NEW HYDROCHEMICAL DATA ON THE DEEP-SEATED GROUNDWATER CIRCULATION IN THE PONTINA PLAIN (CENTRAL ITALY)

We take this chance to propose a model for the groundwater circulation within the karst bedrock underlying the Pontina Plain (Central Italy). We collected brand new data on the chemical composition and the Rn gas content of 13 water samples, among which 8 basal springs, 3 boreholes and 2 spring-sinkholes.

Analyzed data show that the groundwater circulating in the karst bedrock underlying the Pontina Plain can be divided into two different chemical groups. The first group is characterized by a higher value of pH (> 6.8), a SI, with respect to calcite, close to saturation and a lower F- (0.2-0.3 ppm), sulfates (20 - 40 ppm) and Rn-222 contents (roughly 14.000 Bq/m3). The second group shows instead lower values of pH (>6.5), a negative SI with respect to calcite (-0.2 - -0.6) and higher contents of F- (around 0.5 ppm), sulfates (70 - 100 ppm) and Rn-222 (higher than 44.000 Bq/m3).

The whole data set suggests the occurrence of a groundwater circulation that at places, coinciding with the major fault systems, receives a contribution from deep-seated gas fluxes (enriched in carbon dioxide, sulfates, fluoride and Radon). Thus two different pseudo-circuits could be identified in the groundwater circulation. One less mineralized, shallower and emerging closer to the karst ridge. The other much more mineralized, deeper and usually not emerging apart from deeply drilled boreholes or at places in some mineral springs, probably located in coincidence with the major master faults along the karst ridge margin.

The results of this study also support the spring-sinkhole development model proposed by the Authors in 2002 (Tuccimei et al, 2002) and it also fits with the groundwater circulation conceptual model, functional to the sinkhole genesis and development in areas of groundwater discharge, proposed by Salvati & Sasowsky in 2002. This new data set suggests as well that the spring-sinkholes are likely to be representative of the deep-seated groundwater circulation in the karst bedrock. Their genesis and development hence, can be explained only with endogenous processes involving both the groundwater circulation and the hydrochemistry of the deep-seated gas fluxes.