Localized Pollution Impacts on Greenhouse Gas Dynamics in Three Anthropogenically Modified Asian River Systems

Abstract

Despite growing research on greenhouse gas (GHG) emissions from inland waters, few systematic efforts have been made to assess the regional-scale GHG emissions from Asian rivers under increasing anthropogenic stress. We examined factors controlling longitudinal and seasonal variations in the partial pressure of CO 2 (pCO 2 ), and CH 4 and N 2 O concentrations in the Ganges, Mekong, and Yellow River by simultaneously measuring gas concentrations and stable C isotopes, and optical properties of dissolved organic matter (DOM) from 2016 to 2019. The levels of pCO 2 and CH 4 were distinctively higher in polluted tributaries and affected reaches of the Ganges and Mekong than in the Yellow River. The highest levels of N 2 O were found in the Ganges, followed by the Yellow River and Mekong. Across these basins, dry-season mean concentrations of CO 2 , CH 4 , and N 2 O were 1.6, 2, and 7 times higher than those measured in the monsoon season, respectively. This seasonality was consistent with that of δ 13 C-CO 2 , while δ 13 C-CH 4 showed an opposite pattern. GHG concentrations exhibited significant positive relationships with DOM concentrations and optical properties including fluorescence index and proteinlike fluorescence, implying the contribution of anthropogenic, labile DOM to production of GHGs in the polluted reaches. Graphical mixing models of δ 13 C-CO 2 and δ 13 C-CH 4 support the stronger impact of wastewater on the Ganges and Mekong than on the Yellow River. The overall results suggest that neglecting localized pollution impacts on GHG emissions from increasingly urbanized river basins can result in a substantial underestimation of global riverine GHG emissions.Plain Language Summary Rapid urbanization and poor wastewater infrastructure are turning many large rivers in Asia into significant sources of greenhouse gases (GHGs). We examined controlling factors for the concentrations of CO 2 , CH 4 , and N 2 O in the Ganges, Mekong, and Yellow River by simultaneously measuring gas concentrations and stable carbon isotopes, and optical properties of dissolved organic matter (DOM). The concentrations of CO 2 and CH 4 were distinctively higher in polluted tributaries and affected sections of the Ganges and Mekong than in the Yellow River. N 2 O concentration was highest in the Ganges, followed by the Yellow River and Mekong. Across these basins, the concentrations of the three gases were generally higher during dry periods than in the monsoon season. GHG concentrations were positively related to DOM optical properties indicating the contribution of easily degradable DOM from pollution sources. The stable carbon isotope ratios of CO 2 and CH 4 support the stronger impact of wastewater on the Ganges and Mekong than on the Yellow River. The overall results suggest that neglecting local pollution impacts on GHG emissions from increasingly urbanized river basins can result in a substantial underestimation of global riverine GHG emissions. BEGUM ET AL.

Improving Carbonate Equilibria-Based Estimation of pCO2 in Anthropogenically Impacted River Systems

“…The data of pCO 2 measured in the Ganges, Mekong, and Yellow River have been reported by Begum et al (2021). pCO 2 data measured in the Wuding River, a Yellow River tributary, have been reported by Ran et al (2017b), while measurements in the Yangtze River in 2018 have not yet been published.…”

“…While pCO 2 data measured in the Han River from 2014 to 2017 have been reported by Jin et al (2018), more recent measurements from 2018 to 2020 have not yet been published. Field surveys in the Ganges, Mekong, and Yellow River from 2016 to 2019 included concomitant measurements of water temperature, pH, TA, pCO 2 , and DOC (Begum et al, 2021), while field measurements in the Yangtze, Wuding, and Han River from 2014 to 2020 included simultaneous measurements of water temperature, pH, TA, pCO 2 , and DOC.…”

Seasonal shifts in diurnal variations of pCO2 and O2 in the lower Ganges River

“…While sewage drains in India and Bangladesh can also release large amounts labile OM to the Ganges, CO 2 depleted in 13 C (Fig. 1) compared to the 13 C-CO 2 measured in sewage drains in the Ganges Basin during the monsoon (À18.3‰ to À14.6‰) indicate a rather minor contribution or slower transformation of the sewage-derived OM than the natural OM flushed from soils (Begum et al 2021). Monsoonal increases in indices associated with terrestrial DOM, such as HIX, C1, and C2 (Fig.…”

“…While lowered dry-season pCO 2 may be common in impounded reaches of the Ganges, untreated wastewater along the urbanized reaches can create local hotspots for CO 2 by elevating supersaturation levels even during the dry season with high photosynthetic activity (Jin et al 2018;Begum et al 2021). Rising organic pollution in growing metropolitan areas across the basin can also increase monsoonal peaks in pCO 2 via discharges of CO 2 and labile OM from poorly treated wastewater (Park et al 2018;Begum et al 2021).…”

“…The filtered samples were analyzed for DOC concentration (TOC-V CPH ; Shimadzu), ultraviolet-visible (UV-Vis) absorbance (Agilent 8453), and fluorescence excitation-emission matrices (EEMs; F7000; Hitachi). Specific UV absorption at (Begum et al 2021), were calculated from the corrected EEMs: C1 (humic; ex/em 350/460 nm), C2 (microbial-humic; 305/400 nm), and C3 (protein; 275/345 nm; Figure S3). More analytical details are provided in Supporting Information (Text S3).…”