Impacts of riverine pollution on greenhouse gas emissions: A comprehensive review

Many recent studies have revealed that polluted rivers are emerging as potent emitters of greenhouse gases (GHGs). However, the scientific understanding of GHG emissions is still limited. The present review aims to provide a systematic analysis of GHG emissions from polluted rivers in various eco-regions. The analysis highlighted that high nutrient loadings such as total nitrogen (TN), organic carbon (OC), total phosphorous (TP), and the anoxic condition of rivers are the major influencing factors for CO2, N2O, and CH4 emissions. Moreover, a strong positive correlation of trophic level index (TLI) with CH4 and CO2 fluxes was observed. The state-of-art suggested that based on land use and water quality, GHG fluxes from different rivers vary considerably. For instance, the reported N2O fluxes ranged between 0.007 and 3.721 mmol m-2d-1 for urban-impacted rivers and 0.009–0.049 mmol m-2d-1 for agricultural rivers. The CH4 fluxes ranged between 0.001 and 458 mmol m-2d-1, 0.05–1.02 mmol m-2d-1, and 1.40–14.30 mmol m-2d-1 for urban impacted, agricultural and natural rivers, respectively. Further reported CO2 fluxes range between 19.94 and 895 mmol m-2d-1 for urban-impacted rivers. Overall the in-depth analysis of the existing state of the art revealed that water pollution (i.e. eutrophication) enhances the GHG emissions from rivers; therefore, incorporating effective restoration measures can mitigate the intensity of GHG emissions from rivers and assist in achieving sustainable development goals (SDGs).