DEEP PUMPING IN DHAKA FAVORS DISTAL INTRUSION OF MEGHNA RIVER WATER INTO DRINKING WATER AQUIFERS

Deep aquifers (>100m) are increasingly relied upon for arsenic free drinking water in rural Bangladesh. The long term sustainability of this drinking water source is in question due to the massive depressurization by urban pumping. In places where confining clay layers are absent deep aquifer depressurization will draw young water into the deep aquifer. This water may contain high arsenic from shallow aquifers or, if it comes from rivers, labile organic matter released from organic-rich sediments that could promote the reductive dissolution of iron oxides and therefore release arsenic to groundwater. Here we explore the impacts of Dhaka pumping on changing recharge sources in the deep drinking water aquifers of Bangladesh. The study area is Araihazar upazila, a rural 150km² region 20 km east of Dhaka and bounded to the east by the Meghna River. Eighteen pressure transducers were placed within deep community drinking water wells spread across an 8x8km area 2-10km west of the Meghna. One transducer was placed in a river piezometer in the Meghna River. The transducers measured water level fluctuations every 20 minutes from March, 2012 through May 2013. Within the deep aquifer, hydraulic gradients trended towards Dhaka throughout the year. During the early monsoon westward gradients positively correlated with short term fluctuations in the level of the Meghna indicating a strong hydraulic connection to the deep aquifer. In the late monsoon and the dry season however, hydraulic heads in the eastern side of the deep aquifer were higher than the level of the Meghna and the westward hydraulic gradient was not influenced by short term fluctuations in the Meghna. This suggests that Dhaka pumping is creating optimal conditions for enhanced movement of river water into underlying aquifers. This process may buffer the deep aquifer from depletion, but the long term impact on the deep aquifer water quality may be substantial.