Study On Migration, Partitioning And Ecological Risk Of PAHs In A Karst Underground River System In Southwest China

It is well known that karst groundwater is very important in southwest China. However, with the rapid economic development and urbanization, karst groundwater pollution has become a predominant problem. This is not only related to human activities, but also closely related to the vulnerability of karst environment. Polycyclic aromatic hydrocarbons (PAHs), one of a group of persistent organic pollutants (POPs), are ubiquitous in various environmental systems, and have attracted much concern due to their toxicity, carcinogenicity and mutagenicity. The primary environmental source of PAHs is energy combustion. Once they enter into karst environment, it may become their final destination, which will be harmful to karst ecosystem and public health. For these reasons, it is very important to understand the composition, behavior, transport, partitioning of PAHs and their environmental risk to ecosystems, especially in karst underground river systems, which can help recognizing the pollution mechanisms of PAHs in karst underground river systems.In this thesis, the Laolongdong Underground River System (LURS) in Nanshan of Chongqing City has been chosen for studying the distribution, contamination characteristics and sources of PAHs in different medium such as soil, water, particle and sediment, partitioning of PAHs in the underground river, as well as the transporting process of PAHs in the system, based on hydrogeological and pollution source investigation, sampling and monitoring monthly and during rainfall, and measuring by GC/MC or GC/MS. It is found that:(1) Concentrations, composition and pollution levels in environment medium in LURS. In the surface soil of LURS, the total concentrations of PAHs range from277.4～3301ng/g, with a mean value of752.6±635.5ng/g, dominated by2～3ring PAHs, seven carcinogenic compounds of which account for36.17%of the total PAHs with an average. Soil organic matter (SOM) may be the primary factor controlling the concentrations of PAHs. There are differences among monthly variations of PAHs contents in the waters, due to waste water, season and different characteristics of PAH compounds. The concentrations of particle associated PAHs are lower than those of dissolved PAHs, and higher in rainy season than that in dry season, which may be related with the sources and quality of the particles. The PAHs compositions are dominated by low ring compounds in all waters and particles, and high rings PAHs are almost not detected due to relative high solubility of low PAHs. However, high rings PAHs are enrich in the sediments because of their tendency to be adsorbed on the particles with the increase of molecular weight..According to PAHs pollution standard made by Maliszewska-Kordybach, the soil samples of the LURS are slightly, moderately and heavily polluted by PAHs respectively, and most of them are moderately polluted. According to the criteria for classification of Baumard, PAHs pollution level in the sediments is moderate or slight. Based on underground water quality of Netherlands and the water quality standard of Canada, PAHs pollution of LURS are featured by low-ring PAHs. For the monthly total concentrations of PAHs, only several exceeded the limit of drinking water quality standard of China. Compare with other areas, pollution level here is moderate. However, karst features and environments are in favor of enriching PAHs and other POPs, which should attain enough attention. Isomer ratios and molecular weight characteristics show that PAHs in the waters mainly come from petroleum. Isomer pair ratios, molecular weight characteristics and principal component analysis (PCA) suggest that vehicles, coal, petroleum and biomass combustion are the main sources of PAHs, another major source being petroleum. Combustion accounts for56.4%while petroleum accounts for43.6%.(2) Multi-media distribution of PAHs and impact factors. The results show that organic matter is the most important factor governing the behavior and fate of PAHs in the water, particles and sediments of the underground river. The relationships among PAHs and DOC or TOC are unambiguous, while the relationship between PAHs and POC is more complicated. DOM is not only a facilitator for the accumulation of freely dissolved PAHs, but also an important pool for dissolved heavy PAHs in the underground river. Suspend particle matter (SPM) is one of the factors influencing on dissolved PAHs. In contrast with the dissolved PAHs, particle organic matter do not significantly affect PAHs, because of the existing other influencing factors such as DOM. Positive correlation has been found between TOC and PAHs in sediments, except InP, DaA and BgP, which indicates that sediment contaminations are primary controlled by organic matter, and might have some other influencing factors.Studies have shown that particle phase-water phase partition coefficient of PAHs have no relationship with POC, while significantly affected by SPM, because more SPM contents will bring more DOC contents, leading to competitive adsorption PAHs between POM and DOM. Through the research of PAHs partition between sediments and water, the linear free-energy equation between organic carbon normalized partition coefficient (K∝) and octanol-water partition coefficient (Kow) has been built. It is found that sediments have poorly lipophilic for PAHs compounds.(3) PAHs migration and transfer process in LURS. The similar composition of PAHs between the underground river and other waters, tracer tests and the relationship among different waters show that karst landforms play an important role in the migration of PAHs in the system. Different combination patterns of PAHs are controlled by hydrodynamic conditions. It is found through comparing the composition of PAHs between upstream and downstream that2-3rings PAHs have higher transport capability with farther migration distance, while lower transport capability and shorter migration distance for4-6rings PAHs, because4-6rings PAHs are affinity particle with low solubility and can be easily absorbed by sediments or carbonate rocks. The exchange mode of surface soil and underground river sediments indicates that the sediments in the outlet of the underground river mainly come from the transportation of the upstream water and surface soil.It is implied by the rainfall monitoring that sinkhole input and surface water infiltration are two major factors controlling the transport process of PAHs during rain events. Rainfall can promote the migration of PAHs from surface to underground. The more the discharge is, the stronger the migration will be. Once there is enough rainfall intensity, it will not only bring more suspended particles, but also make high ring PAHs easier to migrate. The PAHs distribution among different phases are controlled and influenced by the concentrations of DOC, POC, SPM and characteristics and species of the particles, leading to the different partitioning of PAHs in different phases.(4) Ecological risk assessment of PAHs in LURS. Based on risk quotient (RQ) method for ecological risk assessment of PAHs, it is found that all detected PAHs are in moderate or heavy ecosystem risk. The total PAHs show that the Guihuawan Spring and the outlet of Laolongdong Underground River are severely polluted, reaching high risk, while the Zhaojiayuanzi Spring and the surface water are in level2risk. The ecological risk level of the sediments of Laolongdong is low, leading to little negative ecological impact. However, the ecological risk level of the sediments of Xiannvdong is high. The coincident results are shown according to the methods including ERL/ERM, TEL/PEL and mean ERM quotient. The Ecological risk level of the surface soil is moderate. Low and moderate molecular PAHs presented much more ecosystem risk than high molecular PAHs in the Laolongdong underground river system. The3-ring PAHs have higher contribution to the ecological pressure of the water system of Laolongdong, while2-ring and3-ring PAHs contribute more to the soil and sediments. Therefore, some measurements should be taken to control2-3rings PAHs.Due to thin soil and karst fissure of epikarst system, PAHs can easily enter into karst springs, resulting in high ecological risk in Guihuawan Spring and Zhaojiayuanzi Spring. Affected and unaffected by the sewage of Huangjueya Town, the ecological risk of Laolongdong water is in high risk and moderate risk respectively. The higher the ecological risk of recharge of the underground river in different media is, the higher that in Laolongdong Underground River will be. Difference of migration behavior of different molecular weight PAHs lead to the difference of ecological risk levels in water or sediment between upstream and downstream in Laolongdong Underground River. The high molecular PAHs are enrichment in the sediments of the underground river conduit. Once migrating from upstream to downstream, it will produce ecological threat for the downstream.