Role of humic substances as refractory organic matter in sustainable water quality assessment and management

In northern China, river water originating from or flowing through forests often contains large amounts of oxygen-consuming organic substances, mainly humic substances. These substances are stable and not easily biodegradable, resulting in very high detection values of chemical oxygen demand. However, under natural conditions, the dissolved oxygen demand is not as high. Using experimental values to evaluate river water quality and the impact of human activities on water quality is thus unscientific and does not meet national development goals. In this study, the potential sources of high-concentration chemical oxygen demand in river water in two areas exposed to virtually no anthropogenic activities and strongly affected by humic substances, were analysed. The chemical oxygen demand contributed by humic substances (COD-HSs) was quantified using three methods. The results of water quality monitoring in 2017 and 2018 revealed that the chemical oxygen demand concentrations (5-44 mg/L) predominantly exceeded the standard (15 mg/L). The study results suggest that COD-HSs should be considered separately for objective evaluation and management of water quality, particularly in areas that are seriously affected by COD-HSs, to provide a scientific basis for formulating sustainable water quality management policies.