The Characteristics Of Environment Function And Formation Mechanism Of Cave Hyporheic Zone In Karst Water System

3620 groundwater systems are registered in karst area in South China.The groundwater system discharges as subterranean river or spring,providing domestic drinking water,industrial and irrigation water for numerous of cities and villages,and supporting the unique underground ecosystem.Under the impact of human activities and climate change,most of the groundwater systems suffer from flow reducing or drying and face the problem of water deterioration.This kind of problem can be viewed from two sides.Firstly,the deterioration of water is caused by direct or specific human activities.It can be improved by controlled the activities.Secondly,the water environment degrades quietly,representing by disappearance of original aquatic plant and then trigging problems such as microorganism outburst,water temperature rising,and turbidity increasing,resulting in the loss of groundwater ecological functions.The occurrence and development of the latter problem in karst area have more uncertainty,and the solved measures will be more complicated.The outlet of underground river or spring usually forms karst pool or lake with a certain area,which is not only the location of water resources development and utilization but also carrying significant ecosystem value.Meanwhile,the outlet usually overlaps with the river hyporheic zone?HZ?.Therefore,the outlet of underground river or spring is a HZ of groundwater and surface water.Originally,the HZ is a zone with high active biogeochemistry process,which is the important healthy symbol of groundwater,surface water,and aquatic biological environment.If the interaction function of HZ weakens or loss,the health status of groundwater and surface water will be decreased.Even though the concept of karst hyporheic zone was already raised by researcher in recent years,it was only aimed at the interaction function of karst conduit water and the matrix.The interaction process between karst conduit and surface water,and the relevant detection and monitoring techniques,the hydrochemistry process and the biology function are never involved.This thesis focuses several scientific issues,including how the karst hyporheic zone forms and what roles it plays in improving or deteriorating the water environment and so on.Taking a typical karst basin in South China,Wuming covered karst basin as the case,the environment function of karst hyporheic zone was studied by auto-monitoring and sampling analysis of hydrology and hydrochemistry,by identifying the key community of aquatic ecosystem,and by comparing carbon cycle in different media and hyporheic zones.The following scientific issues were solved emphatically.Firstly,how to define the hyporheic zone in karst area in South China;Secondly,how to classify the karst hyporheic zone;Thirdly,what is the environment function of different karst hyporheic zone,namely,the issue of hydrogeochemistry process and aquatic organism and their interaction in karst hyporheic zone.The objective is to know the environment function of karst hyporheic zone,to understand the role it plays in improving or deteriorating water environment and aquatic ecosystem,and finally to provide scientific evidence for policy-making of unified management in groundwater and surface water system.The main research results are listed as follow.?1?.The concept of karst cave hyporheic zone?CHZ?is defined.Based on the characteristic of flow and discharge of karst groundwater in South China,the cave hyporheic zone is defined as the site or zone where the conduit flow and other types of water are interacting.Breaking the limitation of original concept,this concept focus the issue of how it changes and what is the effect of groundwater after it flowing out to the surface.The concept develops and enriches the scope of karst hyporheic zone.?2?.The types of cave hyporheic zone are classified.According to the karst development and the hydrodynamic condition,the cave hyporheic zone is classified as four types:spring hyporheic zone,sinkhole hyporheic zone,window hyporheic zone,and conduit hyporheic zone.Of which,spring hyporheic zone and window hyporheic zone are the most common and important.The characteristic of each types of hyporheic zone was analyzed.?3?.The hydrogeochemistry process and the main community of aquatic organism in spring hyporheic zone and window hyporheic zone,and the interaction and feedback between them are understood.The hydrochemical features show a significant gradient in horizontal direction in spring hyporheic zone,while there is vertical direction of gradient in window hyporheic zone.After passing through the HZ,the content of NO₃^-decreases as 6.5%-90.9%,indicating that HZ has a good degradation capacity of contaminant.The aquatic macrophyte in CHZ has a good function of water purification performance,but it is easily disturbed,resulting in evolution or vanish.In general,the diversity index of plankton is low in CHZ,and the structures,evenness,and the dominant species in different CHZ are different.Also,the seasonal variations of those indexes are significant,indicating the vulnerability and specificity of CHZ.The species and abundance of plankton in the karst groundwater outlet and in the CHZ are different,and the structures and dominant species change,suggesting the CHZ is an active bioreactor.?4?.The cycle of key nutrient element and its indicator meaning are revealed.Nutrient elements,such as carbon,link up the inorganic environment and biological element in CHZ.Carbon is the key element attending actively in the circulation of materials and energy flow in CHZ.The content and isotope of carbon in different media show CHZ is the source of dissolved organic carbon.The frequency variation of carbon indicates the change of CHZ.The difference of carbon flow reflects the different of environmental evolution in CHZ.?5?.The judge idea of water environmental health including hydrodynamic,hydrochemistry and aquatic ecosystem in the water system is proposed.When the spring HZ and river HZ overlaps,water environment is more easily degeneration.Lingshui is the representative of this kind of HZ.The reason is water quality in surface river usually worse than that in karst groundwater.Even though the vanish of submerged plant would not cause the destroy of aquatic organism food chain in a short time,the water environment and aquatic ecosystem will be changed acutely if the restricted condition of water environment factors are changed.The water quality can be improved by groundwater quickly updating,but the healthy aquatic ecosystem would not be recovered due to the degradation of HZ functions.The research results not only provide new ideas and views for hydrogeology and water environment study in karst area,and propose new observation technique and ways in this field,but also provide reference for effective management and protection of karst groundwater and restoration of aquatic ecosystem.