PS 51-178 - Identification of natural and anthropogenic sources of riverine DOC using dual carbon isotope analysis

Background/Question/Methods

Concentrations of riverine DOC ([DOC]) are rising in many parts of the world. Tracking the sources of DOC is critical not only to understand why riverine [DOC] increases but also to provide management options to improve water quality. We collected water samples in April, July, and September 2021, from upland forest streams to the mainstream Geumho River (GHR) of South Korea, with a variety of wastewater treatment plants (WWTP) effluents. We analyzed concentrations, optical properties, and dual carbon isotope ratios to identify the sources of riverine DOC.

Results/Conclusions

The riverine [DOC] ranged from 1.2 to 11.2 mg L^-1 except livestock WWTP effluent which was up to 408.1 mg L^-1, which became an input to another WWTP. The SUVA₂₅₄ ranged from 1.2 to 4.4 L mg-C^-1 m^-1, and dominant fluorescence components were terrestrial humic substances in upper reaches whereas protein-like materials in lower reaches of the GHR. The Δ¹⁴C-DOC ranged from -504.3‰ to 33.2‰, and δ¹³C-DOC from -28.6‰ to -16.0‰. Significantly lower Δ¹⁴C-DOC was observed in industrial (-372.6±29.2‰) and residential (-158.9±20.2‰) WWTP effluents due to high contribution of fossil-fuel derived organic carbon (OC). Livestock WWTP effluents had a higher δ¹³C-DOC (-19.4±0.2‰) than forested streams (-26.6±0.7‰) and GHR (-25.1±0.7‰), possibly due to the animal feed derived from C4 plants such as corn. Fossil-fuel derived OC contributed 35–56% of [DOC] from industrial WWTP effluents, whereas C4 plant-derived OC contributed about a half of [DOC] from a livestock WWTP effluent. In contrast, the DOC in the residential WWTP effluents, forest streams, and GHR was mainly derived from C3 plants (47–83%). The results suggest that dual carbon isotope analysis can be applied to quantify the contribution of the natural and fossil-fuel derived sources of riverine DOC, providing management options to improve water quality.