Assam's Digaru River under threat: Integrated water quality and human health risk assessment of an industry-laden system through the lens of multivariate statistics and water quality indexing.

River contamination in India has become a critical issue owing to gross mismanagement of resources, poor planning, and limited understanding of contaminant dynamics. In this context, we conducted a systematic yet comprehensive study of the Digaru River, a tributary of the Brahmaputra River, which has been subjected to untreated municipal and industrial discharges for many decades. We analysed thirty physicochemical water quality parameters through periodic monitoring, focusing on identifying pollution sources and contaminant correlations using multivariate statistics. To assess river health, we employed a novel Modified Entropy-Weighted Water Quality Index (MEWQI) and assessed human health risks associated with it. Based on 144 samples collected over four seasons, the study highlighted strong spatiotemporal variability and limited self-replenishment, with monsoon months showing significantly elevated pollutants, including turbidity (> 500 NTU, mean: 56 ± 104), BOD5 (~ 47 mg/L), COD (~ 290 mg/L, mean: 111 ± 85), and NO3- (~ 56 mg/L, mean: 07 ± 10). Among metals, Al (81%), Mn (69%), Fe (97%), and Pb (20%) exceeded permissible limits, while Cr, Ni, Co, Zn, and Pb were regularly detected. The MEWQI revealed that 93.75% of the samples were unsuitable for consumption. PCA bi-plots and HCA showed complex seasonal groupings of physicochemical parameters and stations, indicating a decline in water quality due to sewage and industrial effluents. Carcinogenic and non-carcinogenic risks associated with the water samples indicated higher risks for children compared to adults, driven by Fe, Mn, and Cr. Collectively, the study offers vital insights into spatiotemporal pollution dynamics, providing a scientific basis for targeted water quality control and remediation strategies.