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

摘要

Arsenic concentrations were analysed for 328 water samples collected in the Vicano–Cimino Volcanic District(VCVD), an areawhere severe contamination of groundwater has become a serious problemfollowing the recentapplication of the EU Directive on the maximum allowable concentration level for As in drinking waters. Inaddition, uranium and vanadium concentrations were also analysed in light of the enhanced interest on theirenvironmental toxicity. Waters were collected from springs and wells fed by cold and shallow volcanic–sedimentaryaquifers, which locally represent the main drinking water source. Thermal springs (≤63 °C) relatedto an active hydrothermal reservoir and waters associated with a CO2-rich gas phase of deep provenance werealso analysed. The collected data showed that the As concentrations in the shallow aquifers varied in a widerange (0.05–300 μg/L) and were primarily controlled by water–rock interaction processes. High As concentrations(up to 300 μg/L) were measured in springs and wells discharging from the volcanic products, and about66% exceeded the limit of 10 μg/L for drinkingwaters,whereaswaters circulatingwithin the sedimentary formationsdisplayed much lower values (0.05–13 μg/L; ~4% exceeding the threshold limit). Thermal waters showedthe highest As concentrations (up to 610 μg/L) as the result of the enhanced solubility of As-rich volcanic rocksduring water–rock interaction processes at high temperatures. Where the local structural setting favoured therise of fluids from the deep hydrothermal reservoir and their interaction with the shallow volcanic aquifer, relativelyhigher concentrations were found. Moreover, well overexploitation likely caused the lateral inflow ofAs-rich waters towards not contaminated areas.Uraniumand vanadiumconcentrations ofwaters circulating in the volcanic rocks ranged from0.01 to 85 μg/L and0.05 to 62 μg/L, respectively. Less than 2% of analysed samples exceeded theWorld Health Organization's provisionalguidelines for U (30 μg/L), while none of them was above the Italian limit value of V in drinking water(120 μg/L). Lower U (0.07–22 μg/L and 0.02–13 μg/L, respectively) and V concentrations (0.05–24 μg/L and0.18–17 μg/L, respectively) were measured in the water samples from the sedimentary aquifer and thermalwaters. Local lithology appeared as the main factor affecting the U and V contents in the shallow aquifers, dueto the high concentrations of these two elements in the volcanic formations when compared to the sedimentaryunits. In addition, high U concentrations were found in correspondence with U mineralization occurring withinthe VCVD, fromwhich U is released in solution mainly through supergene oxidative alteration. Redox conditionsseem to play amajor role in controlling the concentrations of U and V inwaters. Oxidizing conditions characterizingthe cold waters favour the formation of soluble U- and V-species, whereas thermal waters under anoxicconditions are dominated by relatively insoluble species. Geostatistical techniques were used to draw contourmaps by using variogram models and kriging estimation aimed to define the areas of potential health risk characterizedby As, U and V-rich waters, thus providing a useful tool for water management in a naturally contaminatedarea to local Authorities.