GEOCHEMISTRY AND MINERALOGY OF CONTAMINANT GROUNDWATER AND SEDIMENTS FROM COASTAL AQUIFERS OF SOUTHWEST BANGLADESH

The high levels of salt concentration and naturally occurring arsenic (As) present in the groundwater of coastal aquifers in Bangladesh pose a significant threat to public health. Especially in the southwest region of the country, both shallow (< 50 m depth) and deep (>150 m depth) aquifers are significantly contaminated with saltwater and As, leading to a crisis for freshwater. Groundwater analysis from 12 monitoring wells from Satkhira district shows that average As concentrations were 45.12 µg/l and 20.65 µg/l during dry and wet seasons respectively which is much higher than the EPA and WHO permissible limits (10 µg/l for As). 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 the groundwater samples are located between zones where goethite and siderite mineral concretions were dominant with pH values ranging from 7.02 to 8.20 in 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. The present study identified factors contributing to the mineral’s reactivity for the enrichment and mobility of As in groundwater from coastal aquifers of Bangladesh.

Textural studies from three core 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 minerals contents suggest these coastal sediments are rather immature. The study explored potential connections between salinity and arsenic contamination with public health concerns among communities living in the study area of southwestern coastal region of Bangladesh.

Keywords: Geochemistry, Sediment mineralogy, Arsenic contamination, Saltwater intrusion, public health.