Arsenic in Groundwater: The Deep Late Pleistocene Aquifers of the Western Bengal Basin

in groundwaters from 145 wells across central West Bengal, India, those from Pleistocene aquifers at depths >70 m beneath paleo-interfluves contain <10 mu g/L As. Pleistocene aquifers beneath deep paleo-channels typically host groundwaters containing 10-100 mu g/L As at depths between 120 and 180 m. The depth profiles of As and SO4 and the conservative tracers Cl/Br, delta O-18, and delta H-2 show that the As in Pleistocene groundwater beneath deep paleo-channels is relict and does not arise from migration downward of As-polluted groundwater in overlying aquifers. We postulate that the As was liberated in situ by reduction of minimal iron oxyhydroxides in the gray Pleistocene sands by organic matter infiltrating from riverbeds during late Pleistocene or earliest Holocene times. Mitigation of the widespread As-pollution in shallow aquifers through exploitation of deep Pleistocene aquifers would improve if guided by an understanding of the distribution of buried paleo-channels and paleo-interfluves and the knowledge that As may be present naturally in groundwater, at depths >150 m beneath deep paleo-channels.