Groundwater circulation and earthquake-related changes in hydrogeological karst environments: a case study of the Sibillini Mountains (central Italy) involving artificial tracers

Abstract

Using artificial tracer tests, this study confirms the presence of a single basal aquifer feeding the springs in the wide and complex hydrogeological boundary of the Sibillini Mountains of central Italy. The tracer was introduced into the sinkhole of the Castelluccio di Norcia plain. The tracer test results, observed at the studied springs, highlight the changes induced by the 2016/2017 earthquake in the water circulation of the aquifer system. In particular, the seismic events increased the hydraulic conductivity of the basal aquifer, with a consequent increase in the springs’ flow rates at the western hydrogeological boundary and a decrease in the flow rates at the eastern Adriatic hydrogeological boundary. This phenomenon is in accordance with the hydro-structural framework of the area. The study also investigated the relation between groundwater circulation and tracer behaviour in the springs during the pre- and post-earthquake periods. The tracer test results led to the formulation of hypotheses about water circulation of the area. The trend of the tracer breakthrough curves demonstrates that the upper portion of the basal aquifer is characterised by fast water circulation similar to that in the epiphreatic area of karst aquifers, while a slow circulation due to fissures with interconnected drains occurs in the deepest portion of the aquifers. The obtained results highlight how tectonics and karst can affect the hydrogeological setting of the Apennine carbonate chain; in particular, the seismicity of central Italy may alter groundwater circulation for a long period of time when great magnitude earthquakes occur.

Comparison between Periodic Tracer Tests and Time-Series Analysis to Assess Mid- and Long-Term Recharge Model Changes Due to Multiple Strong Seismic Events in Carbonate Aquifers

“…However, the fault system was responsible for local groundwater exchanges between different sectors of the aquifer (transversal direction) [28,29]. The Maiolica aquifer, when separated by the Basal one and the Scaglia Calcarea aquifer, is responsible for local groundwater recharge through the minor springs of the area [39].…”

“…After the seismic events, the transversal exchanges between the different hydro-structures seem to be more developed towards the western sector of the domain, while the Apennine flow direction (SSE-NNW) still characterise the Basal aquifer. According to the literature [29,39], the hydrogeological boundaries of the Sibillini Massif are represented in the eastern and southern-east portion of the domain by the Sibillini overthrust, while the western boundary is represented by the Nottoria-Preci normal fault system. As concerns the northern hydrogeological boundary, its features have not been clearly defined in literature yet [29].…”

“…According to the literature [29,39], the hydrogeological boundaries of the Sibillini Massif are represented in the eastern and southern-east portion of the domain by the Sibillini overthrust, while the western boundary is represented by the Nottoria-Preci normal fault system. As concerns the northern hydrogeological boundary, its features have not been clearly defined in literature yet [29]. However, according to the structural framework characterised by a general immersion of the fold axes towards the NW [35], the Nera River can be identified as the main area of groundwater preferential flow.…”

“…In a barycentric position with respect to the study area, there is the Pian Grande plain, which represents a large plateau located almost 1300 m a.s.l. This plain is a tectono-karstic basin filled by Holocenic lacustrine deposits in which several sinkholes are located [29]; the main one, named the Mèrgani sinkhole, has been instrumented by a hydrometric station and starting from 2017 a continuously daily discharge inflowing in the sinkhole has been recorded.…”

“…Hydrogeological section. All the cross-sections are modified from the literature [29,38]. Table 1); 13.…”