Groundwater chemical status assessment considering geochemical background: an example from Northern Latium (Central Italy)

The evaluation of the chemical status of groundwater bodies, pursuant to the European Groundwater Directive 2006/118/EC, requires the assessment of the natural background levels when concentration exceeds the national standards. In large part of Italy, including the case-study region Latium, the presence of substances of natural origin has long been recognized. Nonetheless, a systematic assessment of background levels is still missing. Guidelines have been published both at the European and Italian level, but there are still unanswered questions, including the appropriate geographical scale, the time and spatial variation. In this paper, the evaluation of the groundwater chemical status at the groundwater body scale for the Latium region for the above-mentioned elements is presented. We used the preselection method, choosing the 95th percentile of the preselected datasets for various groundwater bodies. Results show that the natural background levels differ at the groundwater body scale reflecting the complexity of the geological asset. At a smaller scale, the variation of the background even at short distances may be dramatic, and this should be considered especially when dealing with site scale assessment. Conversely, the time variation of the considered parameters seems to be modest. In the case-study region, the chemical status assessment considering the background levels would result "good" as for the exceedances of arsenic and fluoride, solving the problem of erroneous classification of water bodies characterized by contamination of natural origin.