Investigating Sources and Sinks for Water in Deep Aquifers in Bangladesh

The spatial distribution of arsenic in Bangladesh aquifers has been known for years at the country-wide scale. When present, high arsenic water typically occurs at 8-30 m depth. Although the majority of low arsenic wells remain low, in some cases arsenic has been observed to increase in wells placed at intermediate depths (30-100m). It has been suggested that urban pumping from the deep aquifer (>100m) will cause more of this in the future. Multi-level nests located in Araihazar, a 25 km² region 25 km east of Dhaka and bounded to the east by the Meghna River, confirm the depressurization of the deep aquifer in recent years.

Pumping from the deep aquifer will draw water from the river rather than the contaminated shallow aquifer in regions near rivers where the deep aquifer is not separated from the river bed by a confining layer, but is separated from the shallow aquifer by thick confining clay. In these regions depressurization of the aquifer is mitigated by river inflow. River water, however, is high in organic matter and may still cause increases in arsenic concentrations through reductive dissolution of arsenic-binding metal oxides. To understand these processes and determine sources of recharge to the overstressed deep aquifer water levels in 50 wells were measured in Araihazar from March-November, 2012.

Strong vertical gradients were observed across the clay-rich 30-70 m intermediate aquifer (~1x10^-2) in western Araihazar. Within the deep aquifer, lateral hydraulic gradients pointed westward towards Dhaka throughout the year (2.9-3.9x10^-4), but vertical gradients within the deep aquifer were weak, corresponding to fewer confining units. The hydraulic gradient steepness positively correlated with the Meghna River water level implying direct influence of the river on the deep aquifer. Fewer clay layers in the intermediate aquifer near the Meghna may be caused by frequent scouring of adjacent floodplains.