The Santiago-Guadalajara River Basin has historically faced significant pollution pressures from urban, agricultural, and industrial sources. The development and implementation of targeted environmental policies require precise diagnostic tools. In this study, we aim to strengthen diagnostic power by recalibrating the Santiago River Water Quality Index by incorporating updated analytical limits of quantification, enhancing its reliability and sensitivity, particularly at low pollutant concentrations, and by developing three Pollution-Specific Water Quality Indices (PSWQIs), tailored to be sensitive to parameter suites associated with domestic, industrial, and agricultural sources. Spatial analyses showed that PSWQIs captured patterns consistent with land-use mosaics and previously documented trends, flagging areas of concern, including reaches with higher urban land use and sub-basins with extensive cropland and elevated nutrient concentrations. Temporally, the indices captured expected seasonal variations, including wet-season dilution and dry-season accumulation. By characterizing patterns associated with dominant pollution sources across the river basin, these targeted indices can significantly enhance the diagnostic precision available to policymakers, enabling the prioritization of infrastructure upgrades and regulations for agricultural and industrial activities.
