MULTIDISCIPLINARY APPROACH TO ASSESS THE ALLUVIAL AQUIFER HYDRODYNAMIC ROLE IN CONNATE MINERAL GROUNDWATER UPWELLING (VOMANO VALLEY, CENTRAL ITALY)

The so-called “natural mineral waters” or “thermal waters” are generally highly saline groundwater, related to peculiar aquifer geochemistry and underground hydrodynamic. These water resources are worldwide known for their therapeutic properties, that pertain to several human health aspects. However, because of the particular hydrogeochemistry (i.e. high major ions and trace elements concentrations), these natural mineral groundwaters could pose concerns to their management, especially when they mix with fresh water in shallow aquifers. In these cases, considering the current environmental legislation, these interactions between fresh and mineral waters can represent natural contamination events. As a matter of fact, the underground hydrodynamic affects the mixing extent; thus, getting a deeper insight into the aquifer recharge and groundwater flow effect on the mixing can provide vital information for water resource management issues. In light of this need, the main aim of this research is to define the conceptual model of the actual hydrodynamic role in connate mineral groundwater upwelling and mixing with fresh water in shallow alluvial aquifers, by means of a multidisciplinary approach. The data used to this purpose consists of geological and geophysical surveys (i.e. boreholes, seismic refraction method, and electrical resistivity tomography), hydraulic head measurement, groundwater chemical analyses, and physico-chemical water loggings. The selected study area is the Vomano valley, in Central Italy. This area is characterized by a shallow gravelly and gravelly-sandy alluvial aquifer, that shows a thickness of about 5-10 m and whose aquiclude is represented by the marine marly-clayey bedrock (i.e. the post-evaporitic member of the Laga formation). Its recharge is due to the direct rainfall infiltration and the Vomano river leaking in the aquifer. Here, connate natural mineral groundwater ascends through a highly fractured zone of the aquiclude and mixes with shallow groundwater. These connate waters are characterized by very high electrical conductivity, a water-type that is mainly Na-Cl-SO4, high boron, iron and manganese concentrations. Recently, their suitability for exploitation was assessed, and they were considered eligible to be used for therapeutic purposes (i.e. dermatological and inhalation treatment). However, these mineral waters have never been exploited, and they are still mixing in the environment, posing several concerns to the freshwater use for irrigation purposes. The first results show that these connate mineral waters represent old seawater trapped during the last sea-level regression and accumulated in fine marine deposits. They are confined and ascend through the fractured bedrock due to a pressure gradient. Their zone of influence seems to be very limited in space, suggesting a strong dilution due to the fresh groundwater flow. Nevertheless, increases in the alluvial aquifer hydraulic head during the wet season recharge favor, because of the vertical drainage, the connate mineral water upwelling, instead of provoking dilution.