Process-based quantitative description of carbon biogeochemical cycle in a reclaimed water intake area

Reclaimed water plays a pivotal role in addressing water scarcity and pollution. The carbon (C) cycle significantly impacts aquatic ecosystems and water quality, yet the C biogeochemical cycle in nutrient-rich reclaimed water remains enigmatic. This study focuses on reclaimed water, developing a conceptual biogeochemical mass balance model to examine C cycling and assess the C budget in the highly eutrophic Jian and Chaobai rivers. Furthermore, it quantifies the complex biogeochemical interactions among reclaimed water, atmosphere, fluvial sediments, and phytoplankton. The findings elucidate that phytoplankton photosynthesis and the decomposition of sedimentary organic matter (SOM) emerge as the dominant C biogeochemical processes. The phytoplankton photosynthesis fixes approximately 342.24 t of C, while 246.21 t of C are emitted as CO₂ and CH₄ through SOM degradation during the study period. Additionally, damming intensifies C biogeochemical processes. In the Chaobai River, the robust phytoplankton photosynthesis leads to the absorption of CO₂ from the atmosphere, transforming the reclaimed water into a C sink, sequestering 20.78 t of C. The rubber dam also amplifies carbonate precipitation and SOM decomposition. These findings shed light on the role of reclaimed water ecosystems in the C cycle and provide valuable insights for effective reclaimed water quality management and control.