Pollutant sources in an arsenic-affected multilayer aquifer in the Po Plain of Italy: Implications for drinking-water supply

In aquifers 160 to 260 m deep that used for public water-supply in an area ~ 150 km2 around the town of Cremona, in the Po Plain of Northern Italy, concentrations of arsenic (As) are increasing with time in some wells. The increase is due to drawdown of As-polluted groundwater (As ≤ 144 μg/L) from overlying aquifers at depths 65 to 150 m deep in response to large-scale abstraction for public supply. The increase in As threatens drinking-water quality locally, and by inference does so across the entire Po Plain, where natural As-pollution of groundwater (As > 10 μg/L) is a basin-wide problem. Using new and legacy data for Cl/Br, δ18O/δ2H and other hydrochemical parameters with groundwater from 32 wells, 9 surface waters, a sewage outfall and rainwater, we show that the deep aquifer (160–260 m below ground level), which is tapped widely for public water-supply, is partly recharged by seepage from overlying aquifers (65–150 m below ground level). Groundwater quality in deep aquifers appears free of anthropogenic influences and typically < 10 μg/L of As. In contrast, shallow groundwater and surface water in some, not all, areas are affected by anthropogenic contamination and natural As-pollution (As > 10 μg/L). Outfalls from sewage-treatment plants and black water from septic tanks firstly affect surface waters, which then locally infiltrate shallow aquifers under high channel-stages. Wastewater permeating shallow aquifers carries with it NO3 and SO4 which suppress reduction of iron oxyhydroxides in the aquifer sediments and so suppress the natural release of As to groundwater.