Natural background levels for some ions in groundwater of the Campania region (southern Italy)

The aim of the study was the determination of the natural background levels (NBLs) for the ions Na+, Cl-, SO4 (2-), As3+, F-, Fe2+, and Mn2+, in some groundwater bodies of the Campania region (southern Italy). The "Protocol to evaluate the natural background concentrations" proposed in 2009 by ISPRA (Italian Institute for Environmental Protection and Research) has been applied to the chemical data set of groundwater of the examined groundwater bodies. These analyses have also been examined following the guidelines of the BRIDGE project (Background cRiteria for the IDentification of Groundwater thrEshold). These approaches to evaluate the Threshold Values (TVs) and the NBLs, based on probability distribution functions, have been applied in many countries by various authors during the last 5 years. Changes applied to ISPRA Protocol in this study concern mainly the preselection criteria, in particular threshold values of specific ions, deriving from the aquifers geochemical features. The preselection criteria of the ISPRA Protocol have been merged with those of the BRIDGE Project in order to define a procedure suitable for the definition of the NBLs in the examined aquifers. The NBL of fluoride for the "Phlegrean Fields" and the "eastern Plain of Naples" groundwater bodies shows values deeply exceeding the reference value (REF) of 1,500 mu g/L, ranging between 3,600 and 15,000 mu g/L. The cause of this very high fluoride content is in the natural features of the aquifers constituted by volcanic and pyroclastic rocks. The volcanic origin of the aquifers is also the reason for the high arsenic content in "Phlegrean Fields" groundwater. Here the NBL calculated was about 47 mu g/L against the drinking water standard value of 10 mu g/L. The widespread high content of manganese and iron for the groundwater body of the "eastern Plain of Naples" is due to the reducing conditions related to the extensive marshlands present in the past. The very high NBL of all the examined ions for the groundwater body of "Ischia Island" depends on the existence of a geothermal system.