Mineralogical and geochemical profiling of arsenic-contaminated alluvial aquifers in the Ganges-Brahmaputra floodplain Manikganj, Bangladesh

Abstract

The present study investigated three major components – groundwater, sediment and minerals of arsenic-affected Quaternary alluvial aquifers in Manikganj town, which is located in one of the As-hotspots in central 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. Groundwater both in shallow (< 100 m) and deeper (> 100 m) aquifers is mainly Ca-HCO3 type with a mean redox potential of -88 mV and a mean pH value of 6.75. Multivariate statistical analyses revealed that As in groundwater is closely associated with Fe, Mn, Si and pH. Groundwater As is negatively correlated with SO4. Results suggest that the As-contaminated groundwaters are under Fe- and/or Mn-reducing conditions. Several fining-upward sequences form the aquifers in Manikganj. Shallow aquifers are composed mainly of gray sands with silts and gravels toward the bottom. Deeper aquifers are yellow to yellowish-brown, medium to fine sands with occasional gravels. High arsenic concentrations (as high as 8.8 mg/kg) are found in fine-grained sediments, mostly clay and silty clay that were probably deposited as overbank and natural levee deposits mainly by meandering river channels. 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 K-feldspar), lithic fragments and abundant (3-10 wt%) heavy minerals 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 grains, which contain significant amounts of arsenic (as high as 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 also abundant in shallow sediments, are additionally considered as potential As-carriers for groundwater in the alluvial shallow aquifers in Bangladesh.