Peat deposits as bio-geochemical reactors enhancing mobilization of Arsenic and reductive dechlorination of Vinyl Chloride.

Groundwater quality in the Ferrara region (eastern Po Plain, northern Italy) is severely impacted by different types of contamination, two of which are of main human health concern: 1) Arsenic contamination from natural sources, in concentrations between 0.05 and 600 μg/L, is widespread in the shallowest local aquifer; 2) Vinyl Chloride contamination, in concentrations up to 18 mg/L, occurs in the same aquifer from degradation of higher chlorinated compounds of industrial origin. Although the two contaminants are of different nature and likely have different origins, in both cases contaminant occurrence in groundwater is strongly related to shallow peat-rich, swamp deposits. In the case of Arsenic, data from more than 40 sites were treated statistically to verify the natural origin of the compound, investigate its spatial distribution and hypothesize the mechanism of mobilization into groundwater. Good correlations between Arsenic, Iron and Manganese concentrations and the Redox potential suggest that organic-rich sediments allowed a reductive dissolution mobilization that considers Manganese and Iron oxides as Arsenic carriers (sources), and degraded organic matter as the redox driver of the release. Also, the organic matter could be a direct source of Arsenic in addition to its role of redox driver, as suggested by the strong spatial correlation between high Arsenic concentration in sediments and occurrence of the organic-rich deposits. Concerning Vinyl Chloride, previous studies in the same region based on hydrochemical and isotopic evidences pointed out the role of peat-rich deposits as “reactors” for reductive dechlorination of higher chlorinated compounds (mainly PCE and TCE), spilled or improperly stored in several sites of the region until few decades ago. In this study, we analyzed the evolution of contaminant composition over time in 12 sites contaminated by chlorinated ethenes and ethanes. Increase in VC concentration over time and decrease in higher chlorinated contaminants was observed in most sites, thus supporting the effectiveness of the peat-driven reductive dechlorination of parent compounds that led to the accumulation of Vinyl Chloride. At the regional scale, the highest concentrations of Arsenic and Vinyl Chloride showed good spatial correlation with the occurrence of shallow swamp deposits, thus supporting the key-role of organic-rich sediments in the deterioration of groundwater quality