Groundwater diffuse pollution in functional urban areas: The need to define anthropogenic diffuse pollution background levels

Groundwater status in highly urbanized areas is particularly affected by anthropogenic influence due to diffuse pollution deriving from many sources. This makes very often challenging to determine whether the observed groundwater conditions are the result of localized pollutant sources (PS-Point Sources). In the EU legislative framework it is accepted that, when Natural Background Levels (NBLs) of undesirable elements are proven to be higher than specific Groundwater Quality Standards (GQSs), NBLs should be assumed as GQSs. No procedure is instead defined when anthropogenic Diffuse Pollution Background levels (DBPLs) are higher than GQSs and make unfeasible any remediation strategy. Among the many contaminants affecting groundwater, the chlorinated solvents, tetrachloroethylene (PCE), trichloroethylene (TCE) and trichloromethane (TCM) among the organics and hexavalent chromium, among the inorganics, having been widely used in several industries all over Europe, are very often the most prevalent contaminants in soil and groundwater. Aim of this paper is to discuss a multivariate statistical approach to address the issue of identification of anthropogenic Diffuse Pollution Background Levels. With such aim, an area of about 1600 km2, including the Functional Urban Area of Milan, was considered and 10 independent geochemical datasets, provided by local and regional agencies, and covering the period 2003–2014 were merged into a single database after homogenization and multiple quality checks. A total of 618,258 chemical analyses from 3477 sampling wells were considered, being all samples collected and analyzed through internally consistent protocols. The analysis enabled to identify five main clusters, having specific hydrogeological characteristics, different temporal profiles and pollutant background concentration levels, which were also found to respond differently to meteo-climatic changes. This study offers a robust knowledge basis for drafting a diffuse pollution management plan of the area.