Numerical modeling and strontium isotopic signal to assess the arsenic distribution in a low-enthalpy hydrothermal system: the case study of Viterbo geothermal area (Italy)

Several studies on the groundwater geochemistry of the hydrothermal area west of Viterbo, highlight the presence of arsenic and other trace elements. The groundwater of Viterbo area was used as source of drinking water by about 150,000 local inhabitants, until 2010, when it was prohibited the use by EU due to the high level of arsenic. The hydrogeological setting highlights the presence of a shallow volcanic aquifer (composed by alkaline-potassic volcanics), characterized by fresh waters, limited at its base by the semiconfining marly-calcareous-arenaceous complex and low-permeability clays. To the west of Viterbo, vertical upflows of hot waters (with a temperature between 50 and 64°C), are due to the locally uplifted of evaporitic reservoir, the reduced thickness of the semiconfining layer and the high local geothermal gradient. Current study is focused on news geochemical approaches to defining the conditions which control arsenic mobility in groundwater in the low- enthalpy thermal area of Viterbo, related to the interaction between the volcanic aquifer and the geothermal reservoir. In addition to determinate chemical components and chemical-physical properties (T, pH, electrical conductivity) the study provided the isotopic values of 87Sr/86Sr of Viterbo geothermal area. Geochemical modelling is conducted using Phreeqc. The program monitors the significant species and calculates equilibrium concentrations and the pCO2 at desired temperatures. Investigations were undertaken in the area exhibiting thermal manifestations and in the immediate surroundings. On the basis of major ions and temperature, it is possible to subdivide the waters sampled into three main groups: the thermal waters with a sulphate -alkaline-earth facies, the fresh waters with a biocarbonate-alkaline facies, and a group of mixing waters with a undefined facies. The values of strontium isotopic ratio 87Sr/86Sr marks out the different circuits of groundwater. Values lower than 0.70800 are typical of the evaporite rocks, whilst values 0.70950-0.71100 are characteristic of the Cimino-Vicano volcanic rocks. The intermediate values of 87Sr/86Sr has reveled the interaction between the thermal waters and the volcanics. Geochemical modeling have been used to evaluate the changes in thermodynamic conditions due to the interaction between the two aquifers. The results of simulations show that the mixing between thermal waters and the volcanic ones increases the arsenic concentration. Indeed the interaction with hot water and the volcanics dissolves the arsenic which is partially content in the volcanic glasses. However the correlation between arsenic and values of saturation indices of goethite and pCO2, calculated with Phreeeqc, suggests that the sorption of arsenic to goethite play an important role in the control of arsenic concentration in the fresh water. High values of arsenic in shallow aquifer are correlated with the long water-rock interaction. In thermal waters, characterized by high pCO2 values, the high concentration of arsenic is due to the upflowing through the volcanic aquifer which leaches the arsenic contents in volcanic glass, and also dissolved arsenic bound to iron hydroxides.