Spatial resolution of transport parameters in a subtropical karst conduit system during dry and wet seasons

Karst aquifers are characterized by a high degree of hydrologic variability and spatial heterogeneity of transport parameters. Tracer tests allow the quantification of these parameters, but conventional point-to-point experiments fail to capture spatiotemporal variations of flow and transport. The goal of this study was to elucidate the spatial distribution of transport parameters in a karst conduit system at different flow conditions. Therefore, six tracer tests were conducted in an active and accessible cave system in Vietnam during dry and wet seasons. Injections and monitoring were done at five sites along the flow system: a swallow hole, two sites inside the cave, and two springs draining the system. Breakthrough curves (BTCs) were modeled with CXTFIT software using the one-dimensional advection-dispersion model and the two-region nonequilibrium model. In order to obtain transport parameters in the individual sections of the system, a multi-pulse injection approach was used, which was realized by using the BTCs from one section as input functions for the next section. Major findings include: (1) In the entire system, mean flow velocities increase from 183 to 1,043 m/h with increasing discharge, while (2) the proportion of immobile fluid regions decrease; (3) the lowest dispersivity was found at intermediate discharge; (4) in the individual cave sections, flow velocities decrease along the flow direction, related to decreasing gradients, while (5) dispersivity is highest in the middle section of the cave. The obtained results provide a valuable basis for the development of an adapted water management strategy for a projected water-supply system.     

Regional-scale analysis of karst underground flow deduced from tracing experiments: examples from carbonate aquifers in Malaga province,southern Spain

Tracer concentration data from field experiments conducted in several carbonate aquifers (Malaga province, southern Spain) were analyzed following a dual approach based on the graphical evaluation method (GEM) and solute transport modeling to decipher flow mechanisms in karst systems at regional scale. The results show that conduit system geometry and flow conditions are the principal factors influencing tracer migration through the examined karst flow routes. Solute transport is mainly controlled by longitudinal advection and dispersion throughout the conduit length, but also by flow partitioning between mobile and immobile fluid phases, while the matrix diffusion process appears to be less relevant. The simulation of tracer breakthrough curves (BTCs) suggests that diffuse and concentrated flow through the unsaturated zone can have equivalent transport properties under extreme recharge, with high flow velocities and efficient mixing due to the high hydraulic gradients generated. Tracer mobilization within the saturated zone under low flow conditions mainly depends on the hydrodynamics (rather than on the karst conduit development), which promote a lower longitudinal advection and retardation in the tracer migration, resulting in a marked tailing effect of BTCs. The analytical advection-dispersion equation better approximates the effective flow velocity and longitudinal dispersion estimations provided by the GEM, while the non-equilibrium transport model achieves a better adjustment of most asymmetric and long-tailed BTCs. The assessment of karst underground flow properties from tracing tests at regional scale can aid design of groundwater management and protection strategies, particularly in large hydrogeological systems (i.e. transboundary carbonate aquifers) and/or in poorly investigated ones.     

Simulating flow in karst aquifers at laboratory and sub-regional scales using MODFLOW-CFP

Groundwater flow in a well-developed karst aquifer dominantly occurs through bedding planes, fractures, conduits, and caves created by and/or enlarged by dissolution. Conventional groundwater modeling methods assume that groundwater flow is described by Darcian principles where primary porosity (i.e. matrix porosity) and laminar flow are dominant. However, in well-developed karst aquifers, the assumption of Darcian flow can be questionable. While Darcian flow generally occurs in the matrix portion of the karst aquifer, flow through conduits can be non-laminar where the relation between specific discharge and hydraulic gradient is non-linear. MODFLOW-CFP is a relatively new modeling program that accounts for non-laminar and laminar flow in pipes, like karst caves, within an aquifer. In this study, results from MODFLOW-CFP are compared to those from MODFLOW-2000/2005, a numerical code based on Darcy’s law, to evaluate the accuracy that CFP can achieve when modeling flows in karst aquifers at laboratory and sub-regional (Woodville Karst Plain, Florida, USA) scales. In comparison with laboratory experiments, simulation results by MODFLOW-CFP are more accurate than MODFLOW 2005. At the sub-regional scale, MODFLOW-CFP was more accurate than MODFLOW-2000 for simulating field measurements of peak flow at one spring and total discharges at two springs for an observed storm event.     

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.

     

Inference of the structure of karst conduits using quantitative tracer tests and geological information: example of the Swiss Jura

Karst aquifers are known for being particularly heterogeneous with highly transmissive conduits embedded in low permeability volumes of rock matrix. Artificial tracer experiments have been carried out in a complex karst aquifer of the folded Jura Mountains in Switzerland with the aim of deciphering the conduit organisation. It is shown that tracer experiments with multiple injection points under different flow conditions can lead to useful information on the conduits’ structure. This information has been combined with data from structural geology, spring hydrology, and speleological observations. A conceptual model of the conduit network shows that a detailed inference of the conduit organisation can be reached: geology controls conduit location and orientation; spring hydrology, including temporary springs, constrains conduit elevations and relative hydraulic heads in the aquifer subsystems; and tracer tests identify major flow paths and outlets of the system and dilution caused by non-traced tributaries, as well as the presence of secondary flow routes. This understanding of the Aubonne aquifer structure has important implications for the future management of the groundwater resource. Similar approaches coupling geological information, spring hydrology, and multi-tracer tests under various flow conditions may help to characterise the structure of the conduit network in karst aquifers.     

弯曲岩溶管道溶质运移的尺度效应研究

岩溶管道溶质运移的尺度效应研究对穿透曲线的正确解译非常重要,但目前针对单一弯曲管道中溶质运移尺度效应的研究仍比较缺乏。文章将岩溶管道和溶潭分别概化为透明软管和水箱,基于前期建立的水箱-管道系统(简称“管道系统”),在水箱下游设置不同长度的弯曲管道,通过示踪试验研究管道运移尺度对穿透曲线的影响,并采用暂时存储模型模拟试验曲线。结果表明：(1)随着水箱下游管道长度的增加,峰值质量浓度逐渐缓慢降低,穿透曲线上升段斜率无明显变化,穿透曲线拖尾逐渐缩短,表明运移管道长度增加对溶质运移的影响大于下游管道弯曲;(2)穿透曲线偏度系数、后段溶质羽穿透时间和溶质羽穿透时间与管道系统长度呈良好的负线性相关关系(R²>0.96);(3)当对称和不对称水箱管道系统长度分别增加至154.5 m和164.3 m时,偏度系数接近0,穿透曲线分布接近对称;(4)弥散系数、存储区截面积和交换系数与管道系统长度呈良好的负线性相关关系,当对称和不对称水箱管道系统长度分别增加至159.9 m和178.1 m时,存储区截面积接近0,水箱导致的溶质运移滞后效应基本消失。研究结果对野外岩溶管道穿透曲线的...     

Comparison of a karst groundwater model with and without discrete conduit flow

Karst aquifers exhibit a dual flow system characterized by interacting conduit and matrix domains. This study evaluated the coupled continuum pipe-flow framework for modeling karst groundwater flow in the Madison aquifer of western South Dakota (USA). Coupled conduit and matrix flow was simulated within a regional finite-difference model over a 10-year transient period. An existing equivalent porous medium (EPM) model was modified to include major conduit networks whose locations were constrained by dye-tracing data and environmental tracer analysis. Model calibration data included measured hydraulic heads at observation wells and estimates of discharge at four karst springs. Relative to the EPM model, the match to observation well hydraulic heads was substantially improved with the addition of conduits. The inclusion of conduit flow allowed for a simpler hydraulic conductivity distribution in the matrix continuum. Two of the high-conductivity zones in the EPM model, which were required to indirectly simulate the effects of conduits, were eliminated from the new model. This work demonstrates the utility of the coupled continuum pipe-flow method and illustrates how karst aquifer model parameterization is dependent on the physical processes that are simulated.     

Hydrogeological characteristics of some deep siphonal springs in Serbia and Montenegro karst

In terms of hydrogeological, engineering-geological, and hydrotechnical tapping in karst in relation to ground waters, karst channels, springs and ponors, speleodiving is the only research method which enables direct observation, studying and exact geological mapping of karst channels and caverns. Data collected during speleodiving research contribute considerably to the analysis of karst evolution process in the given region, which is very important in evaluating the depth of karstification and determining the main direction of the groundwater flow. In the past 30 years in Serbia and Montenegro, speleodivers have investigated over 40 siphonal springs, active cave channels and ponors, of which more than 20 are proof of deep siphonal circulation in karstic aquifers. The karstic springs are the most interesting phenomenon from a hydrogeological view point, and their investigations need particular attention. Most of significant karstic springs are on the rims of erosion basins—perimeters of karst poljes, river valleys, sea coasts and contact areas between karst aquifers and hydrogeological barriers. General characteristics of the spring regime are the direct correlation between precipitation and spring discharge. Moreover, the hydrogeological regime of these springs also depends on the size of the catchment area, karstic aquifer retardation capacity, total porosity, as well as lithological and structural characteristics.     

Improved regional groundwater flow modeling using drainage features: a case study of the central northern karst aquifer system of Puerto Rico (USA)

In northern Puerto Rico (USA), subsurface conduit networks with unknown characteristics, and surface features such as springs, rivers, lagoons and wetlands, drain the coastal karst aquifers. In this study, drain lines connecting sinkholes and springs are used to improve the developed regional model by simulating the drainage effects of conduit networks. Implemented in an equivalent porous media (EPM) approach, the model with drains is able to roughly reproduce the spring discharge hydrographs in response to rainfall. Hydraulic conductivities are found to be scale dependent and significantly increase with higher test radius, indicating scale dependency of the EPM approach. Similar to other karst regions in the world, hydraulic gradients are steeper where the transmissivity is lower approaching the coastline. This study enhances current understanding of the complex flow patterns in karst aquifers and suggests that using a drainage feature improves modeling results where available data on conduit characteristics are minimal.     

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     

穿透曲线反演管壁溶质通量的实验研究

泉口穿透曲线提供了一个便捷有效的方法来研究岩溶含水层中污染物的运移。在旱季,滞留在石灰岩中的污染物通 过岩壁释放入管道,与管道水相混合并运输到下游泉口,因此,泉口穿透曲线能够反演出污染物释放时的管壁溶质通量。 笔者利用一个基于运移稀释模型的解析解,由40条实验室测量的穿透曲线来反演出相应的管壁溶质通量。结果表明,其基 本形状与穿透曲线相似,管壁透水的非均匀性在穿透曲线上明显表现出来,而在管壁溶质通量上表现为强振荡。因此,管 壁溶质通量比泉口穿透曲线对管壁透水的非均匀性有更高的分辨率。该通量是一个连接泉口穿透曲线和污染物在岩石中早 期分布的中间量,本文提供了一种研究管壁污染物释放过程的新方法。     

An improved method for delineating source protection zones for karst springs based on analysis of recession curve data

A standard method for delineating source protection zones, particularly for karst and carbonate springs, has been improved. The method, based on recession curve analysis, defines four vulnerability scenarios with an evaluation of the appropriate dimensions of the protection areas, accommodating situations where field-test data are not available. The new approach makes it easier to separate the components of the spring discharge hydrograph. The objective is to achieve simplification, and an effective, more rigorous, procedure in the determination of the parameters used by Mangin’s model—Mangin A (1975) Contribution a l’étude hydrodynamique des aquifères karstiques-Troisieme partie: Constitution et fonctionnement des aquifères karstiques (Contribution to the hydrodynamic study of karst aquifers, part 3: formation and work of karst aquifers). Ann Speleol 30(1):21–124. The original procedure, plus the lack of sufficient data, was open to subjective interpretation. With the aid of modern technology, a very large quantity of data is now available and it is necessary to process it using denoise type computer-based filters before passing to interpretation. Working with discharge data series, a statistical approach is proposed to give an analytical solution for determining the values of fundamental parameters of the recession curve model. The new procedure is defined and compared with the original methodology. The new approach has been tested and applied to a number of karst springs in Italy. A case history for a spring located in the Piedmont region of the Maritime Alps, is presented. The proposed new procedure can be utilised to mark the limits of the protection zones of tapped groundwater supplied for potable use, as required by European and local legislation.     

Analysis of karst spring recession curves,west of Iran

The recession hydrographs of karst springs provide important information about aquifer characteristics such as storage properties and drainage potential, karstification degree, and other hydrological features. The Faresban, Famaasb, Gonbad-e-Kabood, and Gian springs are the important karst springs which drain main karst aquifers in west of Iran. In this study, we evaluate hydrological characteristics of these four karst springs using different analytic equations. Almost, all the recession curves were well fitted by a function that consists of three main exponential terms. The initial portion of the recession curve represents the fast drainage of large fractures and conduits, ending with the slowly decreasing curve, where the drainage of rock matrix and small fissures or baseflow is dominant. The Gian spring has the largest storage volume but the lowest drained water (7%). Using Mangin’s equation, it is apparent that the baseflow supplies over 85% of the total drained water to the Gian spring, while a small quantity of total outflow is made up of quickflow periods. Of the water available to the Famaasb, Faresban, and Gonbad-e-Kabood springs, 85, 80, and 70% are drained during period of baseflow, respectively, whereas the quickflow periods have minor importance in some springs. Based on the Coutagne’s equation results, it can be concluded that the catchment of the Famaasb and Gonbad-e-Kabood springs have a karstified saturation zone with drainage channels that provide a rapid discharge with little storage capacity. At Gian catchment area, the initial discharge is not as rapid as karstification but is fairly homogeneous throughout the aquifer.     

岩溶区多重介质水流模型研究进展

数值模拟技术广泛应用于地下水资源评价工作中,为水资源合理开发利用提供科学依据。本文针对岩溶区的数值模拟技术展开研究,综述了岩溶区地下水流建模技术的主要方法和研究进展,介绍了SWMM模型、UGRFLOW模型、CAVE模型、CFP模型等多重介质模型的建模方式和运行原理。岩溶区地下水赋存介质复杂,管道、裂隙和空隙多重介质并存,水流特征多样,等效连续介质模型不能很好地刻画岩溶区复杂的地下水流特征。随着对岩溶水流系统研究的深入,二重和三重介质模型逐渐应用于岩溶区地下水流模拟工作中,取得较好成果,但仍存在如忽略中宽裂隙的导水作用、岩溶管道设置单一、模型适用性不强等问题。今后应从解析岩溶含水介质的结构特征入手,关注中宽裂隙在地下水流系统中的导水作用和水流特征,改进管道流模块以更好地刻画复杂的地下河管道,改进不同介质间水流交换量算法,提高模型精确性和适用性。      

The comparison of hydrodynamic characteristics of karst aquifers: application on two karst formations in Zagros (Asmari and Ilam-Sarvak), southwest Iran

In order to study the function, hydrodynamic behavior, and hydraulic properties of the karst aquifers in Izeh, southwest Iran, time series analysis was applied to the precipitation, spring discharge, and piezometric head data of two representative karst systems of Zagros (Ilam-Sarvak and Asmari Formations). Time series analysis was applied to two karst aquifers, those of Asmari and Ilam-Sarvak Formations. The daily precipitations of anticlines were estimated based on the precipitation–elevation function which was applied on digital elevation model (DEM) of the area. The mean estimated daily precipitations were considered in bivariate time series analysis as input data of each karst system. The total length of time series was about 2.7 years, extending from May 2007 to December 2009. During the research, several one-parameter probe data loggers were installed, which daily measure the water surfaces in karst aquifers. Time series analysis was applied for describing Izeh karst aquifers with a focus on both univariate (autocorrelation and spectral analysis) and bivariate (cross-correlation, gain function, and coherency function) methods. The results show that Asmari karst aquifer in Kamarderaz Anticline has a large storage capacity. Because of lacking a well-organized karst network, in the Asmari karst aquifer, baseflow dominates with low contribution of quick-flow. In the Ilam-Sarvak karst aquifer (Shavish and Tanush Anticlines), the karstification occurred in fractures and small diameter conduits, which caused to quick-flow between dense limestone. The Ilam-Sarvak karst aquifer could be regarded as a transition between two extreme types of karst, e.g., highly karstified system and in the opposite, extremely diffused one. The analysis of well hydrograph in Ilam-Sarvak karst aquifer shows that the karst aquifer has a low storage capacity. Unlike Asmari karst aquifer, the fractures and small diameter conduits in Ilam-Sarvak karst aquifer are more enhanced, producing a better developed karst network. Contrary to the typical karst systems, however, diffuse flow and conduit flow coexist in the Asmari Formation.     

Assessment of key transport parameters in a karst system under different dynamic conditions based on tracer experiments: the Jeita karst system,Lebanon

Artificial tracer experiments were conducted in the mature karst system of Jeita (Lebanon) under various flow conditions using surface and subsurface tracer injection points, to determine the variation of transport parameters (attenuation of peak concentration, velocity, transit times, dispersivity, and proportion of immobile and mobile regions) along fast and slow flow pathways. Tracer breakthrough curves (TBCs) observed at the karst spring were interpreted using a two-region nonequilibrium approach (2RNEM) to account for the skewness in the TBCs’ long tailings. The conduit test results revealed a discharge threshold in the system dynamics, beyond which the transport parameters vary significantly. The polynomial relationship between transport velocity and discharge can be related to the variation of the conduit’s cross-sectional area. Longitudinal dispersivity in the conduit system is not a constant value (α?=?7–10 m) and decreases linearly with increasing flow rate because of dilution effects. Additionally, the proportion of immobile regions (arising from conduit irregularities) increases with decreasing water level in the conduit system. From tracer tests with injection at the surface, longitudinal dispersivity values are found to be large (8–27 m). The tailing observed in some TBCs is generated in the unsaturated zone before the tracer actually arrives at the major subsurface conduit draining the system. This work allows the estimation and prediction of the key transport parameters in karst aquifers. It shows that these parameters vary with time and flow dynamics, and they reflect the geometry of the flow pathway and the origin of infiltrating (potentially contaminated) recharge.     

Increase of vulnerability of karst aquifers due to leakage from landfills

Karst aquifers are very vulnerable to various pollution sources. Among these, landfills can contribute contaminants over long periods. Groundwater quality monitoring is required to assess the impact of a landfill leachate on underlying aquifer water or spring discharge. Tracer tests are useful for selecting the location and frequency of sampling. Three landfill sites in karst areas were studied with tracer tests. Additional insight was accomplished by the comparison of the tracer transfer and the breakthrough curves obtained from these tests with the results of two other tracer tests carried out in the same period in karst areas without a landfill. To explain the differences observed, the hypothesis of increased permeability below the landfill due to the presence of inorganic acids in leachates was further tested. The results of detailed, long-term monitoring of contaminated drip water in the Postojna Cave were used to verify the hypothesis. The analysis of the simultaneously increased content of calcium, magnesium, and contaminants in the drip water indicates a direct correlation between limestone dissolution and contaminants. Increased dissolution increases secondary porosity and thus permeability of the vadose zone resulting in higher vulnerability of underlying aquifers and springs in the vicinity landfills.     

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.     

岩溶地下水脆弱性评价研究

岩溶地下水脆弱性评价是基于保护岩溶含水层从而有效地管理和利用地下水提出的有效方法和手段。岩溶含水层的特殊性导致污染物极易进入地下含水层,并在岩溶管道中快速、远距离地运移。与其他基岩第四系含水层相比,岩溶含水层的保护面临更大的挑战。介绍了地下水脆弱性的概念;以及岩溶地下水脆弱性的相关问题,并讨论了岩溶区脆弱性评价应考虑的一些问题。