The mechanisms through which Etna leaves its mark on the aquifer that flows inside it, the main water resource for eastern Sicily, have been revealed. To discover them, a team of INGV researchers.
The results were published in Chemical Geology and in the Journal of Volcanology and Geothermal Research
The volcanic edifice of Mount Etna hosts an impressive aquifer which represents the most important water resource for eastern Sicily. The understanding of the processes that determine the abundance of elements of volcanic origin dissolved in it is the object of the three researches of the National Institute of Geophysics and Volcanology (INGV): Tracing the circulation of groundwater in volcanic systems using the 87Sr/86Sr ratio : Application to Mt. Etna; Volcanic plume fingerprint in the groundwater of a persistently degassing basaltic volcano: Mt. Etna; Dissolved inert gases (He, Ne and N2) as markers of groundwater flow and degassing areas at Mt Etna volcano (Italy). The results of the work have been published in Chemical Geology and the Journal of Volcanology and Geothermal Research.
“The aquifer of Etna, fed by the rains and by the melting of the snow cover”, says Marcello Liotta, researcher at INGV - Palermo Section, “has been the object of numerous scientific researches in recent years. The characteristics of the volcanic edifice, the peculiar climatic conditions and the presence of a persistent volcanic plume make the Etna stratum a treasure trove of secrets, many of which are yet to be discovered”.
Geochemistry, a branch of Earth sciences that studies the distribution and behavior of the elements that make up the globe, represents a fundamental investigative tool for exploring underground water paths and the migration of elements of volcanic origin through the aquifer. In the last year, some research conducted by the INGV scientific community has revealed how the volcano leaves its mark on the groundwater.
“The summit area of the volcano is characterized by acid rain due to the persistent presence of the volcanic plume which releases large quantities of acid gases and aerosols into the atmosphere. Despite the clear evidence of acid precipitation, demonstrated by previous research, the contribution to the groundwater of the elements from the plume had never been identified until now", explains Marcello Liotta.
Acid rains are neutralized by interaction with the basaltic rocks that make up the volcanic edifice and infiltrate the groundwater.
“From the study of the chemical composition of the water, the contribution due to the plume and that due to the water-rock interaction processes was identified for each element; the latter favored by the high content of carbon dioxide (CO2) of magmatic origin dissolved in the groundwater. The isotopic composition of strontium (Sr) has also made it possible to quantify the contribution to the groundwater of very small fractions of hypersaline solutions that rise through the sedimentary basement, together with large quantities of gas".
Volcanic gases, in fact, are not only emitted from the summit craters, but some of them are released from magma several kilometers deep.
"From the analysis of the chemical and isotopic composition of the inert gases dissolved in the aquifer", adds Antonio Paonita, senior researcher at the INGV-Palermo Section, "it emerges that some gases of magmatic origin reach the aquifer through tectonic discontinuities, dissolve in it and they travel long distances along the flanks of Etna. From the samples taken from wells and sources of the aquifer it was possible to establish the areas in which rising gas intercepts the aquifers and the paths from upstream to the sea followed by the waters laden with magmatic gas, as well as a series of hydrological parameters of the aquifers”.
Hence the idea that the volcano imprints its imprint on the aquifer through at least three different mechanisms: the plume influences the chemical composition of the meteoric recharge; basaltic rocks release large quantities of alkali and alkaline earth metals; finally the inert gases dissolve in the groundwater, carrying valuable information with them.
"The development of ever more refined interpretative models and the fine-tuning of advanced analytical techniques represent an indispensable tool for understanding the processes that take place in the volcanic system, their impact on the surrounding environment and the possible repercussions for society", concludes Marcello Liotta.
Extended
The chemical composition of the groundwater, dissolved inert gases (He, Ne and N2) and He isotope ratio (3He/4He) as well as 87Sr/86Sr ratio was investigated at Mt. Etna in order to determine the extent to which the volcano affects the chemical composition of circulating waters. The content of dissolved elements derives from the bulk deposition (wet and dry deposition) at the recharge areas as well as from the weathering of volcanic rocks during the infiltration and transport of groundwater. In its early phase, the chemical weathering of volcanic rocks and ashes is promoted by the acid rain that characterizes the area and subsequently by the huge amount of deep magmatic carbon dioxide (CO2) coming up through the volcanic edifice and dissolving in the water. The high content of chlorine is mainly derived from interactions between the plume and rainwater, while the total alkalinity can be completely ascribed to the dissociation of carbonic acid (H2CO3) after the hydration of CO2. The 87Sr/86Sr ratio indicates that the Sr-isotope signature of groundwater is determined by the volcanics hosting the aquifer. The volcanic isotopic signature is modified by very small amounts of brines (<1%), characterized by a high concentration of Sr and a 87Sr/86Sr ratio typical of sedimentary environments, but only at sites where the groundwater circulates almost in contact with the sedimentary basement.
Fractions of the volatiles ascending from magma chambers meet groundwaters flowing away from the volcano summit and are carried to great distance as dissolved gases. The complex interactions between ascending magmatic volatiles, tectonic structures, heterogeneities in rock permeability and flow lines of aquifers deeply affect the dispersion of the dissolved species. Studying the spatial distribution of such species can therefore provide valuable information on the circulation of fluids inside volcanic buildings. Our study focused on the composition of dissolved inert gases (He, Ne and N2) and He isotope ratio (3He/4He) because the concentrations of these species differ markedly between magmatic and shallow (crustal and atmospheric) sources, and they do not interact chemically with rocks. We identified groundwaters that flow through anomalously degassing areas associated with clearly evident or known tectonic structures. These waters show a typically magmatic He isotope composition (high 3He/4He ratios) and high proportions of dissolved magmatic gases (He and CO2) compared to the atmospheric ones (Ne and N2). Downstream of the degassing structures, along the hydrological outflows, we found groundwaters that are progressively enriched in atmospheric-derived gases (Ne and N2) and exhibited lower 3He/4He ratios. On this basis, we set up a model of unidimensional dispersion-advection of inert volatile solutes, coupled with a two-layer model for the dynamic exchange of volatiles through the aquifer–atmosphere interface. The model is able to quantitatively explain the progressive dilution of the magmatic signal over distances of several kilometers from the source location of the anomaly towards the final part of the flow lines at the coast.
Typical hydrogeological parameters such as the flow velocity, rock permeability and rate of air–groundwater interaction can be constrained, and underground pathways of waters can be identified. Waters that are anomalously rich in magmatic tracers with respect to their peripheral position along the flowlines reflect arrival of deep gases from below, and they therefore offer a powerful tool for revealing hidden tectonic structures.
Fig.1: Measurement of chemical-physical parameters at a sampling site
Fig.2: Panoramic photo of Etna
Links to the three publications:
http://www.sciencedirect.com/science/article/pii/S0377027317300112
http://www.sciencedirect.com/science/article/pii/S0009254116301632
http://www.sciencedirect.com/science/article/pii/S0009254116304879