Controlling Factors of Water Chemistry along the Zacate Creek, an Urban Tributary of the Rio Grande in Texas

Stream water samples were collected along Zacate Creek, a small urban tributary of the Rio Grande in Laredo, South Texas, to investigate the spatiotemporal variability and controlling factors of water quality across the drainage network. The hydrochemical composition was dominated by sodium among cations and sulfate among anions, with major ions occurring in the order Na⁺ ？ Ca²⁺ > Mg²⁺ ≈ K⁺ and ${\text{SO}}_4^{2？}$ ？ Cl？ > ${\text{HCO}}_3^{？}$ ？ ${\text{NO}}_3^{？}$ ？ ${\text{PO}}_4^{3？}$ . Marine aerosols were identified as a significant source of sodium (49%) and magnesium (44%), while contributing only minor proportions of sulfate, calcium, and potassium. In addition to atmospheric inputs, chemical weathering primarily of carbonate and aluminosilicate minerals plays a key role in regulating the overall water chemistry of Zacate Creek. The results indicate strong seasonal controls on water chemistry, whereas spatial variations along the drainage network were comparatively minor. Magnesium, nitrate, bicarbonate, hardness, and alkalinity displayed pronounced winter-summer differences, reflecting higher ionic strength and nutrient concentrations during winter conditions. Bivariate correlation and regression analyses showed strong linear relationships (|r| ≥ 0.85) among major ions, electrical conductivity, total dissolved solids, hardness, sodium adsorption ratio, and boron, indicating that dissolved ionic constituents exert dominant control over creek hydrochemistry. Multivariate modeling further confirmed that water chemistry is governed by integrated ionic, carbonate, and salinity-driven processes on selected parameters and exceptionally high predictive performance (R² ≈ 1.0). Sodium and