Spatiotemporal analysis and prediction of water quality in the Han River by an integrated nonparametric diagnosis approach

The causes of water environmental degradation in river influenced by intensified human activities are very complex. As an important water source of the middle route of the South-to-North Water Diversion Project in China, the water security of Han River and its related water quality have become a national concern. Here, we conducted spatiotemporal analysis and prediction of the water quality in the middle-lower reaches of the Han River by utilizing an integrated nonparametric diagnosis approach. System models including a Seasonal and Trend decomposition using Loess (STL), Mann-Kendall (MK), Redundancy Analysis (RDA), Coefficient of Divergence (CDiv) and a Long Short-Term Memory (LSTM) model. Results from February to April of 2004-2018 indicated that, the average concentration of total phosphorus (TP) at the Zongguan Station was 0.107 +/- 0.012 mg/L, while the total nitrogen (TN) was 1.945 +/- 0.116 mg/L. The STL and MK analysis showed no significant trend for TP but an upward tendency for TN at the Zongguan station. The variation of TP and TN concentration in 2008 was mainly related to the most serious algal bloom event, while the tipping points in 2014 and 2016 were attributed to the abundant precipitation. Significant spatial distribution were obtained based on the CDiv and RDA methods; four categories were clustered and water quality was deteriorated gradually from upstream to downstream. The future prediction of monthly averaged TN concentrations during the period of 2019-2025 exceeded the limit of the environmental water quality standard (GB 3838-2002) (Class III, TN = 1.0 mg/L), leading to potential water quality risk in the Han River during 2019-2025 under strong effect of human activity. Under the worsening trends for the concentrations of TN and TP, the local departments should strictly control the amount of sewage and wastewater discharged from various industries, especially for phosphate fertilizer and rock enterprises, agriculture and aquaculture. This study expected to provide technical means for the follow-up pollution control and is useful for the sustainable improvement of river ecological environment.