Groundwater arsenic biogeochemistry-Key questions and use of tracers to understand arsenic-prone groundwater systems

Over 100,000,000 people worldwide are exposed to high arsenic groundwater utilised for drinking or cooking. The consequent global avoidable disease burden is estimated to be of the order of 100,000 avoidable deaths or more per annum from just direct exposures-i.e. excluding indirect exposure (from rice and other foods) and excluding morbidity. Notwithstanding 1000s of papers published on arsenic (hydro) (bio)geochemistry, there remain a number of key outstanding questions to be addressed in relation to arsenic geoscience-these include questions related to:(i) the role of human activities-irrigation, agriculture and other land uses-on arsenic mobilisation in groundwaters; (ii) the specific sources, nature and role of organics, minerals and microbial communities involved in arsenic mobilisation; (iii) the relationship to microscopic to macroscopic scale geological (including tectonic) and evolution processes; (iv) unravelling the over-printing of multiple processes in complex highly heterogeneous aquifer systems and (v) using increasing understanding of the controls of arsenic mobility in groundwaters systems to informing improved locally-relevant remediation and mitigation approaches. This article further summarises how the 9 further papers in this Special Issue address some of these questions through the use of chemical and/or isotopic tracers.