Water quality variability in the middle and down streams of Han River under the influence of the Middle Route of South-North Water diversion project, China

Abstract

The middle and down streams of Han River are complex river systems influenced by hydrologic variations, population distributions, and the engineering projects. The Middle Route of China’s South-to-North Water Transfer (MSNW) project planned to transfer 95 billion m3 annually from Han River to north China. The operation of the MSNW project may alter the flow rate and further influence the water quality of Han River. This study used min/max autocorrelation factor analysis (MAFA) and dynamic factor analysis (DFA) to analyze spatio-temporal variations of the water quality variables in three typical tributary-mainstream intersection zones in Han River from June 2014 to April 2017. MAFA results showed that chlorophyll-a (Chl-a), chemical oxygen demand (COD), suspend solid (SS) and phosphate (PO43−) (represented as trophic dynamics) are main concerned water quality variables in densely populated zones (Zones 1 and 3), and total nitrogen (TN), nitrate nitrogen (NO3−), COD, and PO43− (regarded as nutrient formations dynamics) represent the underlying water quality variations in agricultural cultivation zone (Zone 2). DFA results indicated that domestic and municipal effluent pollutants influence the organic concentrations and nutrient formations in the mainstream in Zones 1 and 2. The non-point source nitrogen and phosphorus discharged from the tributaries Tangbai and Hanbei Rivers elevate the nutrient concentrations and increase Chl-a concentration (i.e. promote the algal growth) in densely populated zones. In addition, controlling the flow rates in low and middle flow rate conditions can avoid degrading water quality. The flow rate should be elevated to more than 700 cm (cubic meters per second) in the middle stream and to more than 800 cm in the downstream of Han River for preventing water quality deterioration from high loadings of organic pollutants and nutrients. The integrated MAFA and DFA method establishes an efficient analysis distinguishing spatio-temporal variation of water quality variables and provides useful site-specific management to control water quality in various flow conditions.