Environmental aqueous geochemistry of arsenic in groundwater: occurrence, speciation and biogeochemical processes
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The presence of arsenic and other trace elements within groundwater has become a global concern due to the health risks from drinking water associated with elevated concentrations in the last three decades. Whilst ample information is available in areas of south east Asia (e.g. West Bengal) where arsenic is being mobilised via microbial-mediated reductive dissolution, there is comparatively little research in Europe. Recently, research has demonstrated that elevated arsenic is occurring in fractured bedrock aquifers in many regions, e.g. New England. While not being regarded as sources for large public water supplies in Ireland, these fractured bedrock aquifers are important for private groundwater sources and may show elevated concentrations of geogenic contaminants. The objectives of this dissertation were to assess the spatial distribution of arsenic and other trace elements in fractured bedrock aquifers, understand the geochemical processes giving rise to elevated arsenic and determine the speciation of arsenic in groundwater. Firstly, existing datasets at national-level scale in addition to regional-scales were collated. Due to the presence of left censoring (i.e. arsenic values below an analytical detection limit) and changes in detection limits over time, the application of conventional statistical methods would inhibit the generation of meaningful results. In order to handle these issues, several arsenic databases were integrated and modelled using statistical methods appropriate for non-detect data. In addition, geospatial methods were used to assess principal risk components of elevated arsenic related to differing geological, hydrogeological and land-use classifications. Significant differences were noted between different aquifer lithologies, indicating that Rhyolite, Sandstone and Shale (Greywackes), and Impure Limestone presented a greater risk of elevated arsenic in groundwaters. Significant differences also occurred among aquifer types with poorly productive aquifers, locally important fractured bedrock aquifers and regionally important fissured bedrock aquifers presenting the highest potential risk of elevated arsenic. Comparable results were observed in regional settings. In addition to these previous approaches, a study incorporating secondary geochemical data (collected as part of road construction) found elevated arsenic concentrations (i.e. low-to-moderate levels) within groundwater in north east Ireland (in addition to nitrate, iron and manganese), which was the focus of detailed ultra-trace aqueous geochemical sampling and speciation studies. Aqueous arsenic concentrations showed positive associations with well depth with speciation using field methodology indicating arsenate as the dominant arsenic species in oxic-alkali groundwater. The speciation of arsenic was controlled by groundwater pH, with mobilisation of arsenic occurring at elevated pH through alkali desorption processes. Indeed, elevated arsenic groundwaters are described as oxic-alkali, low Ca/Na ratios, low Fe and Mn, and co-occurrence of several oxyanions (Mo, Se, Sb and U). Low concentrations of methylated arsenicals were detected in groundwaters which suggested microbial mediated mobilisation of arsenic may also be occurring in small clusters. This research contributes to the understanding of arsenic mobilisation processes and will facilitate in guiding future development, testing and treatment requirements of groundwater resources. In addition, it will provide a focus of the arsenic problem (and other geogenic trace element contaminants) arising from fractured bedrock aquifers in Europe which is important with respects to both public health and drinking water resources.
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