Description
The spring is made up of a series of small emergencies, which can be found inside the mine's shaft tunnel at an altitude of 1580 m above sea level, whose waters are conveyed into an intake structure located inside the masonry structure that protects access to the mine.
The interest shown by the Geological Survey in these waters is aimed at understanding the local hydrogeological model, researching any existing relationships between flow rate, temperature, electrical conductivity, pH and rainfall, and defining the supply basin, which does not seem to have the typical connotations of a normally understood catchment basin.
Monitoring system
Until the end of the 1980s, data was recorded and diagrammed manually at irregular intervals.
Starting in 1988, a semi-automatic system for recording chemical-physical variables was installed in order to have a continuous mass of data that could be subjected to statistical investigations capable of shedding light on the laws and relationships governing the relationships between the variables, where they exist. After an initial period of experimentation, a data acquisition system capable of reading and storing on continuous paper, four parameters was installed:
- flow rate
- electrical conductivity
- water temperature
- pH
Since 1993, the semi-automatic acquisition system has been replaced by a fully automatic one that downloads the stored data directly to a portable PC.
Results
Historical data collected in the period from the end of 1988 to December 2001 were graphed of the chemical-physical variables by analysing them in pairs: pH - electrical conductivity and flow-temperature.
An initial visual examination of the flow-temperature graph reveals a direct relationship (an increase in temperature coincides with an increase in flow rate). In contrast, the time series for the pH-conductivity pair shows an inversely proportional relationship: a maximum in electrical conductivity corresponds to a minimum in pH. Looking at the last five years of measurements, an inverse relationship can also be found between conductivity and flow rate. In contrast, no correlation appears to exist between pH and temperature and between pH and flow rate.
However, the statements obtained from this initial examination of an intuitive nature were not borne out by the results obtained from subsequent investigations of a purely analytical nature.
The partial correlation coefficient calculated for each variable in relation to the others neither justifies nor supports the visual intuitions derived from the time series graphs. Even the attempt to introduce a seasonality or other cyclical periodicity factor did not lead to meaningful analytical conclusions.
The very comparison between flow rate and rainfall, for example, did not reveal a clear temporal law capable of highlighting the close relationship that usually exists between rainfall and flow rate, even taking into account the delay times with which the water seeping into the ground takes to reach the source.
This takes the form of a gravitational water drainage structure whose volume is not easy to define. Lacking one of the fundamental parameters for the calculation of the hydrogeological balance (i.e. the area of the feeding basin), the volume of rock that could constitute the natural reservoir of the spring has been estimated at 6,000,000 cubic metres, starting from the volume of water stored, assuming a probable porosity of the rock, and considering the spring depletion curves that represent the flow rate trend in the period of the spring's discharge between a maximum and a minimum.
In this regard, some hydrogeological considerations must be made regarding the type of natural supply reservoir, which is composed of phylladic and paragneissic rocks with very low permeability and exclusively by fracture, and the conditions of the slope strip (NE-SW) immediately above the Acqua Forte tunnel elevation, which appears to be affected by mining voids, remains of ancient cultivation, and boulders and specimens on surface occurrences.
The resulting interpolation curve is a logarithmic curve that makes it possible to estimate, given an initial maximum flow rate, the volume of water existing upstream of the spring prior to the process of progressive emptying of the reservoir and the reserves at the time when recharging begins. The analysis carried out on most of the useful data available between '78 and December 2001 showed that the highest values of accumulation (> 6000 m3 of water) occurred in the 1980s, while in recent decades the average has been around 3-4000 m3.
This differentiation can probably be attributed to the phenomenon of snowfall, which in recent years is manifested by heavy snowfall accompanied, however, by a short stay on the ground and a rapid dissolution, unlike in the early decades when there was a long stay on the ground.
The investigations that have been carried out do not reveal any significant relationship that would make it possible to identify the delay between precipitation and reservoir recharge.
The reason for this could be attributed in part to the effect of snowfall, but above all to the hypothesis that the waters, once they reach the mineralised bank with a percolation velocity comparable to that of the other oligomineral springs in the area (for which there is a clear relationship between flow rate and precipitation), undergo a strong slowdown. In fact, they exploit the preferential pathways constituted by the alteration zones produced by the bacteria that attack the mineralised mass, pass through it with extreme slowness, leaching it and loading it with the salts that make it so unique in the Trentino and world panorama of hydromineral resources.
This prolonged sojourn, quantitatively variable due to various factors essentially linked to the internal hydraulic load, constitutes the non-constant displacement element that makes the flow rate apparently disconnected from the amount and duration of rainfall.
Conclusions
The Vetriolo mineral water represents a unicum that is of interest to scholars all over the world, especially in the field of alternative medicine. The data collected suggest the need to conserve the existing mining environment within which an acid microclimate has been created that favours the neoformation of minerals of great scientific interest.
Continuous measurements of the chemical-physical characteristics of the water can be continued with a view to acquiring further elements of analysis, but they do not constitute a priority aspect for understanding hydrogeological phenomena. It must also be emphasised that such an economic resource, which appears to be undermined by external factors, must be continuously monitored to ensure the temporal continuity of its supply and the constancy of its organoleptic and chemical-physical properties.
Realised by:
Data collection: L. Veronese, R. Gilli, G. Gottardi
data processing, interpretation and publication: L. Veronese, S. Piccioni