IMPACT OF OVER-PUMPING FROM DEEP AQUIFER IN DHAKA CITY ON THE SAFE WATER SUPPLY FOR SURROUNDING RURAL POPULATION

The natural setting of Bangladesh on the Ganges-Brahmaputra-Meghna (GBM) Delta has endowed it with abundant quantities of easily accessible groundwater over most of the country. But dangers of elevated arsenic concentrations and fecal pathogens in shallow aquifers leave rural people exposed to deadly chronic and acute diseases. To mitigate these risks more than 200,000 deep wells (>120 m) have been installed across the country. Rural people living near Dhaka City relying on deep aquifer water, however, are increasingly affected by rapidly declining water levels due to urban pumping in Dhaka. At some wells during the dry season the low water levels cause villagers to switch to drinking high arsenic water from shallow aquifers. To investigate how this urban pumping is changing sources of recharge to the deep aquifer we document the drawdown in a rural area 15 to 35 km east of Dhaka resulting from urban pumping. We installed pressure transducers in 17 community wells 130-240 m deep in Araihazar Upazila and in the adjacent Meghna River. Water levels varied by 3 m seasonally in both the deep aquifer and the river, with lateral hydraulic gradients of 2x10^-4- 4x10^-4 trending consistently westward towards Dhaka throughout the 2-year deployment. An overall increase in the lateral gradient is apparent over 2 years indicating the continued eastward expansion of the drawdown cone. Analytical steady and transient models demonstrate that the deep aquifer in eastern Araihazar is rapidly recharged through vertical infiltration from surface sources during the late wet and early dry seasons. While the short term impacts include creating safe water scarcity in villages reliant on hand pumps for drinking water, in the long term increased Dhaka pumping could accelerate an invasion of elevated arsenic and/or dissolved organic carbon from shallow depths that can promote the release of arsenic, thereby contaminating the deep, currently low-arsenic source of water supply.