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