Hydrochemical response of urban Himalayan rivers to land use change and water quality degradation

Urban Himalayan tributaries are increasingly exposed to land-use transformation, drain-mediated pollutant loading, and seasonal hydrologic stress, yet integrated frameworks linking spatial land dynamics to multi-season water quality degradation remain limited. This study quantifies spatio-temporal degradation and process controls in the Rispana and Bindal rivers (Dehradun, Uttarakhand, India) using a coupled hydrochemical GIS statistical approach. Seasonal monitoring (post-monsoon, winter, summer, monsoon; October 2021–October 2023) was conducted across seven stations (LR1–LR4; LB1–LB3) for physicochemical, ionic, organic, and microbial indicators, followed by statistical validation (one-way ANOVA, Pearson correlation, hierarchical clustering), multivariate time-series decomposition using Dynamic Factor Analysis (DFA), and index-based diagnosis using Weighted Arithmetic WQI (WAWQI), CCME WQI, Horton WQI, and Oregon WQI to evaluate sensitivity to drain-dominated urban stressors. Results reveal strong downstream deterioration with persistent hotspot formation in urban reaches (LR4; LB2–LB3), where WAWQI frequently exceeded the “unsuitable” threshold, reaching extreme values during low-flow seasons (e.g., LB3 ~ 272 in summer and ~ 269 in monsoon). Degradation was characterised by concurrent dissolved oxygen depletion (< 4 mg/L) and elevated organic loading (high BOD/COD), indicating dominance of sewage-driven oxygen-demanding processes under reduced assimilative capacity. DFA resolved a limited set of latent dynamic factors, separating organic–microbial forcing from ionic/mineralisation control and capturing seasonal concentration–dilution behaviour. GIS-based LULC assessment confirmed intensive urban expansion within river corridors, while the Dynamic LULC Impact Matrix (DLIM) quantified net degradation for both rivers (Rispana ≈ − 1.46; Bindal ≈ − 1.15). Markov projections indicate that continued urban expansion is likely to maintain pressure on river water quality unless appropriate management measures are implemented. The integrated framework presented in this study provides a practical approach for understanding present degradation and supporting future management of urban rivers in rapidly developing Himalayan regions.