Geochemistry and Mineralogy of Contaminant Groundwater and Sediments from Coastal Aquifers of Southwest Bangladesh

Abstract

The groundwater in the coastal aquifers of Bangladesh contains high levels of salt concentration and naturally occurring arsenic (As), which may affect human health. Especially in the southwest coastal region of the country, there is a crisis for freshwater due to contamination of both shallow (<50 m depth) and deep (>150 m depth) aquifers with As and saltwater concentration. Groundwater analysis from 12 monitoring wells in Satkhira district shows that the average concentration of As during dry and wet seasons is 45.12 µg/l and 20.65 µg/l, respectively. These levels are significantly higher than the permissible limits set by EPA and WHO, 10 µg/l for As in drinking water. Arsenic is positively correlated with Fe and pH, which suggests that As may be derived from bacterial iron reduction of Fe oxides. XRD and SEM analysis revealed that the groundwater samples are located between zones where goethite and siderite mineral concretions dominate, with pH values ranging from 7.02 to 8.20 in the Eh-pH stability plot. The reactivity of these minerals implies that aquifers were exposed to slightly oxidized to moderately reducing conditions with negative ORP values from -53 mv to -149.5 mv, which may contribute to the enrichment and mobility of As in the groundwater. Textural studies from three core sediment samples suggest major aquifer sediments are mostly subangular to angular and comprise 92% of fine to medium sand and 8% fine-grained particles of silt and clay fractions. More angularity of minerals grains and lower ZTR (zircon-tourmaline-rutile) values of heavy mineral contents suggest these coastal sediments are immature. This study identified factors contributing to the mineral’s reactivity for the enrichment and mobility of As in groundwater and sediments from coastal aquifers of Bangladesh. The study also utilized GIS models to assess temporal trends of water and vegetation extents that may be affected by arsenic and saltwater contamination in the southwestern coastal regions of Bangladesh.