Pollution dynamics in the lower Bengal Delta of Bangladesh

Understanding pollution dynamics in tropical coastal rivers and groundwater is critical due to the complex landscape interface between terrestrial, mangrove and marine processes, especially in the lower Bengal Delta of Bangladesh. We are conducting hydrogeochemical statistics by combining in-situ and ex-situ hydrochemical data at high and low tide to estimate pollution dynamics in 90 km long river networks of three major rivers along with 30 groundwater wells in the catchments. We used the Hydrochemical Facies Evolution Diagram (HFED) to assess seawater intrusion and freshening stages with dominant ions (cation and anion) visualised through Piper Diagram. Results showed that river water in both high and low tide and groundwater chemistry is dominated by Na⁺ and Cl^- during the dry season, with all river water samples (100%) plotted in the seawater intrusion zone (Na-Cl).  In contrast, 50% of groundwater chemistry is dominated by Na⁺ and Cl^- and plotted in the seawater intrusion zone (Na-Cl), whereas 23.33% is dominated by Ca²⁺ + Na⁺ and HCO₃^- and plotted in the mixed zone, 23.33% is dominated by Ca²⁺ + Mg²⁺ and HCO₃^-, and plotted in the temporary hardness zone, and lastly 3.34% is dominated by Na⁺ + K⁺ and HCO₃^- + CO₃^2- and plotted in the alkali carbonate zone. Furthermore, Ca²⁺, Na⁺, and HCO₃^- ions indicated that groundwater chemistry is significantly influenced by rock weathering processes, which is also evident in the Gibbs diagram. Regression analysis illustrated significant positive relationship (r²) between Na⁺ and Cl^- in river (r² = 0.99) and groundwater (r² = 0.93). Further, box and whisker plots illustrated variation in ions’ concentration in three rivers, where 2^nd river has higher variation compared to the 1^st river (upstream) and 3^rd river (downstream). These variations align with diverse land use patterns along 2^nd riverbank including mega coastal city, industries, food processing facilities, agriculture, and aquaculture. The degree of pollution including nutrient parameters such as NO₃^- indicated high pollution levels in river catchments which ranged from 95.6% to 99%. Downstream groundwater samples showed higher pollution levels (89.3%) compared to upstream groundwater (3.4%). Possible reasons for increasing water pollution include variations in freshwater flow associated with precipitation and temperature patterns. Additionally, diverse land use patterns from upstream to downstream in the river catchments have significant impact on water pollution levels and are considered as an important anthropogenic pollution source. The research offers a novel approach for providing in-depth pollution characterization through ion concentration analysis, which aids regional-scale water quality management in the lower Bengal Delta of Bangladesh.