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

Estimating riverine carbon dioxide (CO₂) emissions has been constrained by lacking field measurements of the partial pressure of CO₂ (pCO₂) and inaccuracies in calculating pCO₂ using carbonate equilibria-based models such as CO2SYS. To evaluate potential errors in applying the carbonate equilibria-based pCO₂ calculation to river systems affected by monsoon rainfall and water pollution, we compared pCO₂ values calculated using CO2SYS and those measured by headspace equilibration in five Asian rivers (Ganges, Mekong, Yangtze, Yellow, and Han rivers) undergoing various water pollution stages. Across the five rivers, calculated and measured pCO₂ values exhibited larger discrepancies during the monsoon season, particularly in the low pH range, while in the Han River mismatches were also noticeable during the dry season. In the Han River, pH was negatively correlated with dissolved organic carbon (DOC) during the monsoon, indicating organic acids flushed from soils during rainfalls as a key factor for overestimated pCO₂ at sites with low pH and alkalinity, whereas dry-season overestimation of pCO₂ may be ascribed to non-carbonate alkalinity including organic acids and inorganic anions delivered by wastewater effluents or sporadic rainfalls. The four large rivers exhibited a positive correlation between pH and DOC in tributaries during the monsoon season, indicating that DOC flushed from soils may be diluted by monsoonal floods to such a degree as to exert little influence on pH and hence pCO₂. Therefore, the monsoonal overestimation of pCO₂ at sites with low pH and alkalinity warrants further investigation of other factors than non-carbonate alkalinity to explain the increased sensitivity of pCO₂ to subtle changes in acidity and buffering. These results illustrate the importance of direct measurements of pCO₂ in highly polluted rivers, especially during the monsoon season. For river systems lacking pCO₂ measurements, we suggest that carbonate equilibria-based models be complemented with corrective measures: 1) presenting pCO₂ values calculated from low pH values (pH < 6.5 for monsoon and pH < 6.3 for dry season) together with the pH range to warn potential overestimation; 2) using pre-established regressions between measured pCO₂ and environmental variables to correct pCO₂ values, particularly during wet periods when large changes in pH and acid buffering are expected.