ALBERT

Description: Determination of the solid-phase arsenic speciation in sediments hosting high-arsenic groundwaters, utilized for drinking and irrigation in Bengal, SE Asia and elsewhere is important in order to understand the biogeochemistry of arsenic. Despite this, there is a relative paucity of speciation data for solid-phase arsenic in such systems, due to preservation difficulties, low arsenic concentrations in the sediments, multiple coordination environments and sample heterogeneity. In this study, X-ray absorption near edge structure spectroscopy was used in conjunction with linear least-squares fitting of model compounds to determine the oxidation state of arsenic in sediments from West Bengal and Cambodia. Whatever the collection and storage method used, substantial oxidation of arsenic was commonly observed over periods of weeks to several months. Sands were particularly susceptible to changes in arsenic oxidation state during storage. Analysis within two or three weeks of collection is therefore recommended, whilst on-site storage under a nitrogen atmosphere immediately after collection is particularly recommended for the preservation of sandy samples. Both muds and sands from West Bengal and Cambodia were dominated by arsenite (As(III)) with

Description: Our recent discovery of hazardous concentrations of arsenic in shallow sedimentary aquifers in Cambodia raises the spectre of future deleterious health impacts on a population that, particularly in non-urban areas, extensively use untreated groundwater as a source of drinking water and, in some instances, as irrigation water. We present here small-scale hazard maps for arsenic in shallow Cambodian groundwaters based on 〉1000 groundwater samples analysed in the Manchester Analytical Geochemistry Unit and elsewhere. Key indicators for hazardous concentrations of arsenic in Cambodian groundwaters include: (1) well depths greater than 16 m; (2) Holocene host sediments; and (3) proximity to major modern channels of the Mekong (and its distributary the Bassac). However, high-arsenic well waters are also commonly found in wells not exhibiting these key characteristics, notably in some shallower Holocene wells, and in wells drilled into older Quaternary and Neogene sediments.It is emphasized that the maps and tables presented are most useful for identifying current regional trends in groundwater arsenic hazard and that their use for predicting arsenic concentrations in individual wells, for example for the purposes of well switching, is not recommended, particularly because of the lack of sufficient data (especially at depths 〉80 m) and because, as in Bangladesh and West Bengal, there is considerable heterogeneity of groundwater arsenic concentrations on a scale of metres to hundreds of metres. We have insufficient data at this time to determine unequivocally whether or not arsenic concentrations are increasing in shallow Cambodian groundwaters as a result of groundwater-abstraction activities.