Arsenic in Bangladesh: A Geological and Human Perspective

A decade of research has started to outline some of the complex interactions between hydrogeological and biogeochemical factors that regulate arsenic concentrations in groundwater of Bangladesh. There is growing evidence from the regional to the local scale that flushing of the sedimentary aquifer lowers arsenic concentration in groundwater over geologic time. Older Pleistocene deep aquifers are depleted of arsenic in both groundwater and in the sediment when considering mobilizable arsenic leached by a 1M phosphate solution. Over shorter geologic time, the variability of arsenic in shallow reducing Holocene aquifers appears to reflect flushing rates estimated from a first-order relationship of 250 ug/L dissolved arsenic per 1 mg/kg sediment mobilizable As. Variability around this average relationship observed during batch adsorption experiments can tentatively be attributed to differences in the surface properties. Together with the observation that groundwater arsenic concentrations generally increase with groundwater age over a time span of 1-40 years suggests that flushing simultaneously lowers the groundwater and sedimentary arsenic. The flushing model implies that groundwater discharging to rivers and streams at the end of a flow path should contain high arsenic. This has been observed and causes an enrichment of arsenic in Meghna River bank sediments. Therefore, arsenic is not completely flushed out to the sea and may be recycled as river and stream channels shift their course to re-deposit the arsenic enriched sediment. The same flushing model predicts that widespread irrigation pumping for rice production may on one hand reduce arsenic concentration in shallow groundwater through accelerated flushing while at the same time adding this arsenic to soil. The fate of the arsenic accumulating in soil is uncertain but there is new evidence that its impact on rice productivity may be of greater concern than an increase in human exposure because uptake into rice grains is limited.