Origine e distribuzione dell'arsenico in rocce e acque sotterranee del distretto vulcanico vico-cimino: un esempio di approccio integrato per l'individuazione di risorse idriche per uso potabile
Author(s)
Mazzuoli, Maurizio
Date Issued
June 17, 2016
Type
Doctoral Thesis
Abstract
The research activities carried out and reported in this thesis concern the occurrence of arsenic in groundwater of the Cimino-Vico volcanic area (central Italy). Groundwater resources are currently widely used for local drinking water supply and irrigation. The Cimino-Vico system presents a complex groundwater circulation, which includes a continuous basal aquifer, discontinuous perched aquifers, groundwater flows at high altitude, and local interactions with rising thermal fluids.
Data on arsenic concentrations in 250 water samples from springs and wells, 101 samples from rock outcrops, and 3 rock samples from a core drilled in the thermal area of Viterbo, were taken into account and combined together with existing information.
The approach applied included leaching tests, sequential extraction tests (to investigate the arsenic behavior relatively to the water-rock interaction) and a geostatistical modeling of data. This integrated approach allowed to identify and preliminarly quantify water resources with an arsenic concentration below 10 μg/L, the present limit for drinking water.
The leaching tests showed a low arsenic release (< 1%) for all lithologies analyzed. This result was subsequently confirmed by sequential extraction tests, performed on four samples of Vico Ignimbrite. In addition, the results showed that more than 90% of arsenic is retained until the last step. The behavior of the core drilling sample showed a different response, because of the significant percentage of arsenic which was found to be linked to Al-, Fe- and Mn oxides (56%). This different behavior is probably due to the interaction with the geothermal fluids: during the rise of geothermal fluids, As3+ is oxidized to As5+ as it interacts with atmospheric oxygen. The As5+ species is then sorbed depending on pH and availability of sorption sites of oxide minerals (such as amorphous Al-, Mn- and Fe oxides and hydroxides).
Results highlight that arsenic concentrations in groundwater are influenced by type of aquifer, groundwater flow path, arsenic content of the aquifer rocks, and interaction with fluids rising from depth. Waters circulating in the Vico volcanics have high concentrations of arsenic, both for basal and perched aquifers. A large percentage of the waters associated to these rocks have arsenic contents higher than 10 μg/L (82% for basal, 40% for perched). On the contrary, waters circulating in the Cimino volcanics present lower arsenic contents:
only 30% of basal and 10% of perched aquifers have waters with arsenic concentrations higher than 10 μg/L. Specifically, these good quality waters are found in the basal aquifer constituted by fissured rocks of the Cimino complex, and at high altitude in the lava domes of the same complex.
Based on the results obtained in this study, the following recommendations have been defined to facilitate a sustainable withdrawal of this naturally contaminated resources:
1. To avoid the realization of wells close to hydrothermal areas;
2. To limit the drawdown induced by the pumping from wells;
3. To prefer the exploitation of aquifers characterized by short flow path (i.e. with less water-rock interactions);
4. To prefer aquifers hosted in rocks characterized by low arsenic concentration.
The minimum yield of groundwater suitable for human consumption in the hydrogeological volcanic system is about 5,7 L/s per km2, about 32 × 106 m3/y (about 1.015 L/s) (Baiocchi et al., 2006). The instream flows were calculated using the method adopted by the Provincia Autonoma di Trento, Italy (DPR 22.12.1986), for the NE sector of the system (about 178 km2).
Maintaining at least the 50% of this resource to limit the impact on fluvial ecosystems, a drinking water amount for the needs of about 170.000 inhabitants (250 L/s) is still available, avoiding part of the expensive treatment plants scattered over the area.
Same considerations can be done for the Cimino Mountain (about 56 km2), where 300 L/s would be available, i.e. 150 L/s available for human consumption. In this case the withdrawal of groundwater may occur through horizontal drainage systems, taking into account the topographical and hydrogeological characteristics of the area. Such a solution would have the further advantages of producing limited cones of depression and of energy saving since it would work without the use of any electric pump.
Additional information
Dottorato di ricerca in Ecologia e gestione delle risorse biologiche
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