Wastewater Treatment Plant Effluent Weakens Alkalinity Buffering and Enhances CO ₂ Evasion in a Carbonate-Weathering-Dominated River

High Resolution Image Download MS PowerPoint Slide Carbonate-weathering-dominated rivers typically exhibit high total alkalinity (TA), but wastewater treatment plant (WWTP) effluent is an increasingly important anthropogenic driver that can alter riverine chemistry and carbonate-system dynamics. Its overall impact, however, remains poorly constrained. We investigated the Yanshui River in southwestern Taiwan, which receives substantial WWTP discharge from a major semiconductor park, with effluent inputs exceeding river flow by ∼1.5-fold. Time-series observations spanning dry to wet seasons show a significant shift in dissolved loads from carbonate weathering-dominated headwaters (Ca 2+ -HCO 3 – ) to WWTP-driven salinization in urban reaches (Na + -Cl – ). Upstream waters enriched in dissolved inorganic carbon (DIC) and TA are substantially diluted by effluent characterized by low DIC, TA, and pH. This mixing, together with in-stream organic matter decomposition, increases downstream DIC/TA ratios, lowers pH, and elevates p CO 2 . Thermodynamic modeling indicates that WWTP dilution reduces headwater TA by approximately 2.5-fold and lowers buffering capacity by nearly 45%, thereby enhancing acidification and CO 2 evasion. These results identify effluent alkalinity as a key control on downstream carbonate chemistry and a practical lever for mitigating anthropogenic CO 2 emissions from rivers.