INTERMEDIATE DEEP AQUIFER (IDA): A POTENTIAL SOURCE FOR ARSENIC-SAFE AND LOW-MANGANESE DRINKING WATER

In rural Bangladesh, the drinking water supply is largely dependent upon manually operated suction mode hand tubewells, mostly installed by the local community. Identifying safe aquifer for low cost tubewell installation is of prime concern for scaling-up safe water access in arsenic affected regions in the country. Depth of the tubewell is a key parameter as it is related to groundwater quality and cost of installation. The shallow wells (usually < 80m) are mostly at risk of As contamination. As a mitigation option, deep wells are drilled usually to depths of about 250 m which costs about 4-5 times of conventional shallow tubewells. Compared to safe water demand, the number of deep wells is still very low and one important reason is the installation cost, which is beyond the affordability of local community especially for the poor and disadvantaged section of the society. Although red and off-white sands at shallow depths could be a good source for As-safe water, elevated manganese (Mn) is a concern in safe water issue. Based on observations from monitoring wells installed at relatively intermediate deep aquifers (IDA), drinking water tubewells were installed around a depth range of 120 m in Matlab, Bangladesh for exploiting As-safe and low-Mn water. Baseline water quality analysis of 243 Intermediate Deep Tubewells (IDTW) provided promising results which support the strategy of exploiting IDA as safe sources for installation of drinking water tubewells. Arsenic, manganese and other trace elements along with the major ions were analyzed by high-precision ICP-OES and ion chromatography. Bangladesh drinking water standard for As (50 µg/L) was exceeded only in 3 wells and more than 91% (n=222) were within the WHO guideline value of 10 µg/L. For Mn, 89% (n=216) wells show the concentration within or below the previous WHO guideline value of 0.4 mg/L with a mean value of 0.18 mg/L. Availability of similar sand at this depth range could be targeted by local drillers at almost half-cost of deep tubewell installation and hence seems quite encouraging for the local community considering their affordability for installation of tubewell by their own initiative.