Evaluating total organic carbon as an indicator for organic pollutant management in the marine environment: A case study on wastewater treatment plant effluent input into the coastal ocean

South Korea recently shifted its assessment indicator for organic matter in terrestrial environments from chemical oxygen demand (COD) to total organic carbon (TOC) due to the increase in refractory organic carbon levels. However, in the marine environment, where the inflow of refractory organic matter is also on the rise, COD is still used in some instances to assess organic pollution in contaminated areas. Our findings reveal that the main source of dissolved organic carbon (DOC) is terrestrial-derived refractory organic carbon, which enters through nearby wastewater treatment plant (WWPT) outlets. The low oxidation efficiency of COD to TOC (approximately 4 %) prevents it from being an accurate measure of terrestrial-derived refractory DOC. Contrasting results were observed when comparing the organic pollution index (OPI), which we calculated using TOC, with the currently employed water quality index (WQI) for ocean water quality evaluation, particularly in areas influenced by WWPTs. This discrepancy arises because the WQI primarily evaluates autochthonous organic carbon through chlorophyll measurements, whereas the OPI incorporates both autochthonous and allochthonous organic carbon through TOC measurements. Our findings demonstrate that TOC can effectively replace COD as an organic pollution indicator in marine environments.