Riverine Hydrochemical Characteristics of a Typical Karst Urban Watershed: Major Ion Compositions, Sources, Assessment, and Historical Evolution

Abstract

Urban watersheds suffer from great pressure concerning poor water carrying capacity in the karst area with a fragile ecology system and urbanization disturbance. The hydrochemistry of karst urban rivers plays a vital role in the maintenance of water ecology. This study determined the sources of major ions (Na+, K+, Mg2+, Ca2+, F–, Cl–, NO3–, SO42–, and HCO3–) in the Nanming urban watershed. The ion concentrations were dominated by the HCO3–-Ca2+·Mg2+ type and complied with the compoment characteristics of major ions in karst rivers with the order of Ca2+ (mean: 83.1 mg/L) > Mg2+ (mean: 17.6 mg/L) > Na2+ (mean: 15.2 mg/L) > K+ (mean: 6.7 mg/L) and HCO3– (mean: 252.7 mg/L) > SO42– (mean: 101.5 mg/L) > Cl– (mean: 17.8 mg/L) > NO3– (mean: 17.7 mg/L). The chemical composition of the Nanming River was possibly affected by carbonate weathering (Ca2+, Mg2+, and HCO3–) and anthropogenic inputs (SO42–, NO3–, Cl–, Na+, and K+) through methods of principal component analysis (PCA) and ion ratios. The irrigation water quality and health risk were evaluated by the calculation of sodium adsorption ratio (SAR), soluble sodium percentage (Na%), residual sodium carbonate (RSC), and hazard quotient (HQ) and the comparison of corresponding guidelines. The results showed that the Nanming River was marginally suitable for agricultural irrigation but dangerous for direct drinking intake, and the non-carcinogenic health risk of riverine ions should also be of concern. The control of urban wastewater discharge should be paid more attention to improve the water quality in such an urban watershed.