Study And Application Of The Karstification Model With The Concept Of Dissolution Potential

Analysing the karst distribution and evolution is extremely difficult because of the heterogeneity of karst aquifer media. While studying the local karst, the near surface survey and engineering investigation are effective. A few numerical models of karst evolution, with the constraint of limited fracture network, are not match with actual karst spatial trends. And their visualization and applicability are dissatisfied as they are source code.Karstification Index ΦK, a new parameter to illutrate the concept of the carbonates dissolution potential in groundwater, is point out in this study depending on the groundwater flow system theory and carbonates chemical dissolution mechanism. Karstification Index represents the max amount of carbonates that can be dissolved in groundwater per unit time [MT-1], which can be described as ΦK= q·s Here q [L3T-1] is the flow rate at the point and s[ML-3] is the amount of dissolved carbonates in unit volume groundwater. Karstification Index is 0 in vadose zone.The positive feedback between the heterogeneous aggresive groundwater and unequal dissolved carbonate rock is the important property of the karst aquifers, self organization mechanism. The model of karst aquifers evolution is the way to implement the self aoganizaion, and describe the karstification processe and scale.Karstification Index can be calculated by the flow rate, water temperature, concertration of Ca2+, CO2 partial pressure. The porous media diameter enlargement during a time step are functions of Karstification Index, which related to porous media conductivity and dissolved carbonates. The concertration of Ca2+ from the coupled groundwater flow-Ca2+ transport model with the function of dissolving carbonates, the flow rate and the water temperature from the flow-heat model with new porous media conductivity are used for Karstification Index calculation of the next time step. As Karstification Index is the bridge of the self organization mechanism between karst media and aggressive gourndwater, the karstification model is presented as a new way for karst development research by repeated simulation and iteration.Small continuum circular pipes that connecting with each other in any direction represent the initial permeable media in this approach. The groundwater flow in the initial karst aquifer follows the Darcy law. FEFLOW, a software with multiple functions, is applied to simulating groundwater flow, heat, solute transport in this study.The changes of porous media and groundwater flow in karst aquifers are insignificant in a given period which is relative short refer to the whole karstification processes in long geologic time. Therefore, the karstification model assumes that the velocity of carbonates dissolution is uniform and the karst aquifer remain unchanged within a time step. In this way, the real-time karstification is ignored while the stages of karstification is involved, which is reasonable and meaningful for karst aquifer evolution study.On the other hand, since physical erosion is dominant in mature karst aquifers with large tubes and caves, the model based on the dissolution potential no longer applies. Turbulent flow occours at the convert time from early karst to mature karst, so Reynolds number is the critical threshold for the simulation time of karstification model. However, the model of early karst development is the prototype of karstified media pattern, which determines the general trend of karst aquifer evolution if no tectonic activities changing the basic geologic construction. As a result, the model based on the concept of dissolution potential is a feasible way to describe and predict the macro characteristics and the spatial diveristy of karst quantitatively.An area between two rivers which is consist of homogeneous isotropic limestone is designed as the hydrogeological theoretical karstification model for studying the general karst development process. Turbulent flow appeared in the theoretical model at 100ky. In the model, the maximum karstification potential was found at the discharge location at the basic level (250m), and the early karst channel was found to be horizontally extended along the basic level.Early karst development can be divited into 3 stages according to the theoretical karstification model:(1) Incubation stage (0-40ky), the water table lowered gradually untill it kept stable. The time of the water table variation was the divition criteria. (2) Development stage (40-90ky), limestone rock was dissolved by groundwater slowly at the basic level. In this stage, the increase of Karstification Index was less than 10%of the whole Karstification Index increment. (3) Surging stage (90-100ky), early horizontal karst channel which extented from discharge area to inner aquifer formed. The increase of Karstification Index was more than 90%of the whole increment. Time sections of the 3 stages are variable at diffirent area, but the above divition method conforms the order of early karstification.The theoretical model is applied to study the influence of some basic hydrogeological and geochemical parameters on the karst development, including permeability of limestone rock, insoluble aquiclude, water-conductive structures, concentration of Ca2+in recharge sources. As a result, karstification potential was proportion to the limestone peameability and inversely proportion to concentration of Ca2+in recharge sources, and the influence of the former was 100 times greater than the latter.Once adding a insoluble aquiclude or water-conductive structure in limestone aquifer, the scale and distribution of karstification both changed remarkable. Karst development was layered different upon or beneath the horizontal aquiclude in the theoretical model. The upper layer formed horizontal karst tubes along the impermeable base, while the lower layer was found no karst developed. If a water-conductive structure was designed in the aquifer, karstification concentrated at this structure, which enhancing the heterogeneity of aquifer media and accelerating the karst process. The locations of karst also differed with the location of water-conductive structures. Horizontal karst tube extended along the condunctive structure which was overlap the discharge basic level (250m). Small caves could be found in the model with a horizontal conductive structure at the elevation of 350m. While in the model with the structure at 150m, tubes formed both along conductive structure and beneath the river bed vertically, caves also developed at basic discharge area. Once the water conductive structure skew the aquifer, a tube could be found at the bottom part of the structure and caves formed at the discharge area on both sides.The author researches the karst development of Xiangxi river basin in Xingshan, Hubei province and collects a lot of data from the hydrogeological survey project in the scale of 1:50000. On the basic of dividing karst groundwater flow system of Xiangxi river basin, this study established 2 karstification potential model of Wulongdong sub-basin and Xianglong sub-basin, which in order to analyze the karst development characteristics of 2 different geological systems, and to explain some problems that cannot illustrated via surface survey and investigation, and to verify the reliability and feasibility of the model by comparing the survey phenomenons and simulation results.The results of Wulongdong sub-basin karstfication model showed that the upper Cambrian karst aquifer formed multi-layered horizontal tubes which connected via vertical channels between layers, and probably formed beaded little caves along faults. In contrast, the karstification potential in the lower Sinian aquifer was 1/105-1/106 of the potential in the upper aquifer, which caused weak dissolution at Gaolan river bed and faults only. In terms of Xianglongdong sub-basin model, a cave occured at the discharge area while few karst developed in intenal aquifer. Macro features of karst development in the karstification model of Wulongdong and Xianglongdong sub-basins reproduced the real karst phenomenon, including the locations, basic scale and shape of springs, caves and karst tubes. In addition, the hydraulic dynamic of the spring in upper Cambrian karst aquifer of Wulongdong sub-basin model was similar to the monitoring dynamic data. Therefore, the application of karstification model in Xiangxi river basin is practical and feasible.The karstification model can illustrate constructions and levels of complex karst groundwater systems, which is helpful for hydrogeolocal researchers to predict the major karst locations and scale, and to analyze the spatial distribution of deep karst water resources. In this way, they can target the positions which need more survey and investigation with a high degree of accuracy, and create scientific water exploration projects. Karsttification refers to complex physical and chemical processes which can also be expressed by the concept of potential and expanded as new modulesin the karstification model. The development of karstification model will be from simple singer module to multi complex modules.