CO₂ Outgassing from an Urbanized River System Fueled by Wastewater Treatment Plant Effluents

Continuous underway measurements were combined with a basin-scale survey to examine human impacts on CO₂ outgassing in a highly urbanized river system in Korea. While the partial pressure of CO₂ (pCO₂) was measured at 15 sites using syringe equilibration, 3 cruises employing an equilibrator were done along a 30 km transect in the Seoul metropolitan area. The basin-scale survey revealed longitudinal increases in surface water pCO₂ and dissolved organic carbon (DOC) in the downstream reach. Downstream increases in pCO₂, DOC, fluorescence index, and inorganic N and P reflected disproportionately large contributions from wastewater treatment plant (WWTP) effluents carried by major urban tributaries. Cruise transects exhibited strong localized peaks of pCO₂ up to 13 000 μatm and ¹³CO₂ enrichment along the confluences of tributaries at an average flow, whereas CO₂ pulses were dampened by increased flow during the monsoon period. Fluctuations in pCO₂ along the eutrophic reach downstream of the confluences reflected environmental controls on the balance between photosynthesis, biodegradation, and outgassing. The results underscore WWTP effluents as an anthropogenic source of nutrients, DOC, and CO₂ and their influences on algal blooms and associated C dynamics in eutrophic urbanized river systems, warranting further research on urbanization-induced perturbations to riverine metabolic processes and carbon fluxes.