Source, Partition and Ecological Risk of Polycyclic Aromatic Hydrocarbons in Karst Underground River Environment, Southern China

As the main source of drinking water in the karst areas of southern China, underground rivers play an important supporting role in local economic and social development. However, due to the special aquifer structure in the karst areas of southern China, polycyclic aromatic hydrocarbons (PAHs) can easily enter the water environment of underground rivers and cause serious pollution, which will affect the water quality safety. In this study, Qing-shui Spring underground river in Nanning city was selected as a representative of the typical underground river in southern China, and the pollution characteristics, source analysis, ecological risk assessment and diffusion law of PAHs were studied. The results showed that the total concentration of PAHs in groundwater and surface sediments was in the range of 282.42–464.88 ng/L and 400.14–1194.69 ng/g, respectively, and the overall concentrations of PAHs were below the moderate pollution level. Pollutant discharge and physical and chemical properties of PAHs caused the concentrations of PAHs to decrease gradually from upstream to downstream, but the proportion of high-cyclic PAHs increased gradually. According to the source analysis results of the ratio method, PAHs sources in groundwater were mainly biomass combustion sources in the upstream, oil sources in the middle and mixed sources in the downstream. Sediment and groundwater had some differences. The middle and upper reaches were the source of biomass combustion, while the middle and lower reaches were the mixed source. The ecological risk assessment results showed that the ecological risk of groundwater was at a moderate level, while the ecological risk of sediment was at a low level. Benzo[a]anthracene (BaA) and benzo[b]fluoranthene (BbF) in groundwater and acenaphthylene (Acy) and fluoranthene (Flu) in sediment should be monitored intensively. Appropriate control measures should be put in place to prevent further pollution. Partition coefficients varied from 0.73 to 3.85 L/g in the sediment–water interface, increasing with the rise of PAH compounds. All of the organic carbon partition coefficients (Koc) in the sediment–water interface were higher than predicate values based on the typical model of equilibrium distribution; this indicated that PAHs were strongly adsorbed in the sediment. The linear free energy relationship coefficient between the Koc value and the n-octanol-water partition coefficient (Kow) was 0.85 but the slope was lower than 1, indicating that sediment in the Qing-shui Spring underground river had weak lipophilic characteristics and adsorption ability for PAHs. The results can provide a reference for the risk assessment and pollution control of PAHs in the karst underground rivers in South China.