A holistic water quality risk assessment and key parameter identification approach coupling comparative water quality index (WQI) models: a six-year monitoring study in Danjiangkou Reservoir, China

The Danjiangkou Reservoir (DJKR), the source of the world's longest inter-basin water diversion, the Middle Route of the South-to-North Water Diversion Project (MRSNWDPC), has attracted global attention for its water quality. This study collected water samples monthly from 31 monitoring sites between January 2017 and December 2022, assessing 18 water quality parameters. Multivariate statistical methods and five water quality index (WQI) models were employed to comprehensively evaluate water quality, optimizing critical parameter identification and enhancing the predictive reliability of water quality changes. Compared to traditional WQI approaches, our method offers improved accuracy in assessing water quality variations and risks. The findings reveal that the tributary river systems were classified by pollution degree, while reservoirs were categorized based on adjacent geographical locations. WQI values in the Danjiangkou Reservoir basin ranged from 82.70 to 91.24, indicating that the water quality was consistently "good" to "excellent". Key water quality indicators, i.e., ammonia nitrogen (NH3-N), permanganate index (CODMn), dissolved oxygen (DO), chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN), and heavy metals, i.e., selenium (Se), copper (Cu), mercury (Hg), and cadmium (Cd), were used to construct WQI(min) models for different clusters. These models demonstrated explanatory solid power for evaluating water quality (R-2 > 0.9). Four weighting methods-equal weight, rank sum, rank reciprocal, and rank order centroid-were applied to determine the WQI(min) and WQI models. The results identified equal weight as the optimal WQI(min) model construction method. The carcinogenic risks of chromium (Cr) and arsenic (As) ranged from 4.03 x 10(-5) to 4.66 x 10(-5), within the acceptable level of 1 x 10(-4) to 1 x 10(-6). The hazard quotients for all heavy metals were below "1", and non-carcinogenic risks were higher for children than for adults. Overall, the heavy metals in DJKR presented no significant human health risks. After the MRSNWDPC became fully operational, the DJKR maintained excellent water quality, with the reservoir's water deemed safe for drinking and other uses. This study provides a scientific basis for managing water quality in DJKR and other large-scale hydroengineering projects.