Influence of hydrostratigraphy and structural setting on the arsenic occurrence in groundwater of the Cimino-Vico volcanic area

Dottorato di ricerca in Ecologia e gestione delle risorse biologicheLa presenza naturale di arsenico nelle acque sotterranee del distretto vulcanico Cimino-Vicano, nota da molti anni, è diventata un argomento di grande rilevanza ambientale in seguito alle nuove disposizioni legislative in materia di acque destinate al consumo umano (Direttiva comunitaria 98/83 CE). La massima concentrazione di arsenico ammessa per le acque potabili, infatti, è stata modificata da 50 μg/L a 10 μg/L, principalmente a causa delle conseguenze sulla salute umana riscontrate sulle popolazioni esposte ad acque potabili ricche in arsenico. Poiché numerosi punti d’acqua, pozzi e sorgenti, nell’area Cimino-Vicana superano questo nuovo limite di legge, l’arsenico è diventato la più urgente problematica degli organi governativi locali. Uno studio integrato, basato su idrostratigrafia, idrogeologia e chimica analitica, è stato applicato al fine di comprendere i processi geo-ambientali alla base di questo fenomeno. L’indagine scientifica è stata prevalentemente focalizzata sulle acque che confluiscono negli acquedotti dei vari comuni dell’area in esame. Tuttavia, anche alcune acque termali sono state prese in considerazione, poiché è noto che i processi idrotermali possono essere legati alla mobilizzazione geochimica dell’arsenico. Un totale di 65 campioni, suddivisi in acque fredde e termali, di cui 43 prelevati da pozzi e 22 prelevati da sorgenti, sono stati raccolti e sottoposti ad analisi chimico e chimico-fisiche (svolte sia in campo sia in laboratorio). Un range di arsenico pari a 1,6-195 μg/L è stato riscontrato nelle acque fredde, evidenziando che circa il 62% di esse ha un valore di concentrazione superiore al limite di legge di 10 μg/L. Per le acque termali, invece, è stato ricavato un range di concentrazione di arsenico pari a 176-371 μg/L. Integrando i risultati delle analisi chimiche con le informazioni sull’assetto idrogeologico e idrostrutturale, è stato concluso che l’arricchimento in arsenico delle acque sotterranee è principalmente legato alla risalita di fluidi idrotermali profondi e alla loro miscelazione con le acque d’infiltrazione superficiale. I risultati ottenuti rappresentano una risorsa fondamentale per la corretta gestione delle acque sotterranee e per la valutazione della vulnerabilità degli acquiferi dell’area Cimino-Vicana.The occurrence of natural arsenic in the groundwater of the Cimino-Vico volcanoes (Central Italy) is know from many years, and become recently a primary topic following the new maximum concentration limit of 10 μg/L (98/83 CE Directive). Arsenic has many potential health effects, and serious chronic long-term effects were observed in populations exposed to arsenic via the drinking waters. An integrated approach, based on hydrostratygraphy, hydrogeology and analytical chemistry, was used in order to characterise and to explain the setting of the studied area and the reasons behind the natural arsenic occurrence. The study was primarily focused on cold, drinking waters, that represent the main drinking water supply in the area. However, some thermal waters were also taking in account, since the hydrothermal processes, present in the Cimino-Vico, were important to understand the arsenic hydrogeochemistry. A total of 65 waters, 43 wells (42 cold waters and 1 geothermal water) and 22 springs (18 cold waters and 4 geothermal waters), were sampled. On field and laboratory chemical analyses were performed, revealing a wide range of arsenic concentration (1.6-195 μg/L for cold waters, and 176-371 μg/L for thermal waters). About the cold waters, the water used as drinking resource by the population resident in the studied area, about 62% of the result over the current maximum limit of 10 μg/L. By overlapping the chemical analyses with the hydrogeology background, it was understood that the arsenic occurrence is mainly related with the deep-rising fluids that characterize the hydrothermal system. In fact, the relatively high arsenic concentrations (20-100 μg/L) found in cold drinking waters are the result of mixing between deep groundwater and the recharging infiltration waters from the top of the volcanic aquifer. The waters sampled where the volcanic aquifer recharge is affected only by top infiltration, concentrations less than 10 μg/L were found. The obtained results represent a solid and key resource useful in the groundwater monitoring and in the evaluation of the aquifer vulnerability. Moreover, the results will be useful to the administrators of the drinking water resources of the area, in order to better manage the existing supplying points and in order to better select the future ones. Other than arsenic, also the occurrences of many other minor elements were studied. Among the other trace elements analysed, the uranium distribution called attention. The cold waters showed a richer amount in uranium (0.04-49 μg/L) than the thermal ones (0.06-0.09 μg/L). The highest values of uranium concentrations (>10μg/L) concern the south-western zone of volcanic aquifer, constituted mainly by the Vico products, that it is exactly located in the area where uranium mineralizations in the alkaline volcanic deposits were documented. Even if there is not a maximum concentration value for uranium in drinking waters, it is important to monitor the presence of this trace element. In fact, the drinking water represents usually the major intake source of uranium, since the intake from air and food is extremely low. However, until now, the exposure to soluble uranium from drinking waters has not been shown to increase the risk of developing serious health effects