ELEMENT TRANSPORT AND PARTITIONING ALONG TIDAL CHANNELS IN SOUTHWEST BANGLADESH (Invited Presentation)

Estuarine environments are a complex array of interactions between freshwater, seawater, and sediment. Thus, environmental transport and cycling of inorganic metal(loids) of human and ecological concern within estuarine waterways requires a holistic understanding of the geochemical processes occurring within these media. Full characterization of water and suspended sediment samples with increased seawater mixing in coastal waterways can provide this important insight and thus enhance understanding of heavy metal(loid) mobility and transport. This is particularly essential to quantify in protected ecological areas and within agricultural areas downstream of large urban centers that may be sources of pollution. Therefore, water samples within the upper 1m of the water column along a tidal channel transect in Southwest Bangladesh were collected during the dry season (May) and analyzed for dissolved and suspended sediment element concentrations and other geochemical parameters. The transect contained an increased proportion of seawater as it ran to the south towards the Bay of Bengal (BoB), with the transect beginning upstream of a large urban center, Khulna City. While most elements in the suspended load were either depleted relative to upper continental crust (UCC) or similar in concentration to UCC, several trace elements such as Sb, As, Cd and Se were slightly enriched. Additionally, several trace elements in the dissolved load were well above world average riverine concentrations, particularly Se and As, which were also above World Health Organization (WHO) drinking water guidelines. Dissolved load Ba and Se displayed mostly conservative mixing trends with seawater after originally being sourced from nonconservative processes, with Ba likely sourced from sediment desorption in addition to groundwater exfiltration, while Se may have been anthropogenically sourced from Khulna City. Dissolved As did not display any obvious conservative mixing trends, and may ultimately be geogenic in origin, such as from groundwater. Both Ni and Co show trends consistent with desorption from competitive seawater cation exchange along the transect from north to south, similar to an earlier study conducted in the nearby Hooghly Estuary in West Bengal (Samanta and Dalai in Geochim 228:243-258, 2018).