Water quality analysis and evaluation of irrigation applicability in Nandong underground river basin, Southwest China

The karst surface water and groundwater of Nandong underground river basin are vital water sources for agricultural and industrial production, and lives of residents in Honghe Hani and Yi Autonomous Prefecture in Yunnan, Southwest China. With a growing public awareness of water environment and water safety, the evaluation of karst water quality and irrigation applicability especially in agricultural regions is becoming increasingly necessary. In this study, 32 samples of karst surface river water and 24 samples of underground river water were collected from Nandong underground river basin during dry and rainy seasons. 11 inorganic regular ions (i.e., K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, ${\rm{SO}}_4^{2-}$, ${\rm{HCO}}_3^{-}$, ${\rm{NO}}_3^{-}$, NO$_2^{−}$, NH$_4^{+}$, and F⁻) and 9 heavy metals (i.e., Al, Cu, Pb, Zn, Cr, Cd, Ni, Mn, and As) were analyzed in these samples to investigate the water quality and irrigation applicability. Based on the measured concentrations of the hydrochemical composition, the Nemerow composite index in combination with the four irrigation assessment systems of sodium concentration (SC), sodium adsorption ratio (SAR), residual sodium carbonate (RSC), and permeability index (PI) were applied to evaluate the karst water quality and irrigation applicability, respectively. Results show that Ca²⁺ and ${\rm{HCO}}_3^{-}$ were identified as the major ions in both karst surface water and karst groundwater, indicating weakly alkaline karst water in the study area. For karst surface water, among the 11 inorganic regular ions, only NH$_4^{+}$ was found exceeding the maximum acceptable level for drinking water recommended by General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China. Meanwhile, among the 9 heavy metals, the concentrations of Al, Pb, Mn, and As were evaluated as exceeding the maximum acceptable level. Moreover, the rates of measured ions exceeding permitted levels in the dry season were generally higher than those in the rainy season. For karst groundwater, the concentrations of Al, Pb, Zn, Cr, Mn, and As exceeded the maximum acceptable levels, and the rates of these metals exceeding permitted levels in the rainy season were generally higher than those in dry season. The nitrogen synthetic fertilizers might be a main cause of the increase of NH$_4^{+}$ concentration in karst surface water. High concentration of Al, Mn, and Cd in karst water were mainly caused by large amounts of discharge from metal smelting, waste residue of mining, and open-pit mining. Furthermore, an exposed surface environment of bedrock in some parts of this study area might be responsible for these components exceeding the permitted levels due to the lack of surface decontamination of pollutants. The karst water quality belonging to the good grade or above accounted for 89.29% and 85.71% of the total water samples collected during the dry season and the rainy season, respectively, and the quality belonging to poor grade or below accounted for 7.14% and 14.29%, respectively. Hence, the karst water quality in Nandong underground river basin was generally high based on the results of the assessment of karst water quality. However, some cases of poor-quality karst water were found in parts of our study area (e.g., G1 and S4 sampling sites in the dry season and G1, G2, and G3 sampling sites in the rainy season), accompanied by the seasonal differences in karst water pollution. According to the permitted level for irrigation water and the results of SC, SAR, RSC, and PI assessments, most of the karst water in Nandong underground river basin was generally suitable for irrigation. However, it is noted that the concentration of As (54.70 μg·L⁻¹) in the sampling site of surface water (i.e., S3) exceeded the permitted level for the irrigation of vegetables and water crops in the dry season, thereby being no longer suitable for irrigation. Our results suggest that the high concentrations of metal compositions (e.g., Al, Pb, Mn, and As) should be controlled by some targeted measures to prevent further metal pollution in karst water of Nandong underground river basin. Our study also provides an integrated method for the comprehensive understanding of water quality in karst surface water and groundwater.