The Hydrogeochemical Characteristics And Transportation Of Suspended Particle Matters In Qingmuguan Underground River System, Chongqing, China

Groundwater in karst aquifers in southwestern China including Yunnan, Guizhou, Guangxi, Chongqing, Hunan, Hubei, Sichuan, and Guangdong is a fragile system, especially when these areas are subject to intense karst rocky desertification processes. According to geologic survey, there are 3066 underground rivers in this region. With an annual discharge of～1482 m3/s, those karst aquifers provide a vital water resource for numerous communities. In recent years, impact of human activities on groundwater quality in the karst area has become serious. Irresponsible land uses for agriculture, industry, city construction, and mining have been strongly impacting groundwater quality. Hence, protection of groundwater quality is paramount to human health, economic development, and environmental interests of southwestern China.Karst aquifer media with varied shape is heterogeneous and the storage space for the groundwater. So it is necessary to understand the geometrical characteristics of the karst aquifer media prior to researching on karst groundwater of storage, transport law and hydrogeochemical features. The theory of Karst Dynamics has fully illustrated the source of the carbonate-derived ions, such as calcium, magnesium, and bicarbonate. But most of the research on the source of noncarbonate-derived ions has only been descript shallowly. Besides, the data often are random. Thus a high-resolution monitoring and quantitative method to determine the source of noncarbonate-derived ions are extremely necessary. The suspended particle matters (SPMs) play an important role in the groundwater. The domestic basic research in this sphere is blank, whereas most of the related studies abroad are concentrated in the interaction of SPMs and microorganisms, or organic matters. It should be noted that few efforts have been devoted to investigating the source of the SPMs.In this dissertation, Qingmuguan underground river system (QURS) which is a typical karst valley and located at southwest Chongqing, China, has been obtained as a case for the study.The characteristics of the karst aquifer have been fully investigated by the high-resolution technique (flourometer) that is the first importation from Switzerland. Based on the results of hydrogeological surveys and using flourometer, several times of tracer tests were carried out to obtain the breakthrough curves of dye. The symmetrical breakthrough curves, the geometrical morphology and hydraulic parameters of karst aquifer calculated by the Qtracer2 (U.S. Environmental Protection Agency) suggest that there is a large-scale single karst conduit in QURS. Using groundwater attenuation equation, we found that the water volume stored in caves-conduits and karst fissures accounted for 8.6% and 91.4%, respectively. This result demonstrates that there is multiple media including conduit and fissures.The source of inorganic ions in the groundwater during storm events has been investigated in detail. With development of laboratory test and field automatic monitoring technology, data of groundwater hydrogeochemical dynamics have been recorded continuously. Hence, it consequently results in large sets of hydrogeochemical parameters which are relevant to each other and often difficult to be interpreted and drew firm meaningful conclusions. High-frequency samples have been collected at Jiangjia Spring. Principal component analysis (PCA) of the 20 variables is employed to interpret the relationships with specific processes that control the groundwater hydrogeochemical formations. Through PCA,85% of the total amount information is extracted to indicate the formations of groundwater hydrogeochemical features in QURS during storm events. The first component separates the soil erosion (i.e., increases in discharge, turbidity and concentrations of A13+, TFe, TMn, Ba2+ and NO₂), and dilution effect (i.e., decreases in specific conductance and concentrations of HCO₃, Ca²⁺and Sr2+), accounting for 41.5% of the variability in the data. The second component indicates residual fertilizers and duck's waste from farmlands (i.e., increases in specific conductance and concentrations of Na+, NO₃, PO(₄³), K⁺and Cl), contributing for 37.4%. The dissolution of dolomite and dolomitic limestone makes up 6.0%. The precipitation had a crucial impact on the groundwater hydrogeochemical variations in QURS. The groundwater quality is the comprehensive reflection for hydrogeochemistry. The dominant factors controlling the groundwater quality were obviously varying at different hydrologic processes. Therefore the high-frequency sampling strategy was essentially required, and the comprehensive hydrogeochemical parameters including the discharge were necessarily considered to fully assess the karst groundwater quality.The source of SPMs in the groundwater has been identified. We used the particle counter to record the size distribution of SPMs. We found that at the beginning of the storm, the SPMs were mainly allochthonous substances. When the water in the karst fissures recharged the underground river, the re-suspension of sediment from the karst fissure played a dominant role. When the rainwater recharged the groundwater, the dilution effect dominated, and the SPMs were allochthonous. The transported ways of different size of SPMs are subject to the size of particle, i.e., the size of sediment was significantly negative with the loading capacity of groundwater in which the critical diameter of the particles was 4μm. Soil erosion and nutrient losing not only strongly destroyed the fragile karst ecological environment, but also lead to non-point source pollution, and seriously threatened the drinking water safety of locals.