however the existence of mixing process with deep fluids and/or sea water cannot be excluded in some cases as supported by saturation indices of halite and anhydrite. The analysis of time series allowed the identification of trends in groundwater ions concentrations. Near the sea, these variations can be attributable to the increase in the salinization process in the coastal aquifer, while far from that it could be related to the upwell of deep fluids across major faults. Most water samples are suitable for irrigation and drinking purposes, except for the samples influenced by seawater and prolonged water-rock interaction. This study highlights the need of a detailed hydrogeological characterization for strategic groundwater management policy and planning, to avoid the decline of groundwater quality due to a decrease in recharge rates induced by climate changes.