SEDIMENT CHARACTERS AND STRATIGRAPHY OF ARSENIC-CONTAMINATED ALLUVIAL AQUIFERS OF MANIKGANJ, BANGLADESH

Groundwater in alluvial aquifers of Bangladesh is contaminated by naturally occurring elevated arsenic. Manikganj, a district of Bangladesh, is located at an arsenic-contaminated hotspot. This study focuses on stratigraphy, sediment composition of the Quaternary alluvial aquifers, and distribution of groundwater arsenic in Manikganj township and adjoining areas.

Groundwater samples from 83 tube wells have been analyzed for arsenic and other elements. Arsenic concentrations in the surveyed wells range from 0.25 µg/L to 191 µg/L, with a mean concentration of 33.17 µg/L. Well depths studied range from about 7.6 m to 228.6 m, with a mean depth of 52 m below the surface.

Sediment core samples were collected from two wells down to a depth of 152 m each that penetrated several aquifers within the study location. Sediments are composed mostly of sand, silt, clay, and gravels. The uppermost aquifer occurs between 10 m and 80 m below the surface and has a mean arsenic concentration of 35 µg/L. Deeper aquifer (> 100 m depth) has a mean arsenic concentration of 18 µg/L. Sediments from the upper aquifer are mostly gray to dark-gray in color likely indicating a reducing environment, whereas sediments from deep aquifer are mostly yellowing-gray to brown in color with less organic matter. Quartz, feldspar, mica, hornblende, garnet, kyanite, tourmaline, magnetite, ilmenite are the major minerals in sediments from both aquifers. Biotite and K-spars are dominant in shallow aquifer, although plagioclase feldspar and garnet are abundant in deep aquifer sediments. Sediment composition suggests a mixed provenance both from orogenic belts and cratons. Occurrence of thick graveliferous layer (about 25 m) in one drilled site within the deeper aquifer suggests deposition within a large paleo-channel, possibly during low stand of sea-level in the last glacial maximum. A relatively thinner (< 7 m) graveliferous layer with clay is also recorded from the other drilled well, suggesting deposition in adjoining floodplain of the major paleo-channel in study area.

Quaternary sedimentary sequences in Manikganj represent several fining-upward alluvial cycles. Arsenic concentrations in these sediments seem to be related to the presence of organic matter, peat, and argillaceous material that are mostly observed in shallow sediments.