AUTHIGENIC AND DETRITAL MINERALS IN THE QUATERNARY SEDIMENTS AND THEIR CONNECTION TO ELEVATED ARSENIC CONCENTRATIONS IN ALLUVIAL AQUIFERS IN BENGAL BASIN

The present study investigated the petrology and mineralogical characteristics of the Quaternary alluvial aquifers in the Ganges-Brahmaputra floodplains in central part of Bengal basin, Bangladesh. Approximately 60% of surveyed tubewells (n=88) within an area of 40 km² in and around Manikganj town contain arsenic concentrations exceeding 10 µg/L. Measured arsenic concentrations are as high as 191 µg/L with a mean value of 33 µg/L. Multivariate statistical analyses revealed that As in groundwater is closely associated with Fe, Mn, Si and high pH. Arsenic is negatively correlated with SO₄. Arsenic-contaminated groundwaters are under Fe- and/or Mn-reducing conditions.

Shallow aquifers (< 100 m) are composed of gray sands with silts and gravels toward the bottom. Deeper aquifers (> 100 m) are yellow to yellowish-brown, medium to fine sands with occasional gravels. High arsenic concentrations (~ 8.8 mg/kg) are found in fine sediments, mostly clay and silty clay that were probably deposited as overbank and natural levee deposits by meandering channels. Single extraction of sediments by hydroxylamine hydrochloride (NH₂OH.HCl) suggests that most extractable amounts of arsenic (1.4 mg/kg) are associated with amorphous and crystalline Fe-oxyhydroxides phases. Alluvial sediments, which were probably deposited during the Holocene sea-level lowstand, form the deeper aquifers that are As-free (< 50 µg/L).

Sediments both in shallow and deep aquifers are composed of quartz, feldspars (mainly potassium feldspar), lithic fragments and heavy minerals (3-10 wt%) including magnetite, ilmenite, biotite, amphibole, pyroxene, garnet, kyanite, sillimanite, apatite, sphene, epidote and zircon. Authigenic Fe-oxyhydroxides (goethite), and siderite concretions are found in drill-core sediments. Goethite concretions that contain high arsenic (~341 mg/kg) are abundant in shallow sediments. Results suggest that microbially mediated reductive dissolution of Fe-oxyhydroxides is the principal mechanism for releasing arsenic into groundwater. Detrital magnetite, apatite, biotite and amphibole, which are abundant in shallow sediments, are additionally considered as potential As-carriers for groundwater in shallow aquifers in floodplains and delta plains in Bengal basin.