Importance of Ssurface water background values for objectively assessing water quality in a unique basin.

Ignoring differences in surface water background values of different basins is a prominent shortcoming in water resource assessment and management systems in China. This study described the impact of this shortcoming on the results of water quality assessments by analysing water quality and the intensity of anthropogenic activities in 10 water resource basins in China. In particular, the Songhuajiang River Basin had a relatively undegraded natural environment with minimal anthropogenic activity. However, water quality assessment results showed the lowest water quality, and this basin is the most seriously affected by background values. The Huma River source water reserve was selected to conduct research of background value identification, concentration characteristics, migration and transformation laws, and output mechanism characteristics to enable the acknowledgment of the severe background value problem. Most data collected in basins with minimal anthropogenic pollution failed to meet Chinese water quality standard requirements, and high background values mainly resulted from litter leachate and soil erosion by rainfall or snowmelt runoff. A revised water quality assessment method was proposed in view of the particularity and heterogeneity of background values in the Songhuajiang River Basin. This study therefore clarified the negative impact of background values on water quality assessments, so that these values can be properly considered in Chinese water quality assessment and management systems.