Quantifying Effluent Contributions to Dissolved Ions and Discharge in a Karst River: Insights From a 12‐Month Continuous Observation

Effluents of wastewater treatment plants (WWTPs) have increasingly recognised as an important component of river flow and biogeochemical conditions; yet, its influence on ionic composition and dynamics in riverine systems remains poorly understood. This study monitored changes in major dissolved ions in a karst river downstream of WWTPs to evaluate the hydrochemical contribution of effluent. We found that effluent substantially increased the concentrations of Na + , K + and Cl − in the receiving river reach, while declining the proportions of Ca 2+ , Mg 2+ and HCO 3 − . Seasonal variations in the major ion concentrations were closely associated with rainfall and river discharge. On a monthly basis, the effluent contributions varied largely, ranging from less than 10% to over 70%, reflected by hydrological conditions. Conservative estimation showed that effluent‐derived dissolved inorganic carbon (DIC), with HCO 3 − as the dominant form, accounted for approximately 52% and 57% of the riverine total DIC lateral export at the two monitoring locations. The high effluent‐derived CO 2 fraction (often > 100%) further implied the potential for carbon emission, especially under low‐flow conditions. These findings highlight that effluent‐fed baseflow can play an increasing role in river discharge, hydrochemistry and carbon dynamics, especially in regions with intense urbanisation and hydrological regulation.