Degradation of chlorinated ethenes in a peat-rich hydrogeologic system: opportunities and environmental issues

Many aquifers throughout Europe are contaminated by chlorinated solvents and the occurrence of these contaminants is commonly related to improper industrial or disposal practices taking place few decades ago. The region around the city of Ferrara (eastern Po plain, northern Italy) is an area strongly impacted by old industrial activities, with the occurrence of chlorinated compounds at several sites. A peculiarity which lumps together the contaminated sites identified around Ferrara is the predominant occurrence of Vinyl Chloride (VC), which in most of the cases represents the only chlorinated compound detected in groundwater, with the formation of VCA (Vinyl Chloride Alone) plumes. The City of Ferrara is located on a lowland alluvial plain, originated by the River Po, which consists of a cyclic alternation of aquifers and aquitards, down to a maximum depth of 200 m b.g.s. The study presented here is focused on the shallower confined regional aquifer, located between 15 and 50 m b.g.s. and exploited at several locations for drinkable water purposes. Within this aquifer, three sites strongly contaminated by chlorinated solvents were investigated by means of a multidisciplinary, hydrogeologic and stratigraphic approach. High-resolution sedimentological analyses from the Ferrara subsurface show the ubiquitous occurrence of peat-rich layers between 10 and 15 m depth, interpreted as early Holocene transgressive swamp deposits. An hydrogeologic investigation carried out along a vertical profile at one of the sources of contamination, proves that reductive dechlorination, from PCE and TCE to VC, takes place during the migration of DNAPLs through the fine peat-rich Holocene deposits, which locally act as a “reactor” for biodegradation. The results of this study highlight a significant criticality peculiar to the territory of Ferrara: the co-occurrence of peat-rich deposits and scattered sources of chlorinated solvents lead to recurring accumulation of VC, thus enhancing the degree of human health risk related to this kind of contamination.