Hydrogeological investigation for assessment of the sustainability of low-arsenic aquifers as a safe drinking water source in regions with high-arsenic groundwater in Matlab, southeastern Bangladesh

Exploitation of groundwater from shallow, high prolific Holocene sedimentary aquifers has been a main element for achieving safe drinking water and food security in Bangladesh. However, the presence of elevated levels of geogenic arsenic (As) in these aquifers has undermined this success. Except for targeting safe aquifers through installations of tubewells to greater depth, no mitigation option has been successfully implemented on a larger scale. The objective of this study has been to characterise the hydrostratigraphy, groundwater flow patterns, the hydraulic properties to assess the sustainability of the low-arsenic aquifers at Matlab, in south-eastern Bangladesh, one of the worst arsenic-affected areas of Bangladesh. Combining groundwater modelling with monitoring hydraulic heads in multi-level piezometer tests, 14C-dating of groundwater, conventional hydraulic testing and assessment of groundwater abstraction rate proved to be a useful strategy. A model comprising of three aquifers covering the top 250 m of the model domain showed to best fit the evaluation criteria for calibration. Matlab is a recharge area, even though it is adjacent to the great Meghna River. Irrigation wells are placed in clusters and account for most of the groundwater abstraction. Even though the hydraulic heads are affected locally by seasonal pumping, the aquifer system is fully recharged during and after the monsoon period. Groundwater simulations demonstrated the presence of deep regional and horizontal flow systems with recharge areas in the eastern, hilly part of Bangladesh and shallow small local flow systems driven by local topography. Based on modelling and 14C groundwater data, it can be concluded that the natural local flow systems reach a depth of 30 m b.g.l. in the study area. A downwardvertical gradient of roughly 0.01 down to 200 m b.g.l. was observed and reproduced bycalibrated models. The vertical gradient is mainly the result of the aquifer system and-properties rather than abstraction rate, which is too limited at depth to make an imprint. Although irrigation wells substantially change local flow pattern, targeting low-As aquifers seems to be a suitable mitigation option for providing people with safe drinkingwater. However, installing new irrigation- or high capacity production wells at the same depth is strongly discouraged as these substantially change the groundwater flow pattern. The results from the present study and other similar studies can further contribute to develop a rational management and mitigation policy for the future use of the groundwater resources for drinking water supplies.QC 20150114. Updated from manuscript to article in journal.