Urban Wastewater and River Health in Bangladesh: A Systematic Evidence Map Revealing a Dhaka Bias and a Neglected Barnoi River

Background: Urban and industrial wastewater discharge has contributed to river pollution in Bangladesh, raising concerns about water quality and ecosystem health. However, existing research remains unevenly distributed across geographical regions and environmental compartments, particularly in rivers associated with secondary cities. Objective: To map primary field studies of effluent-affected rivers by region, sampled compartment and pollutant group, with particular attention to the Barnoi River near Rajshahi. Study Design: Systematic evidence map, reported in line with PRISMA 2020 and ROSES. Methodology: DOAJ and PubMed were searched, and 547 unique records were screened against predefined criteria; of these, 204 primary field studies were included and coded by region, compartment sampled and pollutant group; 21 reviews were kept for comparison. The completeness of reporting on sampling design and analysis in each abstract was appraised with a seven-item checklist; this is a reporting appraisal, not a validated risk-of-bias assessment. Results: Rivers of the Dhaka region accounted for 90 studies (44%), and Dhaka, Chattogram and Khulna together for 72%; Rajshahi and the north-west accounted for 12 (6%). Water was sampled in 70% of studies and sediment in 31%, but fish in 20%, other aquatic life in 6%, soil or crops in 5% and people in 2%. Metals dominated (63%), and microplastic studies rose from none before 2020 to 34. Of the 189 studies with an abstract (15 had none and could not be appraised), reporting completeness was high in 30%, moderate in 50% and low in 21%; only 14% of these abstracts reported the number of samples. The Barnoi River, which has received the wastewater of Rajshahi since 2008, had three studies against 42 naming the Buriganga, and none examined its sediment, plankton, bottom-dwelling animals, riverbank soil, crops or riverside communities. Conclusion: Current knowledge describes the water chemistry of a few industrial rivers near the capital far better than the ecological and human consequences of effluent elsewhere. Baseline studies that follow effluent from the drain to the community in secondary cities, with fuller reporting of sampling design, are the clearest priority. Because only two databases were searched and coding was based on titles and abstracts, the counts are minimum values.