Preliminary geochemical characterization of the Mts. Simbruini karst aquifer (Central Italy)

Mts. Simbruini karst aquifer feeds important springs whose capture contributes to the water supply of Rome City. To improve the geochemical characterization of this aquifer, we analyzed 36 groundwater samples, 29 from springs and 7 from shallow wells, collected in 1996 and 2019. Atomic adsorption spectroscopy, titration, ionic chromatography and mass spectrometry were the used analytical methods. Groundwater is bicarbonate alkaline-earth type and HCO3- dominance confirms that the aquifer is hosted in carbonate rocks. Total alkalinity vs. cations plot indicates that CO2 driven weathering controls the water chemistry. The probability plots of HCO3-, cations and Ca2+ +Mg2+ indicate four groundwater populations with the less represented one (9 samples) characterized by the highest PCO2 values (> 0.3 atm). Most anomalous values of the dissolved PCO2 are from springs located near the center of the studied area. Four samples have negative values of delta C-13(CO2) (about -22 parts per thousand vs. PDB), indicating its organic origin, but two other samples have positive values (1.6 and 2.6 parts per thousand vs. PDB), similar to those observed in the CO2 of deep origin discharged at the close Colli Albani volcano. Therefore, geochemical evidence indicates that the Mts. Simbruini aquifer is locally affected by the input of deep originated CO2, likely rising up along fractures, interacting with a recharge of meteoric origin, as evidenced by its delta H-2 and delta O-18 isotopic signatures.