Evaluation of river restoration efforts and a sharp decrease in surface runoff for water quality improvement in North China

Rapid urbanization, population growth, and other intensive human activities have greatly altered natural hydrological conditions and matter cycling, which are the main causes of water quality deterioration in North China’s rivers. With the help of a 15-year (2005–2019) dataset of river water quality (1043 records from nine sites), this study investigated the spatiotemporal water quality patterns in the Yongding River Basin (YRB) in North China using a new water quality index (WQI-DET), which has been customized for China’s water quality classification scheme. Our results showed that the river water quality of the YRB has significantly improved due to the decreased surface runoff and an abrupt change of WQI-DET was observed in 2011. The elimination of anoxic conditions and the mitigation of nitrogen and phosphorus resulting from the construction of wastewater treatment plants and the improvement of treatment capacity are the main reasons for the improvement in river water quality. We also found that eutrophication is still not completely eradicated because of the high concentrations of NH 4 + and total phosphorus. Our study suggests that for rivers in which runoff has decreased sharply, the water quality could be improved significantly by wastewater treatment facilities. At present, for the YRB, more effort is needed to eliminate eutrophication and dried-up river sections and thereby finally improve the river ecosystem. We concluded that more attention and effort should be given to river hydrological conditions, specific river ecological characteristics, and the increasingly important non-point source pollutants during the design of river restoration measures in North China.