Avoiding high concentrations of arsenic, manganese and salinity in deep tubewells in Munshiganj District, Bangladesh

Over 85% of shallow tubewells (STWs) in the district of Mushiganj, Bangladesh, are affected by As concentrations above 50 μg/L. Deep tubewells (DTWs) are among the preferred mitigation options for reducing exposure to As. Around the town of Sreenagar (30 km South of Dhaka) water samples were collected from existing STWs and DTWs, monitoring wells (5–210 m depth) and newly installed DTWs. Analysis of water samples from 2005 to 2010 indentified three types of groundwater currently used for drinking: (1) Shallow water from 20 to 100 m deep dark-grey sediments with high As-concentrations (100–1000 μg/L), intermediate-high Fe (2–11 mg/L), intermediate Mn (0.2–1 mg/L) relatively low EC (400–900 μS/cm) dominated by Ca–Mg–HCO3-. (2) Water from 140 to 180 m deep light-grey sediments with low As (<10 μg/L), intermediate Mn (0.2–1.0 mg/L), intermediate Fe (1–5 mg/L), low total organic C, N and NH4+, and intermediate EC (1200–1800 μS/cm) dominated by Ca–Mg–HCO3-–Na–Cl. (3) Deep water from 190 to 240 m deep brown sediments with low As (<10 μg/L), high Mn (2–5 mg/L), low Fe (<3 mg/L), and high EC (2000–3000 μS/cm) dominated by Ca–Mg–Na–Cl with high Ca and Cl concentrations. Drillers have traditionally used the transition from grey to brown sediments as an indicator for the depth from which safe drinking water can be obtained. However, in most of the tubewells in the studied area of over 190 m depth, Mn exceeds the WHO-limit of 0.4 mg Mn/L by a factor 2–5 and the water tastes noticeably saline. Based on the depth-resolved water compositions and a small survey of DTWs in a 2.5 × 2.5 km2 area, a depth range of 150–180 m with light-grey sediments is recommended for the construction of new DTWs. Pumping tests have shown that the deeper aquifer is largely separated from the upper aquifer, such that small volume water abstraction for drinking with hand pumps can be deemed safe as long as wells are periodically tested. DTWs are an excellent option for avoiding high As concentrations. However, before installing larger numbers of new DTWs, monitoring wells should be installed to determine the optimal depth for water withdrawal in each area.

► Deep tubewells are the second most successful arsenic mitigation option in Bangladesh. ► Manganese concentrations and salinities are often high and not systematically tested. ► Distinctly different groundwater types are typically found at different depths. ► Monitoring wells are advisable to find the optimal depth for new tubewells. ► Groundwater abstraction by hand pumps should not lead to draw-down of As-rich water.