Study On The Boundary And Structural Characteristics Of The Underground River System In Nandong, Yunnan

The Nandong underground river system is the largest and most typical karst underground river system in Yunnan province.From the persepective of systematic scientific theory and based on field investigation,exploration and sampling data,this paper studied the foundation and formation conditions of karst development,the boundary location,properties and the structure of Nandong underground river system by using the methods of modern karst,geology and hydrogeological analysis.The similarities and differences between the two underground river subsystems(No.1 and No.2)were examined by from the fields of hydrodynamic,hydrochemistry and water temperature.It is believed that the results of the study can provide scientific basis for the exploitation and utilization of this unique underground water system.Major conclusions are given as follows:1.The landform of the Nandong karst underground river system mainly include the hill depression,peak-cluster depression(funnel),karst middle-mountain,karst low-middle mountain,karst fault block mountain and karst fault steep slope.The peak-cluster depression(funnel)and hill depression are the main geomorphologic types in the study area.They are mainly distributed on the eastern karst plateau,shallow karst hills and near the northern Dazhuang basin.Observation shows that the formation lithology,tectonic,groundwater runoff and neo-tectonic movement control the development and distribution of karst underground river.The underground river is most developed in the carbonate rocks of Gejiu formation(T2g),while weak or not developed in other carbonate rocks due to low carbonate content and weak karstifacation.Karst development and underground river distribution is controlled by Northwest and Northeast geologic structure.The underground river is most developed along the fault zone which extends in the direction of NW-SE and NE-SW.There is good hydrodynamic condition,rapid flow circulation and strong karstification in the eastern plateau mountainous area of Caoba basin with comparison to the other areas,It have poor hydrodynamic condition,slow water circulation and weak karstification in the basin areas,the underground karst shapes dominated by solution holes and fissure.The karst strongly developed along the faulted fracture zones which mainly consists by conduit and caves.The neo-tectonic movement has the characteristics of the terraced features in the study area.Three layers of karst caves are developed and two drainage datum plane are formed along the Dazhuang,Caoba,Mengzi and Kaiyan basins.The formation of each drainage datum plane brings the karst growth into a strong period.The formation of Dazhuang,Caoba and Mengzi basins creates the condition for the development of Pingshiban,Heilongtan,Shidong-Daheishuidong underground rivers.The formation of drainage datum plane in the Kaiyuan basin provides the precondition for formation of No.1 underground river.It also causes the retrogressive erosion of No.2 underground river which gradually captured and merged the Shidong-Daheishuidong conduit and finally formed the mainstream of No.2 underground river.2.The boundary of Nandong underground river system was reanalyzed.The observed boundary type mainly includes the surface watershed,the underground watershed,the water-blocking faults tectonic belt,the non-soluble rock aquiclude,the buried magmatic ridge watershed and karst aquifers deep stagnant boundary.The northeast boundary,which original restricted by Kaiyuan-Madiaodoupo fault(F14),was adjusted to the surface watershed boundary of this area.This is a revision of previous understanding.The southeast boundary is consistent with previous studies,i.e.the eastern segment is restricted by the surface watershed boundary and the southern segment is a mixed boundary of the water-resistance fracture and the clastic rocks.The southwest boundary is newly calibrated,which greatly differs from the previous understanding.We think the southwest boundary can be divided into two sections:the impermeable boundary formed by the marlstone sedimentary deep groove from Neogene in the Mengzi basin area,It is mixed boundary surface watershed and underground watershed form Magmatic basement for the Guanjiashan-Gejiu City;the western boundary is the newly established boundary of this study which breaks water resistance fracture as the western boundary.The internal boundary between No.1 underground river subsystem and the No.2 underground river subsystem is the underground watershed in the northern Dazhuang basin.It should be the karst aquifers deep stagnant boundary,and suppose to be consistent with the area of surface watershed.For the top and bottom boundaries of the Nandong underground river system,it is revealed that the top boundary is karst water table in those exposed karst mountains,while the upper boundary should be the interface between soil and stone in those buried karst basin areas.The bottom boundary of the system is limited by the karst development.3.The Nandong underground river system consists of input subsystem and output subsystem.The input subsystem performs the input function of matter and energy which consists by the middle mountain depressions,the peak-cluster(hill)depressions,and sinkholes in the the slope-hill and the basin areas subsystem.The output subsystem performs the output function of matter and energy which composes by the underground conduits,mainly the two parallel underground rivers(No.1 and No.2)in the study area.No.2 underground river subsystem is the main body of the Nandong underground river system with complicated structure and karst conduits.For example,there exists a parallel relationship and multiple outlets within the upper conduits of Daheishuidong area.The Pingshiban underground river,Xiaoheishuidong spring and Heilongtan underground river are the three underground rivers that located on the top of the mainstream of No.2 underground river.4.There is a great difference in hydraulic gradient between No.1 underground river subsystem and No.2 underground river subsystem.The profile of No.1 underground river seems like a graded profile,the upstream hydraulic slope is larger and the downstream hydraulic slope is smaller,forming a concave curve.The hydraulic slope of the outlet section is only about 2-7 ‰,while hydraulic gradient at the upstream of Kaiyuandongshan peak-Qingtanzi is 46.78‰.Influenced by the intermittent movement of the earth crust,multi-layeres of underground river were developed.The profile changes of the No.2 underground river mainstream are more complicated.Average hydraulic slope is 10.9-14.5 ‰,the hydraulic slope of flood period is obviously larger than the dry period.The average hydraulic slope between Shidong and Daheishuidong is 23.38-28.16 ‰,and the hydraulic slope of the eastern edge slop zone of the Caoba basin can up to 83.24%.The hydraulic gradient in the basin and hilly areas is relatively smaller,the hydraulic gradient is only 4.32-4.89 ‰ from Huitudi to Caobayicun and only 2 ‰ at the outlet section.The reasons for the change of hydraulic gradient in the above longitudinal profile are mainly caused by the gradual uplift of tectonic movement and the formation of different discharge datum plane.5.The structure characteristics of karst water-bearing media in the Nandong underground river system are deeply analyzed.The karst water-bearing media of the No.l underground river subsystem is mainly consists by karst conduits with beehive-like dissolved holes,caves and fissures in both sides.The conduit plays a role of water transptation,while the beehive-like dissolved holes,caves and fissures plays a role of water storage.The structure of karst water-bearing media of No.2 underground water subsystem is very different in space:the water-bearing medium is dominant by straight conduit in the upstream,which discharge quickly.The groundwater flow velocity is fast and the karst conduit and fissure medium are fully developed.The downstream karst water-bearing medium is mainly composed by the network structure of conduit and wide fissures,which leads to better drainage.The midstream is mainly buried faulted karst basin,the karst water-bearing medium is dominated by network fissures such as dissolved-fissures and fissures,and the conduit is not well developed.The underground water flow is not unblocked and quite slow.Flow attenuation analysis of the Nandong underground river shows that the dissolved-fissure and conduit are the two kinds of water storage space of the Nandong underground river system which form the double water-bearing medium.In the earlier recession period,the total karst water storage volume was 115.17 × 106m3 with conduit water storage of 11.75 × 106m3(accounts for 10.20%of the total volume)and dissolved-fissure storage of 103.42 ×106m3(89.80%).The latter is significantly bigger than the former,indicating that dissolved-fissure is the main water storage space of the Nandong underground river system.It is also the important reason why the Nandong underground river system can maintain the discharge of 2-3m3/s without rainfall supplies in the near half year.6.It is revealed that there exist obvious differences of chemical field between the the No.l underground river subsystem and the No.2 underground river subsystem.Temporary hardness and mineralization of No.1 underground river subsystem is relatively low,outlet water SIC and SID values are low negative,the content of 3H is 3-4tu higher than the No.2 underground river subsystem,while ?D and ?18O are smaller.This result shows that No.1 underground river subsystem has larger medium space of the karst aquifer,shorter water transportation time,faster water circulation and the outlet groundwater also has strong erosion ability.In addition,groundwater age is relatively new and the stored water can be discharged annually.The temporary hardness and mineralization of No.2 underground river subsystem is slightly higher than No.1 underground river subsystem.From the recharge area to the discharge area,the mineralization of the conduit water is increasing.The value of SIC is positive and SID is negative at outlet,whic is higher than No.1 underground river subsystem.The 3H content is 3-4tu lower than the No.1 underground river subsystem,but it is 0.6 TU higher than precipitation.The 3H,?D and ?18O of No.2 underground river subsystem are-14.17 ‰,-0.342 ‰and 2.6 + 1TU deviate from atmospheric precipitation,respectively.This result indicates that karst water-bearing medium space of No.2 underground river subsystem is quite complicated,the groundwater staying long in the karst voids,while water circulation is slower than No.1 underground river subsystem.The outlet water SIC has reached saturation,the conduit water is not exactly the rainfall in the rainy season and the waterage is relatively old.The water temperature of No.1 underground river subsystem is always higher than the No.2 underground river subsystem for about 0.5 ?,It shows that the underground water comes from different systems.The water temperature doesn't change very much from the recharge area to the discharge area in the No.1 underground river subsystem.The water temperature of the No.2 underground river subsystem is gradually increasing from the upper plateau mountainous area to eastern edge of basin and then the final outlet.The catchment area of No.1 underground river subsystem is smaller and runoff field is quite simple.Groundwater flows generally from northeast to southwest with groundwater flows from east to west along the Kaiyuan-Madiaodoupo fault(F14)in between the Wabaibai-Adeyi area.The catchment area of No.2 underground river subsystem is bigger,groundwater flows generally from southeast to northwest,but the runoff direction changed in different places.According to tectonic influence,the groundwater flows from southeast to northwest for the Pingshiban and Heilongtan underground river,while the groundwater flows from east to west was observed for the Shidong-Huitudidong(Daheishuidong).The karst water in those plateau mountainous and depression areas are connected through Huitudi-Daguoxi fault(F31),Laoyanzi-Yuguopu fault(F28)and Dahongdi-Chenghongzhai fault(F4).In other words,the main path of conduit runoff is along the way the Huitudidong?the north Yongning sinkholes ? the east No.32 drill ? Leigongshao ? Laoyanzi ?Chenghongzhai.Karst water from the eastern edge Dazhuang basin is running towards the direction of the Yakou village.Similarly,the karst water in the north of Xiaoheishuidong is also flowing to the Yakou village.After then,both water flows together at Yakou village running along the Xiaobajiao-Bohei fault(F7)zone and finally meet with the mainstream of No.2 underground river at Leigongshao.