SEDIMENT AND MINERALOGICAL CONSTRAINTS ON ELEVATED GROUNDWATER ARSENIC IN ALLUVIAL AQUIFERS IN BANGLADESH

Elevated dissolved arsenic in alluvial aquifers in Bangladesh is one of the worst environmental problems in the world. Geochemical study of groundwater, aquifer cores and minerals from the central Bangladesh provides interesting results on groundwater-sediment interactions and the occurrence of arsenic in groundwater.

ICP-MS analysis of 83 groundwater samples from Manikganj, Bangladesh reveals a broad range of arsenic concentrations in the surveyed wells (from 0.25 µg/l to 191 µg/l), with a mean concentration of 33.17 µg/l. Screen depth in the tubewells ranges from about 7.6 m to 228.6 m, with a mean depth of 52 m.

Aquifer sediments are composed of sand, silt, clay, and gravels varying in color from yellowish-brown to dark gray. The uppermost aquifer occurs between 10 m and 80 m below surface and has a mean arsenic concentration of 35 µg/l. Deeper aquifers (>100 m depth) have a mean arsenic concentration of 18 µg/l. Quartz, feldspar, muscovite, biotite, amphibole, pyroxene, garnet, kyanite, sillimanite, apatite, magnetite, hematite, ilmenite, sphene, zircon and chlorite are the significant detrital minerals in aquifer sediments. Minerals were identified with scanning electron microscopy using backscattered electron and energy-dispersive X-ray spectrometry methods. Authigenic goethite grains are found in sediments mostly at shallow depths (<50 m). However, authigenic siderite grains are found only at intermediate to deeper depths (80-120 m). At deeper depths, where arsenic concentration is low, the Fe-oxides comprise of hematite and magnetite. Electron microprobe analysis detected arsenic in Fe-oxyhydroxide minerals. The concentration of arsenic in goethite is measured at 341 mg/kg (weight %), which is just below the minimum detection limit of 366 mg/kg of arsenic by electron microprobe. The maximum arsenic concentration in bulk sediments at shallow depths is found about 8.8 mg/kg (ICP-MS) that supports the presence of arsenic in the Fe-oxyhydroxides minerals. Sediment composition suggests a mixed provenance, both from orogenic belts and cratonic uplifts. Sediment chemistry and mineral assemblages in the aquifers suggest that the authigenic Fe-oxyhydroxides are the principal carriers of arsenic in groundwater within shallow to intermediate (<120 m) aquifers.