Seasonal Water Quality, Trace Element Concentrations, and Estuarine Salinity Dynamics in Two Urban Rivers of Panama with Contrasting Urbanization Levels

Urban rivers face increasing degradation from wastewater, runoff, and land-use change, yet multi-season tropical estuarine datasets remain scarce. This study compared physicochemical water quality, metals, and salinity dynamics over one year in the Juan Díaz and Pacora rivers, two contrastingly urbanized Panama Bay watersheds. Juan Díaz showed higher nutrient concentrations, lower dissolved oxygen, and greater variability than Pacora. Dissolved oxygen in Juan Díaz fell below 5 mg/L in five of nine campaigns and ammonia nitrogen exceeded 3.7 mg/L in all three dry-season campaigns, versus two of seven campaigns below 5 mg/L in Pacora. Total dissolved solids exceeded 500 mg/L in four of nine Juan Díaz campaigns, all in the wet season, but stayed compliant in Pacora. Total nitrogen and total phosphorus were up to 10-fold and seven-fold higher, respectively, in Juan Díaz during the dry season. Cu exceeded its USEPA criterion in two of three detections; Cd was detected once, in Juan Díaz, below its saltwater criterion (0.0079 mg/L). Salinity confirmed stronger tidal influence in Juan Díaz (up to 24.7 g/kg) than Pacora (0.03–2.74 g/kg). PCA separated the two rivers along a nutrient–oxygen gradient explaining 53.6% of variance. Overall, Juan Díaz shows greater degradation, while Pacora remains less contaminated but requires continued monitoring amid rapid urbanization.