A multi-(hazard) risk approach for maximizing the efficiency of a hydrogeochemical monitoring network in the Strait of Messina (Italy) area.

The Strait of Messina (hereafter SoM), separating Sicily from continental Italy, is prone to different, high-grade, geological hazards, including energetic seismicity (as the 1908 M 7.1 Messina-Reggio Calabria earthquake) and diffuse mass movements, triggered by intense rainfalls, as the 1 October 2009 landslide, which destroyed several little villages immediately southward of Messina, causing 37 causalities. Climatic changes pose further treats, caused by the intensification of rainfalls, which modifies the runoff/infiltration ratio, and sea level rise, fostering the intrusion of the saline wedge into coastal groundwater bodies. The landscape of the SoM area is dominated by a complex interdigitation of natural and built environments, where the evolutive dynamics of a part could trigger profound perturbations in the other, and vice versa.

Efficient environmental monitoring networks represent an indispensable tool for developing correct multi-risk assessments, and related mitigation plans. Their implementation in the SoM area is one of the aims of the WP5 “NEMESI” of the Italian PNRR project MEET, leaded by INGV, financed in the framework of the European Next Generation EU initiative.

An important part of this network will consist of hydrogeochemical stations, acquiring near real time data from state-of-the-art sensors, able to produce reliable information over time.

The strategy for designing this network has been developed as follows.

First, only parameters potentially influenced by the different hazard-generating processes acting in the SoM area have been selected, excluding those for which no efficient, or too much expensive (for the project budget) sensors are presently available.

Second, geological, geomorphological, hydrogeochemical and seismic data have been analysed, also applying geostatistical tools, for extracting a first general list of potential sites (springs, wells, piezometers, drainage galleries, surface water bodies) candidate to host the network.

Third, all sites affected by unsurmountable logistic (absence of mobile network coverage for data transmission, impossibility of building structures hosting the instrumentation, etc.) and/or administrative (time to obtain permission of using the site not compatible with the project deadline) limitations have been excluded.

The remaining sites have been equipped with low cost dataloggers, integrated by periodic surveys, for verifying that, over a complete hydrological year at least, the recorded variations were compatible with the network aims: presence of transients emerging over a pure seasonal cycle.

After the preliminary monitoring, the final list was extracted, preferring sites where variations of one or more physic-chemical parameters should represent a proxy of one or more hazard-generating processes. Some examples are: i) changes in water electrical conductivity due to saline wedge intrusion, ii) variations of temperature and piezometric levels induced by permeability changes driven by seismic and aseismic deformations, iii) changes in oxygenation, turbidity and dissolved CO₂, which can be controlled by both eutrophication and mixing with deep volatiles, whose flux is driven by neotectonic activity.

The final aim is producing open access data of interest for the different stakeholders, including, but not limited to, the scientific community, the shellfish food industry, urban planners, water companies, public agencies, general contractors involved in civil infrastructures construction.