Groundwater as a limited carbon dioxide source in a large river (the Yangtze River)

Groundwater discharge to river networks makes up a major source of riverine CO2 emission, available evidence however comes mainly from headwater streams which are directly connected to terrestrial ecosystems and spatially limited in terms of system size. Here relying on coupled water and CO2 mass balances, we quantified the groundwater-mediated CO2 input to the Yangtze River mainstem on an annual basis, where the mass balance of water provided physical constraints on CO2 exchange between the river and groundwater. A landscape topographic control of the groundwater-river interaction was proposed where mountain reaches preferentially receive water and CO2 discharge from the groundwater while plain alluvial reaches predominantly lose water to the aquifers. Groundwater CO2 inputs were however small in magnitude on all reaches (0.3-14% of the total CO2 emission and transport by the river) and unable to account for the discrepancy between surface evasion and internalmetabolismin the river. Minor direct groundwater discharge to the reaches in comparison to smaller streams (negative to < 3.5% of the surface water flows) was concluded to be the main reason for low groundwater-sourced CO2 in the large river reaches. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Groundwater discharge to river networks is a major source of riverine CO2 emission. Through water and CO2 mass balances, it was found that mountain reaches preferentially receive groundwater discharge while alluvial reaches lose water to aquifers. However, the magnitude of groundwater CO2 inputs is small, unable to account for the discrepancy between surface evasion and internal metabolism in the river.