Radon groundwater monitoring in the Gran Sasso aquifer

Full text: Since May 1996, groundwater monitoring (222Rn, pH, electrical conductivity, pressure of dissolved gases and temperature) at the Gran Sasso National Laboratory of the National Institute for Nuclear Physics has been performed by a multiparametric equipment. The automatic multiparametric equipment is made of stainless steel with pneumatic valves and it consists of a system for groundwater geochemical analysis (temperature, electrical conductivity, pH) and extraction of dissolved gases (pressure of dissolved gases) and of a detection system for alpha radioactivity due to the decay of radon and its daughters (218Po, 214Po). The groundwater analyzed flows in a tube introduced into the rock down to a depth of about three meters and a sampling period of twelve hours has been selected. The measurement site is located at the Gran Sasso National Laboratory near the main overthrust fault, and, particularly, in the well drained cretacic formations This monitoring activity is aimed to better define the geophysical and geochemical properties of the Gran Sasso aquifer, the most important of central Italy, and its radon variation source(s). The hydrological system is CaMg(CO)3)2-CO2-H2O with a high dynamics behaviour due to high permeability of cretaceous limestones. From the analysis of the raw data may be observed: a) the groundwater temperature is characterized by a mean deviation of 0.2 deg.. C; b) the electrical conductivity had a small range of variability of the order of 0.2 mS/m but two spike-like events occurred in October-November 1996 and August-September 1998, respectively; c) during the period from September 1996 to March 1997 pH increased of 0.4 pH units [about 5% of Average Background Value (ABV)], then from April 1997 to August 1997 a pH buffering effect occurred, and then decreased to its initial value in November 1997; d) a spike-like event of radon content in groundwater in November 1996 occurred. Furthermore, the mean value of the 222Rn from January 1997 to March 1997 decreased (about 3% ABV), then from April 1997 to September 1997 decreased (15% ABV), and then increased (September 15th, 1997) till November 1997 (about 16%-14% ABV). The occurrence probability of hydrogeochemical spike-like anomalies has been estimated with a time series approach and has been computed avoiding the influence of non tectonic factors, i.e. meteo-climatic and stratum parameters. Then, selecting an autoregressive linear model to describe the residual time series, the presumed anomalies on the white noise series obtained were identified when the modulus was greater than three times the estimated standard deviation. In order to correlate the groundwater chemical-physical spike-like anomalies with seismic activity, we estimated the ε deformation parameter in an area of 150 km from measurement site by the ING catalogue. We first selected seismic events and clusters assuming a lower limit for ε of 0.1·10-8. The time series analysis shown strong anomalies in pH and radon highly correlated with the known 1997-1998 Umbria-Marche seismic sequence occurred in central Apennines (Italy). This phenomenology may be justified by the occurrence of transient compression phase changing the carbon dioxide content in groundwater and then modifying the carbonate equilibria and the radon transport properties. Also, electrical conductivity spike-like anomalies correlate with low magnitude local seismic events and probably due to variations in groundwater dynamics induced by strain processes have been emphasized. (author)