Spatial distribution of arsenic, uranium and vanadium in the volcanic-sedimentary aquifers of the Vicano–Cimino Volcanic District (Central Italy)

Abstract

Arsenic concentrations were analysed for 328 water samples collected in the Vicano–Cimino Volcanic District (VCVD), an area where severe contamination of groundwater has become a serious problem following the recent application of the EU Directive on the maximum allowable concentration level for As in drinking waters. In addition, uranium and vanadium concentrations were also analysed in light of the enhanced interest on their environmental toxicity. Waters were collected from springs and wells fed by cold and shallow volcanic–sedimentary aquifers, which locally represent the main drinking water source. Thermal springs (≤63°C) related to an active hydrothermal reservoir and waters associated with a CO2-rich gas phase of deep provenance were also analysed. The collected data showed that the As concentrations in the shallow aquifers varied in a wide range (0.05–300μg/L) and were primarily controlled by water–rock interaction processes. High As concentrations (up to 300μg/L) were measured in springs and wells discharging from the volcanic products, and about 66% exceeded the limit of 10μg/L for drinking waters, whereas waters circulating within the sedimentary formations displayed much lower values (0.05–13μg/L; ~4% exceeding the threshold limit). Thermal waters showed the highest As concentrations (up to 610μg/L) as the result of the enhanced solubility of As-rich volcanic rocks during water–rock interaction processes at high temperatures. Where the local structural setting favoured the rise of fluids from the deep hydrothermal reservoir and their interaction with the shallow volcanic aquifer, relatively higher concentrations were found. Moreover, well overexploitation likely caused the lateral inflow of As-rich waters towards not contaminated areas.Uranium and vanadium concentrations of waters circulating in the volcanic rocks ranged from 0.01 to 85μg/L and 0.05 to 62μg/L, respectively. Less than 2% of analysed samples exceeded the World Health Organization's provisional guidelines for U (30μg/L), while none of them was above the Italian limit value of V in drinking water (120μg/L). Lower U (0.07–22μg/L and 0.02–13μg/L, respectively) and V concentrations (0.05–24μg/L and 0.18–17μg/L, respectively) were measured in the water samples from the sedimentary aquifer and thermal waters. Local lithology appeared as the main factor affecting the U and V contents in the shallow aquifers, due to the high concentrations of these two elements in the volcanic formations when compared to the sedimentary units. In addition, high U concentrations were found in correspondence with U mineralization occurring within the VCVD, from which U is released in solution mainly through supergene oxidative alteration. Redox conditions seem to play a major role in controlling the concentrations of U and V in waters. Oxidizing conditions characterizing the cold waters favour the formation of soluble U- and V-species, whereas thermal waters under anoxic conditions are dominated by relatively insoluble species. Geostatistical techniques were used to draw contour maps by using variogram models and kriging estimation aimed to define the areas of potential health risk characterized by As, U and V-rich waters, thus providing a useful tool for water management in a naturally contaminated area to local Authorities.