Abstract
Expanding knowledge about the origin and mixing of deep fluids and the water–rock–gas interactions in aquifer systems can represent an improvement in the comprehension of crustal deformation processes. An analysis of the deep and meteoric fluid contributions to a regional groundwater circulation model in an active seismic area has been carried out. We performed two hydrogeochemical screenings of 15 springs in the San Vittorino Plain (central Italy). Furthermore, we updated the San Vittorino Plain structural setting with a new geological map and cross-sections, highlighting how and where the aquifers are intersected by faults. The application of Na-Li geothermometers, coupled with trace element and gas analyses, agrees in attributing the highest temperatures (>150 °C), the greatest enrichments in Li (124.3 ppb) and Cs (>5 ppb), and traces of mantle-derived He (1–2%) to springs located in correspondence with high-angle faults (i.e., S5, S11, S13, and S15). This evidence points out the role of faults acting as vehicles for deep fluids into regional carbonate aquifers. These results highlight the criteria for identifying the most suitable sites for monitoring variations in groundwater geochemistry due to the uprising of deep fluids modulated by fault activity to be further correlated with crustal deformation and possibly with seismicity.
Highlights
Groundwater hydrogeochemistry is closely related to the nature of the aquifer and to the length and depth of the groundwater flow paths, and it strongly depends on the residence time [1,2]
Previous investigations based on the hydrogeochemistry of the fluids [43,65] highlighted that the groundwater hydrogeochemistry of the San Vittorino Plain is the result of the addition of deep fluids to the shallow groundwater circulating in a regional carbonate fractured aquifer
The San Vittorino Plain, an intramontane plain where a large amount of groundwater resources converges from large mountain fractured aquifers to several springs affected by different degrees of mineralization, represents one of the more interesting areas for evaluating groundwater mixing of direct rainfall recharge components with deep fluids uprising along high-angle faults reaching a depth of at least 5 km b.s.l
Summary
Groundwater hydrogeochemistry is closely related to the nature of the aquifer and to the length and depth of the groundwater flow paths, and it strongly depends on the residence time [1,2]. Deep fluids play an active role in the seismic cycle and fault activation mechanisms, which are increasingly highlighted in geoscience studies [10,11,12,13,14,15,16,17,18] For this reason, in recent decades, works aimed at identifying signals that can anticipate strong seismic events in field and laboratory activities have been intensified [3,19,20,21,22,23,24,25]. These studies have identified variations in the concentration of gases such as He, Rn, CO2 , H, and CH4 [26,27], groundwater chemical content [3,21,22], and the physical properties of rocks [28,29]
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