Mise-a-la-masse method,tracer tests and cave detection analysis of underground-river karst-conduit distribution and structure: a case study of the Dafengdong conduits in Guizhou Province,China

In karst areas, groundwater movement is dominated by conduit flow; thus, understanding the distribution and structure of karst conduits has great significance for water research, groundwater protection and engineering construction. With the Dafengdong (Guizhou Province, China) underground river karst as an example, a mise-a-la-masse method, tracer tests and cave detection were conducted to study the distribution and structure of karst conduits. Combined with information on the geological and hydrogeological conditions, the geological factors that form karst conduits were determined. It can be concluded that: (1) Under the influence of faults, karst conduits usually develop towards or along faults; (2) multiple karst conduits form easily on both sides of tectonic fracture zones; (3) both lithology and geological structure affect the formation of blue holes, and when the directions of karst conduits change, blue holes easily form in weak parts of the structure; (4) at springs where two aquifers intersect, with strong chemical dissolution of the lower karst aquifer and mechanical collapse of the upper aquifer, blue holes also form easily; and (5) integrated mise-a-la-masse method, tracer tests and cave detection can accurately discern the distribution and structure of karst conduits. Geological factors can be used to preliminarily delineate the distribution and structure of karst conduits in similar areas based on hydrogeological conditions. Such methods hold great significance for groundwater extraction and protection and engineering construction in karst areas.

     

层流—紊流共存流场中岩溶裂隙网络演化过程的数值模拟方法

研究岩溶水系统的演化过程对许多资源与环境问题有着重要意义。岩溶地区水资源、油气资源的预测与开发、水土流失成因及防治等课题都与此密切相关。岩溶管道是地下水对裂隙的逐渐溶蚀扩展形成的,岩溶演化初期,所有裂隙宽度不大,流场整体成层流状态。随着溶蚀的进行,一部分裂隙优先扩大使其中的水流进入紊流状态,而另一部分裂隙中水流仍呈层流状态。文章提出一种数值方法,能够模拟层流-紊流共存流场的岩溶演化过程。利用蒙特卡洛(Monte-Carlo)方法模拟初始裂隙网络,通过非连续介质方法模拟裂隙网络中的渗流。裂隙扩展的速度通过岩石表面溶蚀速度经验公式计算,使用迭代方法求解层流-紊流共存条件下的水头非线性方程。构建了能够利用解析法求解的模型,把数值解和解析解的结果进行对比,验证了本研究的数值法及软件的可行性。      

Karst flash flooding in a Mediterranean karst, the example of Fontaine de Nîmes

Karst flash flooding, identified as one of the hazards in karst terrains, is directly linked to the structure and hydraulic properties of karst aquifers. Due to the characteristics of flow within karst aquifers, characterized by a dual flow – diffuse flow within fissured limestone and conduit flow within karst conduits networks – flash flooding may be important in volume and dynamics. Such phenomenon may cause serious damages including loss of lives, as it occurred on 3rd October 1988 in Nîmes (Gard, South France). Flash floods there have been considered to be the result of very intensive rainfall events conjugated to runoff due to the geomorphologic context of the city located down hill. However, preliminary results of recent studies of the hydrologic behaviour of groundwater and surface water for a specific event (September 2005) show that the karst plays an important role in the flood genesis. The main characteristics of the Nîmes karst system leading to karst flash flooding are presented in this paper. A methodology comprising modelling of the karst system allowed proposing simple warning thresholds for various part of the karst (water level threshold for the karst conduits and cumulative rainfall threshold for the overflowing fissured karst). These thresholds can be included in the flash flood warning system of the Nîmes city.     

岩溶管道结构影响泉流量变化的数值模拟研究

采用MODFLOW-CFP建立管道流数值模型,模拟岩溶含水系统在暴雨期的响应过程,以泉口流量峰值作为因变量,分别对控制管道结构的4个参数以及落水洞集中补给比例进行调整,研究管道结构如何控制泉流量变化过程。结果表明:管道直径、管壁渗透系数和落水洞补给比例对泉口流量峰值均表现为正相关,其中管壁渗透系数影响最大;管道弯曲度值较小时其增大促进出口流量峰值变大,当径流途径变长引起的流量减小幅度超过管壁面积变大引起的流量增大幅度时,流量峰值逐渐变小;管壁粗糙度在达到两个相对应的雷诺数临界值时水流状态发生突变,导致流量峰值也表现为两次突变。      

Modeling hydrological responses of karst spring to storm events: example of the Shuifang spring (Jinfo Mt., Chongqing,China)

Reproduction of hydrographs at karst springs has been an approach of understanding the karst aquifer, which normally acts as drains for the groundwater flow. However, its numerical modeling is difficult since factors for the internal geometry and connectedness are unknown and hard to quantify. Hydrographs of the karst aquifer with well-developed conduits in Shuifang spring catchment were obtained from the automatic gauging station at the spring orifice. Data as to the conduit system were also obtained based on results and analyses of tracer tests. With these data, the hydrological responses of Shuifang spring to storm events were simulated by storm water management model (SWMM) developed by USA EPA (Environmental Protection Agency). Nash–Sutcliffe efficiencies are used to compare the computed flow to the observed, which are 0.95 and 0.92 for calibration and validation. SWMM was verified and applicable in karst conduit drainage system. The model illustrated correctly quick recharge through conduits and slow and low inflow from the fissured aquifer matrix. The SCS-CN (soil conservation service-curve number) infiltration method was used for computation of losses and runoff. Field tests indicated that permeability was extremely high but different in karst area, which was less sensitive to the computed runoff when exceeded the common value provided by SWMM. Therefore, an improved quantitative infiltration model for karst area will make SWMM possible to be a useful tool for assessing and reproducing spring hydrographs.     

Hydrological modeling in the karst area,Rižana spring catchment,Slovenia

Karst aquifers are known for their heterogeneity and irregular complex flow patterns which make them more difficult to model and demand specific modeling approaches. This paper presents one such approach which is based on a conceptual model. The model was applied in a karst area of the catchment of Rižana spring (200 km2). It is based on the MIKE SHE code and incorporates the main hydrological processes and geological features of the karst aquifer (diffuse and concentrated infiltration, allogenic recharge, quick and slow groundwater flow, shifting groundwater divides and groundwater outflow from the catchment area). Modeling of evapotranspiration and flow in the upper part of the unsaturated zone is more detailed. For the modeling of groundwater flow in the karst aquifer, a conceptual model was applied which uses drainage function for the simulation of groundwater flow through large conduits (karst channels and large fissures). The model was calibrated and validated against the observed Rižana spring discharge which represents a measured response of the aquifer. The results of validation show that the model is able to adequately simulate temporal evolution of the spring discharge, measured by Nash–Sutcliffe coefficient (0.82) as well as overall water balance.     

The karst contagion model: Synopsis and environmental implications

The contagion model of karst terrane evolution focuses on the environmental implications for a large karst depression population on the Pennyroyal Plain (southern Kentucky) and the adjacent Western Highland Rim (Tennessee) immediately south of the Mammoth Cave Plateau. In karst terranes where the contagion model applies, there is a well-defined infrastructure comprised of hydrologic, structural geologic and geomorphic interacting elements that result in clustered depressions underlain by a radial conduit system. Clusters tend to be randomly distributed and typically contain a parent depression surrounded by numerous daughters.Groundwater flow is assumed to be turbulent and confined largely to conduits that are 3-dimensionally configured between clusters in a dendritic to trellis network. Parent depressions serve as conduit nodes for collecting groundwater migrating from beneath daughter depressions.Flow velocities in the 3-dimensional cluster-cell conduits exceed those in granular media by several orders of magnitude making pathogen and chemical contaminant migration rapid. Groundwater quality assessment in karst conduit hydrogeologic settings is difficult because monitoring wells are inappropriate. Monitoring wells may have a low probability of intercepting a major conduit and therefore the sampling regime must take into consideration the pulse discharge of pollutants in karst conduits. Representative water quality data must come from springs located near the local base level.     

Surface and subsurface environmental degradation in the karst of Apulia (southern Italy)

Karst environments are highly vulnerable to a variety of degradation and pollution problems. Geology (fractured carbonate rocks), morphology (presence of a network of cavities produced by karst processes), and hydrogeology (rapid concentrated flow through fractures and conduits) of karst carbonates strongly favour the movement of contaminants towards the water table. In particular, poor quality of subsurface water can derive from polluting substances flowing at the surface, and/or by direct immission of liquid and solid waste into the water table through the systems of conduits and joints in the rock mass. As a consequence, water quality can deteriorate severely, which implies very high economic and social costs in order to clean the polluted sites and restore the original situation. In some cases, such as when the original karst morphology is changed because of anthropogenic interventions, the variations created in the landscape are not recoverable, and a loss of sites of naturalistic interest has to be registered. High vulnerability of a typical karst region of the Mediterranean area is illustrated in this paper by describing some case studies from Apulia, southern Italy. The Apulia region, where karst processes have had a prominent role in the development of the present landscape, is mostly underlain by intensely karstified limestone. Two cases of pollution due to solid waste into karst cavities (Grave di S. Leonardo in the Gargano Promontory and Grave Pelosello in the Murge plateau), landscape transformation in the Minervino Murge area, and degradation of Gravina Monsignore, a typical karst valley in southeastern Murge, are described in the paper. In two out of four cases, degradation of the sites was discovered thanks to activity from local speleologists, who also acted as promoters for cleaning and safeguarding the polluted sites. These examples underline well the mismanagement of karst territories (in particular, the common practice to dump refuse into sinkholes and caves), the pollution of limestone aquifers, and the effects that such pollution in karst areas might have in terms of the risk to public health.     

岩溶水系统结构和水文响应机制的定量识别方法——以三峡鱼迷岩溶水系统为例

岩溶水系统结构复杂难以刻画,介质结构对地下水文特征的控制机制不清.以三峡鱼迷岩溶水系统为研究对象,利用人工示踪试验和地下水动态监测技术,建立了对流-弥散模型和扩散模型,求取了标准衰减曲线,构建了电导率和流量的高斯混合分布模型.结果表明:该系统中存在“单源单汇”、“单源多汇”和“多源单汇”3种地下水循环模式;水文事件的响应阶段地下水运动以对流为主,管道流占优势,衰减阶段则以扩散为主,裂隙流和孔隙流占优势;鱼泉洞在丰、枯水期分别识别出5种和6种地下水径流组分,以电导率180 μS/cm和流量0.6 m3/s为界,低电导率和大流量组分为快速流,其平均时间贡献占比为3.5%,径流量占比为19%.岩溶水系统中,管道和溶洞是快速流的储蓄和运移空间,空间尺寸大、开放性强、易受降雨影响;裂隙和孔隙是慢速流的储蓄和运移空间,空间尺寸小、开放性弱、对降雨有一定调蓄作用.研究可为岩溶水系统结构识别和水文机制研究提供参考,为岩溶流域水文地质调查提供理论依据.      

The formation of sinkholes in karst mining areas in China and some methods of prevention

 Mining of coal, lead and zinc, gold, and iron ore deposits in karst areas has been closely associated with sinkholes in China. Surface collapse causes an increase in mine water drainage and the possibility of major water inflow from karst aquifers, which threatens the environment in mining areas and endangers mine safety. The origin of such sinkholes is analyzed quantitatively in this paper and a combination of factors including soil weight, buoyancy, suffosion process and vacuum suction can contribute to surface subsidence. The key measures to prevent sinkholes in mining areas are to control the amount of mine drainage, reduce water-level fluctuation, seal off karst conduits and subsurface cavities in the overlying soil, prevent water inflow, and to increase gas pressure in the karst conduits. Received: 2 May 1996 · Accepted: 29 July 1996     

重庆市龙泉村—庆丰山村岩溶塌陷分布规律与成因机制

文章以地质环境与隧道建设活动为基础,总结出重庆市沙坪坝区龙泉村—庆丰山村岩溶塌陷分布规律及发育特征:（1）岩溶塌陷集中分布于槽谷与横向冲沟交汇地带、槽谷中地势低洼地带以及地表溪沟附近;（2）岩溶塌陷形成演化呈现从地势低洼处往高处扩展、从隧道疏干区核心部位往边缘外围逐渐扩展的特征。研究区岩溶塌陷受隧道施工疏干排水及降雨影响控制,为隧道施工疏干后期降雨型岩溶塌陷。研究区岩溶塌陷的形成演化可分为三个阶段,其中前两个阶段为塌陷高峰期。不同阶段诱发岩溶塌陷的主控因素不同:第一、第三阶段主控因素为渗流潜蚀、增荷软化效应;第二阶段主控因素为渗流潜蚀及岩溶管道水气压力突变效应。     

基于MODFLOW的岩溶管道水流模拟方法探讨与应用

针对岩溶管道水流特征,选取广西寨底岩溶流域为研究对象,分别采用三维有限差分地下水流动模型(MODFLOW)中的Drain和River模块概化模拟岩溶管道特征,结合管道内上游、中游及下游三个观测孔对比分析两种方法的适用性,结果表明:末期等水位线图中位于管道两侧处流线均有形状突变,符合实际管道水流动态特征,且能模拟出管道内水位变化趋势。最后分析两种方法模拟管道水流的原理,Drain模块中岩溶管道仅起排水作用,不允许管道内水流流向周围含水层,而River模块允许岩溶管道水与周围含水层水流交换,因而相对Drain模块而言,应用River模块概化模拟岩溶管道更加精确。       

利用示踪试验时间-浓度曲线分析岩溶管道结构特征

岩溶管道结构特征对岩溶水资源的合理取用、保护及地下工程安全施工均具有重要影响。目前在利用示踪试验曲线分析岩溶管道结构特征时,难以通过曲线叠加、钝锋、不规则上升和下降等形态准确解释多条岩溶管道连接关系、地下湖所处位置和地下水状态。运用地下水溶质运移理论,推导出岩溶管道流溶质运移模型,根据模型绘制三维溶质运移形态和理论时间-浓度曲线,结合水力学相关知识,对岩溶管道连接关系、地下湖位置及岩溶管道流形态变化对应的时间-浓度曲线进行解释。得出以下主要结论:（1）曲线出峰个数对应岩溶管道条数,由于管道径流长度及流速存在差异,双管道并联曲线存在3种模型,分别为2个孤立的单峰、下降曲线存在双峰叠加和2个连续上升的叠加峰;（2）单管道曲线下降梯度个数对应地下湖个数,多管道需结合管道个数和地下湖位置具体分析下降梯度和地下湖个数关系,根据地下湖位置将双管道并联岩溶管道划分4种类型,即地下湖存在于未分支管道、主管道、支管道、主管道和分支管道上;（3）曲线形态极速变化标志着管道流发生表流和承压流的相互转化。研究结果可为岩溶地区地下水保护及地下工程安全建设提供保障。     

初始裂隙对岩溶水紊流形成的影响

岩溶地区地下发育着大量的溶洞和地下河管道,地下水流状态既有层流也有紊流,而紊流是溶洞管道形成的重要条件。紊流的形成受到岩石初始裂隙的影响,初始裂隙的张开度、分布、走向、迹长、密度等因素都影响着裂隙发育过程中水流状态的变化。通过对不同统计特征的初始裂隙网络进行水流和溶蚀的数值模拟发现,以张开度标准差反映的裂隙网络非均匀性越强,模拟紊流出现的时间就越早;主要裂隙的存在使裂隙网络的非均性增强,主要裂隙与水力梯度总方向的角度越小,紊流出现的时间就越早;当裂隙平均迹长过小时会导致裂隙连通性较差,影响裂隙水流和溶蚀作用;裂隙密度,尤其是主要裂隙密度,对岩溶发育的影响较大。相对于次要裂隙,如果主要裂隙密度偏小,紊流形成时间会大大增加,甚至很难形成紊流。当初始裂隙张开度小于0.001 cm,增大水力梯度仍没有紊流发生,岩溶几乎不发育。      

基于CFP的岩溶管道流溶质运移数值模拟研究

多重岩溶含水介质的复杂性导致岩溶地下水流动及溶质运移的数学模拟成为地下水研究难点之一。为了探讨岩溶多重含水介质中地下水流溶质运移特征,文章构建了管道流CFP水流模型和MT3DMS溶质运移三维耦合数值模型。在阐述管道流CFP和MT3DMS基本原理的基础上,通过建立水文地质概念模型算例（1个落水洞、4个直管道）,探讨岩溶管道水流及溶质运移规律,分析讨论不同水文地质参数对浓度穿透曲线的影响。研究结果表明:管道流CFP模型能够刻画岩溶管道与基岩裂隙水流交换特征,MT3DMS模型能够模拟穿透曲线的拖尾现象,符合实际岩溶区特征。随着水力梯度、管道直径及管道渗透系数增大,孔隙度减小,浓度曲线峰值越大,峰值到达时间越快,浓度穿透曲线越对称。得出结论:耦合CFP水流模型和MT3DMS溶质运移模型能够刻画岩溶管道流溶质运移规律,为研究岩溶复杂介质污染物运移特征提供一种思路和途径。     

Relative importance and chemical effects of diffuse and focused recharge in an eogenetic karst aquifer: an example from the unconfined upper Floridan aquifer, USA

Karst aquifer studies often focus on allogenic water inputs and large conduit flow. However, diffuse recharge can be significant, particularly in unconfined eogenetic karst aquifers that retain high matrix permeability. This study examines an unconfined region of the upper Floridan aquifer (USA) that hosts a sinking stream, its resurgence, and a large conduit system. Daily diffuse recharge was approximated using a water-budget method and ranged from 17% of precipitation during a low precipitation year to >53% during the highest precipitation year, illustrating the highly variable nature of diffuse recharge in this region. The total allogenic input via the sinking stream over the 5 years of the study was significantly larger than the volume of diffuse recharge. However, only about 2% of the allogenic recharge flows from the conduit into the surrounding aquifer. That flow is restricted to storm events when hydraulic heads in the conduits exceed those in the surrounding aquifer. The estimated volume of dissolution is similar for allogenic recharge and diffuse recharge to the unconfined region surrounding the conduits, but dissolution from the diffuse recharge is distributed over a larger area than dissolution from allogenic recharge. These results exemplify how recharge type impacts flow and water–rock interactions in eogenetic karst aquifers.     

贵州乌江南源上游流域不同岩溶地貌单元的流量衰减分析

选取云南高原向贵州高原过渡地带上的乌江南源——贵州省三岔河上游流域作为研究对象，分别提取其流域内的花渔洞地下河流域与三塘地下河流域的地形指标，分析两者的流量衰减现状，并探讨了岩溶地表形态和岩溶含水介质结构之间的关系。结果显示:（1）花渔洞地下河流域的平均起伏度为73.2 m，可分为高起伏度的Ⅰ区（均值122 m）和低起伏度的Ⅱ区（均值64 m）两个区，其分别占整个流域面积的14.9%和85.1%；三塘地下河流域的平均起伏度为87.6 m，可分为高起伏度的Ⅰ区（均值106 m）、中起伏度的Ⅱ区（均值81 m）和低起伏度的Ⅲ区（均值48 m）三个区，其分别占流域面积的40.2%，49.7%和10.1%；（2）花渔洞地下河流域各阶段的衰减系数为0.000 3，0.000 1，0.000 06（0.5 h）-1，总蓄水量达2 199.356×104 m3，各亚动态蓄水量分别占总蓄水量的0.1%，9.9%、90.0%；三塘地下河流域各阶段的衰减系数为0.001、0.000 3、0.000 1（0.5 h）-1，总蓄水量为2 310.902×104 m3，各亚动态蓄水量分别占总蓄水量的9.4%、30.8%、59.8%。这表明地面起伏度和衰减动态之间可能存在着关联性，即高起伏度对应岩溶发育强烈的管道流，低起伏度对应导水通道较小的裂隙、溶隙等含水层。      

A parsimonious distributed model for simulating transient water flow in a high-relief karst aquifer

A mathematical model of a highly heterogeneous functioning karst aquifer is described. The aquifer is in a high-relief karst massif and, as is common for such locations, data are scarce and there are no borehole, piezometer or pumping-test data. The scarcity of data in this case required a parsimonious approach to ensure that the level of complexity of the model was commensurate with the amount, type and quality of the available data. Parsimony also requires the model to include the minimum essential components that account adequately for the data, which in this and similar cases are the functional dualities of the karst system: duality in recharge, flow and discharge. The model is three-dimensional (3D) in the sense that the aquifer is discretized into 3D voxels, although the flow is one-dimensional (1D) and vertical in the vadose zone, and horizontal and two-dimensional (2D) in the saturated zone. The parsimonious model was designed by coupling a 1D unsaturated gravity-driven flow along the vertical (along each column of voxels that discretize the aquifer) and a 2D unconfined Darcy flow in the saturated zone. In the context of this type of aquifer, preferential recharge through the network of karst conduits implies a rapid rise in the water table, the location and extension of which are model parameters. The karst springs are simulated by drains. The methodology, which is completely general, is illustrated by application to the karst aquifer in the Sierra de las Nieves mountains in southern Spain.     

Development of a Darcy-Brinkman model to simulate water flow and tracer transport in a heterogeneous karstic aquifer (Val d’Orléans, France)

Darcy’s law is the equation of reference widely used to model aquifer flows. However, its use to model karstic aquifers functioning with large pores is problematic. The physics occurring within the karstic conduits requires the use of a more representative macroscopic equation. A hydrodynamic model is presented which is adapted to the karstic aquifer of the Val d’Orléans (France) using two flow equations: (1) Darcy’s law, used to describe water flow within the massive limestone, and (2) the Brinkman equation, used to model water flow within the conduits. The flow equations coupled with the transport equation allow the prediction of the karst transfer properties. The model was tested by using six dye tracer tests and compared to a model that uses Darcy’s law to describe the flow in karstic conduits. The simulations show that the conduit permeability ranges from 5?×?10^?6 to 5.5?×?10^?5?m² and the limestone permeability ranges from 8?×?10^?11 to 6?×?10^?10?m². The dispersivity coefficient ranges from 23 to 53 m in the conduits and from 1 to 5 m in the limestone. The results of the simulations carried out using Darcy’s law in the conduits show that the dispersion towards the fractures is underestimated.