Sources and mobilisation pathways for geogenic arsenic contamination in groundwaters; a case study from eastern Ireland

Reconnaissance sampling of domestic water supplies in the Brittas area of Co. Dublin, Ireland detected arsenic concentrations up to 69 μg/L, with 55% of samples exceeding the World Health Organisation (WHO) limit of 10 μg/L, indicating a potential health risk to the local population. Follow-up sampling using low-flow techniques confirmed arsenic concentrations up to 54 μg/L and 52 μg/L in unfiltered and filtered water samples, with median values of 24.8 μg/L and 24 μg/L respectively and >70% of samples exceeding the WHO drinking water limit for arsenic. Waters from small surface streams in the area also exhibit high arsenic concentrations (up to 85 μg/L) with a median value of 54.3 μg/L. Elevated arsenic in the surface- and ground-waters reflect regional-scale elevated arsenic in arsenopyrite-bearing Lower Palaeozoic greywacke-slate bedrocks, that also produced stream-sediment arsenic anomalies up to 217 mg/kg. SEM-EDX investigations of the greywackes identified arsenopyrite in quartz veins that occur within bedrock fractures as the primary mineralogical source of arsenic. Groundwaters with low pH, low alkalinity, low electrical conductivity, and relatively high Eh values are associated with the highest arsenic. Deeper wells show elevated dissolved arsenic, but multiple mobilisation and remobilisation steps are required to explain the hydrochemical data. Dissolved arsenic and sulphate are decoupled in the groundwaters by secondary barite precipitation. Separately, analyses of unfiltered tap waters, sampled to represent waters used by well owners, indicate mobilisation of adsorbed arsenic on entrained iron-manganese oxyhydroxides particulates by high-speed submersible in some wells. Highlights : • >55% of groundwater samples have arsenic contents more than the 10mg/l WHO limit • Quartz vein associated arsenopyrite is the primary mineralogical source of arsenic • High arsenic in surface- and ground-waters linked to arsenopyrite weathering • Deeper fractured bedrock aquifer wells show more elevated dissolved arsenic • Unfiltered tap waters show As mobilisation on entrained Fe-Mn oxyhydroxides