Development of water quality management strategies based on multi-scale field investigation of nitrogen distribution: a case study of Beiyun River, China.

Accurately quantifying the distribution of nitrogen (N) contaminants in a river ecosystem is an essential prerequisite for developing scientific water quality management strategy. In this study, we have conducted a series of field investigations along the Beiyun River to collect samples from multiple scales, including surface water, riverbed sediments, vadose zone, and aquifer, for evaluating the spatial distribution of N; besides, column simulation experiments were carried out to characterize the transport behavior of N in riverbed sediments. The surface water of the Beiyun River was detected to be eutrophic because of its elevated total N concentration, which is 33 times of the threshold value causing the potential eutrophication. The hydrodynamic dispersion coefficient (D) of riverbed sediments was estimated by CXTFIT 2.1, demonstrating that the D of upstream section was lower than that of midstream and downstream sections (D < D < D), with the estimated annual N leaching volume of 130,524, 241,776, and 269,808 L/(m·a), respectively. The average total N concentration in vadose zone and aquifer of upstream Sect. (297.88 mg/kg) was obviously lower than that of midstream Sect. (402.62 mg/kg) and downstream Sect. (447.02 mg/kg). Based on multi-scale investigation data, subsequently, water quality management strategies have been achieved, that is, limiting the discharge of N from the midstream and downstream banks to the river and setting up the impermeable layer in the downstream reaches to reduce infiltration. The findings of this study are of great significance for the improvement of river environmental quality and river management.