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Evaporative environments are known to produce high dissolved arsenic concentrations, particularly in systems characterized by pH > 8. In areas with shallow groundwater, dissolved arsenic accumulates in response to evaporation at the ground surface and within the vadose zone. The distribution of arsenate (the pentavalent form of arsenic) in the vadose zone is distinct from other dissolved ions including other oxyanions, presumably due to specific chemical properties unique to the arsenate species in saline, high pH environments.