pCO2 and CO2 evasion from two small suburban rivers: Implications of the watershed urbanization process

The Impacts of Nitrogen Pollution and Urbanization on the Carbon Dioxide Emission from Sewage-Draining River Networks

“…Rural rivers generally accepting a large amount of domestic/industrial sewage have overwhelmingly altered aquatic ecosystems, while urban rivers could be considered as landscape waters with drastic disturbance by human activities. Higher p CO 2 values were generally accompanied with the low pH and high DIC in direct sewage-draining waters [ 11 , 25 ]. In this study, sampling sites in sewage-draining waters in Longfeng River (R8 and R9), Beijing-drainage River (R20), and Beitang-drainage River (R45) all presented higher p CO 2 (approximately 4 times that of averaged p CO 2 in worldwide rivers, i.e., 3100 μatm) [ 5 ] with lower pH (ranged from 7.29 to 7.85) and higher DIC (ranged from 4.05 to 6.85 mol·L − 1 ) when compared with other waters in this study.…”

“…In this study, pH presented negative correlations with DIC in all the river waters (including lake waters and river waters) (R = −0.38, p < 0.01) including Haihe River (R = −0.78, p < 0.01) ( Figure 6 ), revealing the potential contributions of photosynthetic processes to DIC uptake and CO 2 consumption in water, but they have obviously been affected by other processes and influencing factors [ 13 ]. Specifically, most of the urban rivers and lakes have very low water flow and long residence time, providing ideal static conditions for biogeochemical carbon transformations, including photosynthesis, respiration, and organic matter degradation [ 25 ]. On the one hand, in rural rivers and sewage rivers, frequent runoff processes in the rainy season during the sampling period transferred CO 2 from the soil aeration zone to river water, increasing the p CO 2 levels in the rural river water; on the other hand, a large amount of organic carbon/nitrogen input from drainage waters would promote CO 2 production by stimulating the organic matter degradation and biological respiration in sewage waters.…”

“…The principal buffer mechanism in a freshwater system is carbonate equilibrium. H 2 CO 3 , HCO 3 − , CO 3 2− , and aqueous CO 2 make up the total dissolved inorganic carbon (DIC) in water, which is affected by pH, water temperature, and ionic strength [ 24 , 25 ]. Previous studies have reported that the alkalinity–pH–temperature method might overestimate the values of p CO 2 at low pH, while they could be accurately described with pH > 7.2 and total alkalinity > 1 mmol·L −1 [ 25 , 26 ].…”

“…H 2 CO 3 , HCO 3 − , CO 3 2− , and aqueous CO 2 make up the total dissolved inorganic carbon (DIC) in water, which is affected by pH, water temperature, and ionic strength [ 24 , 25 ]. Previous studies have reported that the alkalinity–pH–temperature method might overestimate the values of p CO 2 at low pH, while they could be accurately described with pH > 7.2 and total alkalinity > 1 mmol·L −1 [ 25 , 26 ]. Based on the pH (ranging from 7.29 to 10.41) and alkalinity (exceeding 1.43 mmol·L −1 ) in water samples in this study area, the alkalinity–pH–temperature method is employed to calculate p CO 2 .…”

“…Among these, k is uncertainty that should be cautiously confirmed by specific environmental conditions. Researchers have made great efforts on combining multiple factors (such as the wind speed, temperature, stream slope, discharge, and depth) with estimation models that have been widely employed in previous studies to optimize the calculation model of gas transfer velocity [ 25 , 27 , 28 ]. Wind speed is a primary cause of turbulence in low-gradient river systems that strongly relate to k 600 [ 7 , 29 ].…”