HEAVY MINERALS IN SEDIMENTS FROM ARSENIC-CONTAMINATED AQUIFERS IN CENTRAL BENGAL BASIN, BANGLADESH

High levels of naturally occurring arsenic in Bengal basin groundwater have become a major public health issue in Bangladesh. This study analyzed sediment samples from two 150 m drill cores from Quaternary alluvial deposits in the Manikganj area, ~70 km northwest of Dhaka city, using heavy mineral microscopy to find potential drainage pathways and geologic source rocks.

The Manikganj area is located east of the confluence of the Ganges and Brahmaputra (Jamuna) rivers. Former studies of this area have found that arsenic levels in groundwater decrease with depth (Shamshudduha et al., 2008). Both drill cores (MG and MN) represent multiple alluvial cycles bounded by aquitards and aquicludes. Nine representative samples were selected from different depth levels on the MG core and 6 from the MN core.

Heavy mineral percentages increased with depth but decreased at the second alluvial system. Heavy mineral analysis revealed the following minerals in decreasing order of abundance in both cores: amphibole, biotite, aluminosilicates, chlorite/chloritoid, garnet, zircon- tourmaline-rutile (ZTR), epidote, zoisite/clinozoisite, apatite, staurolite, muscovite, opaques, and hypersphene. Zoisite/clinozoisite levels decreased with depth in the MG core and drastically decreased at about 50 m in the MN core. Both garnet and ZTR levels increase with depth in both cores. Epidote and apatite decreased with depth in the MG core, and biotite decreased with depth in the MN core.

ZTR levels suggest that the older sediments traveled farther from their source area. The epidote/garnet ratio (Heroy et al., 2003) decreases with depth, suggesting that the sediments below ~50m depth were deposited by the Ganges and younger sediments could involve the Brahmaputra. The epidote family of zoisite and clinozoisite levels suggests that sediment in the youngest alluvial cycle is derived from the high-pressure rocks in orogenic belts in the Himalayas. This area could contribute to the high arsenic levels in the shallow groundwater. Future heavy mineral analyses should be conducted on more cores throughout the Bengal basin and on potential geologic source areas. Additionally, detrital geochronology should help understand sedimentation predominantly by the Ganges, by the Brahmaputra, or by both rivers.