Reseach On The Controlling Factors Of Formation And Distribution Of Subterranean Karst Streams And Its Hydrogeochemistry Regionality, Chongqing, China

Karst groundwater is an important strategic resources and life-support system for sustainable development in China. The storage type of karst groundwater in south China is mainly separate subterranean stream system. According to the latest geology survey, there are 3066 subterranean streams (SS) in south China, which are the important water resources for local people. However, karst aquifer in this region is facing some threats such as the water quality deterioration, water sources exhaustion due to the fragility of karst environment and the disturbance of human activities. The karst areas in Chongqing distribute mainly in the northeast and southeast with the area about 3.0×104km2 is an important area of the southwest karst China. Chongqing have suffered serious and rare drought in the summer 2006 and the spring 2010. The demand for the rich groundwater as an emergency water supply in karst areas was very urgent for the fighting against drought. Therefore, researching on the formation, distribution and hydrogeochemistry of subterranean streams in Chongqing is very important for exploitation, utilization and protection of karst groundwater resources of Chongqing.The formation and distribution of SS is combined controlled by the carbonate rock, structure and relief in karst areas. In south China including Guizhou, Yunnan. Guangxi, many researchers have focused on the controlling factors of formation and distribution of SS. But the same researches are seldom in Chongqing. Hydrogeochemistry of SS is combined influenced by the geological and environmental characters and it has significance spatial variation features. It is very important for the utilization and protection of SS to study on the regionality of SS's hydrogeochemistry and its coupling relationships with the geology and environmental features.Based on the historical references analysis and field surveys, we researched the formation and distribution of SS of Chongqing and sectioned their types in this dissertation. There are approximately 380 subterranean streams with a total length of about 2155km, the multi-annual mean discharge about 151.5m3/s, and the water resources about 47.77×108m3/a in Chongqing. The mean coefficient of development density of subterranean streams is 116.64m/Km2, and the multi-annual mean coefficient of runoff modulus of subterranean streams is 6.56L/s·Km2. Most of the length of SS is short and medium length, and the numbers of SS of which the length is less than 5km account for about 50% of the total SS numbers. Most of the discharge of SS is medium and small discharge, and the numbers of SS of which the discharge is less than 200L/s account for 58.9% of the total discharges. Overall, the SS exists mostly in the southeast and northeast of Chongqing. Its distribution is consistent with the area of carbonate rock in Chongqing.In this dissertation, a new founding is that the formation and distribution of SS is controlled by the lineament structure in Chongqing. As a whole, the SS distributes mostly in the Eastern Sichuan fold belt, Southeastern Sichuan fall-fold belt and Daba Mt. arc-shaped fault-fold belt of Chongqing. The bearing of trend of SS in Daba Mt. arc-shaped fault-fold belt in northeastern Chongqing is NW-SE direction because of the control of the NW trend lineament structure. Due to the influence of the NE and NNW trend lineament structure, the bearing of trend of SS in Southeastern Sichuan fall-fold belt in southeastern Chongqing is NE-SW direction. The SS mainly locates in the core part of a series of anticlines in Eastern Sichuan fold belt in the main districts of Chongqing city and central area of Chongqing municipality, which is the carbonate rock area, and its trends is along the anticline extension direction (NE direction). According to the structure features and groundwater movement features of SS, it is divided into the following three types:afflux-flow type, distributary-flow type and parallel conduit-flow type. The afflux-flow type SS largely distributes in the syncline areas, wide-spaced shallow-dip anticline areas and ridge-trough valley areas. The distributary-flow type SS mainly locates in the anticline areas and syncline chapada areas, and the parallel conduit-flow type SS in the appressed fold areas which is the areas of soluble rock alternating with non-soluble rock.By analyzing the hydrogeochemistry of 70 typical SSs in Chongqing, this study reveals that the water quality is deteriorating and the hydrochemistry type is complicating from the northeastern Chongqing and southeastern Chongqing to the main districts of Chongqing city. The water quality of subterranean streams existed in the main districts of Chongqing city and central area of Chongqing municipality is poorer than that existed in the southeast and northeast Chongqing municipality. In the northeastern and southeastern Chongqing, the solutes of SS mainly originated from the dissolution of carbonate rock by water-rock interaction, which lead to the fact that most groundwater chemistry of SS were typical Ca-HCO3 type or Ca+Mg-HCO3 type. However, some of subterranean karst streams have been changed into the type of Ca+Na-SO4, Ca+Na-HCO3+SO4, Na+Ca-Cl+HCO3 or Ca-SO4+HCO3 in the main districts of Chongqing city and central area of Chongqing municipality due to the strong impact of human activities. The water quality of SSs, which is mainly Ca+Na-SO4, Ca+Na-SO4+HCO3 or Ca-SO4+HCO3 type, is worse in the triangle areas of Wansheng district-Nanchuan district-Qijiang county, where is called the "black triangle" area, of hydrogeochemistry.Multi isotopes were used to study the solute origin and regionality of hydrogeochemistry of SSs, Chongqing. All of the SS water origin from the modern precipitation because theirδ18 O andδD plot along the line of GMWL or LMWL, which shows that the evaporation of groundwater does not occur or is not strong. Under the control of movement of rain cloud cluster from south to north (atmospheric circulation effect) and the regional relief, the regionality ofδ18O and 8D for SS in Chongiqng is shown in the following way: the northeastern Chongqing< the main districts of Chongqing city< the central area of Chongqing municipality< the southeastern Chongqing (the symbol "<" means "more minus"). In dry season, the regionality ofδ18O and 8D for SS is not clear due to the slower movement velocity in karst aquifer. The hydrogeological implication of d-excess value is not clear for karst aquifers because the movement velocity of groundwater in it is faster than pervious aquifers and fissure aquifers with the lower degree of water-rock interaction. Considering the relations between the altitude and the mean value forδ18O or 8D of subterranean streams within 100m altitude differences, we established the second order polynomial of karst groundwater betweenδ18O orδD and altitude. According to the equations, theδ18O-elevation gradient of karst groundwater in Chongqing is -0.34‰/100m in dry season and-0.31‰/100m in wet season, as well as the 8D-elevation gradient is-2.3‰/100m in dry season and-2.9‰/100m in wet season. In most test sites, theδ13C values of SS in wet season is more minus than in dry season, except in main districts of Chongqing city, which shows the concentrations of biogenic CO2 in groundwater may be more in wet season. Based on theδ13C isotope mass balance equation, we calculated the amounts of DIC which came from the carbonate rock dissolution. The calculated results show that 45.1%-79.7% of the DIC in the dry season and 17.2%-82.1% in the wet season is from dissolution of carbonate rocks in karst groundwater. The range of calculated results also shows that the DIC proportion of karst groundwater originated from the carbonate rock dissolution is not necessarily the 50% according to the molar ration of karst chemical reaction equation, but has a range of changes at some extend. As a result, this dissertation suggests that we should count first the rate of carbonate rock dissolution in groundwater DIC using theδ13C when we calculate the carbon sink of karst processes, and then calculate further the karst carbon sink.According to the results of 87Sr/86Sr, the Sr element in test sites mainly origin from the carbonate rock dissolution. Also, Sr element may come from the silicate rock weathering in some test sites. By analyzing the relationship of the 87Sr/86Sr and [K++Na+]/Sr2+, some SSs are greatly impacted by human activities, which exist mainly in the Triassic limestone aquifer in southeast Chongqing. Some subterranean streams are also affected by human activities in Cambrian karst aquifer in Southeast Chongqing. Based on the analysis ofδ34S values, this research found that the SO42- in SS has multiple sources and has obvious regional differences. The sources of SO42- include the source of gypsum solution, oxidation of sulfide minerals, atmospheric acid deposits and human activities by the relation of 87Sr/86Sr vs.δ34S. With the source of the evaporating SO42-, the SSs mainly develop in the Triassic karst aquifer. With the source of the oxidation of sulfide minerals, the SO42- mainly develops in the Permian karst aquifer. The S in rainwater and sewage also contributed to the SO42 of SSs. The SO42- originated mainly from the human activities in the main districts of Chongqing city and in east Chongqing. The SO42- of SSs originated mainly from the atmospheric acid deposits in the areas of higher altitude and smaller disturbance of human activities in southeast and north east Chongqing. Overall, the multiple sources of S clearly contribute to the existence of the SO42- in some SSs, so that those SSs tend to lie in the combined influenced area of multiple S sources.Generally, the controlling factor of formation and distribution of SS is the regional lineament structure in Chongqing. The water quality of SS is deteriorating and the hydrochemistry type is complicating from the northeastern Chongqing and southeastern Chongqing to the main districts of Chongqing city. The regionality of isotope hydrogeochemistry and the solutes origin were revealed using the multi- isotope coupling technology. The research productions of this dissertation will provide science and technology supports for the rational exploitation and protection of subterranean streams in Chongqing municipality.