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Abstract and Figures
The contaminant transport processes in karst water systems have a direct impact on the quality and utilization of karst water resources. The storage and release of contaminants or conservative solutes during the solute transport process is a common phenomenon in karst aquifers. The impact of the storage and release is more prevalent after focused recharge events. In this study, laboratory experimental and numerical studies were conducted to quantify the storage and release processes of conservative solute. The results showed that, conduit water recharges into fissures under high water head conditions, and the fissure water drains back into the conduit while the hydraulic gradient reverse. The conservative solute storage-release process controlled by hydrodynamic conditions produces strong asymmetry, long tailing, or bi-peak in the breakthrough curves (BTCs). The BTCs change from single peak to bi-peak with enhanced hydrodynamics under focused recharge conditions. The dual heterogeneous domain model was calibrated to simulate the long-tailing of the BTCs and their noticeably bimodal characteristics. The flow velocity and dispersion coefficient are the major variables that regulate the bimodal structure of the BTCs, which also control the solute storage-release differences between the conduit and fissures. The bimodal structure of the BTCs becomes more pronounced for large discrepancies in flow velocities. The total BTCs are a superposition of solute transport in the conduit path and storage-release path. A method to evaluate the mass of conservative solute transport in storage-release paths was proposed by segmenting the transport curve in the conduit from the total BTCs, thus quantifying the effects of the groundwater storage-release mechanism on the solute transport process in the karst water system.
The karst development makes aquifer have strong anisotropy and heterogeneity. In order to reveal the characteristics of solute transport in the karst fissure–conduit aquifer system, this study presents a physical model of fissure–conduit in laboratory experiments to carry out the solute transport simulation. In this paper, the tracer tests of fissure–conduit combination, fissure, and conduit solute transport process in saturated flow are designed. We found that different aquifer structures and tracer injection points have an influence on the shape of the breakthrough curve. Besides, the two-dimensional dispersion model of tracer injection of the instantaneous point was used to calculate the dispersion parameters of each group of experiments. Then, the dynamic responses of the linear distance (x) between the injection point and the receiving point, initial time (t0), peak time (tm), peak concentration (cm), average tracer transport velocity (V), and porosity (p) of aqueous media to the longitudinal dispersion coefficient are discussed. In addition, according to the measured data, Gaussian multi-peak fitting can be used to reflect the overall shape and change trend of the multi-peak BTC. These results demonstrate the solute transport behaviors in the saturated karst aquifer system, which have important reference significance for solving the engineering environmental problems in the karst area.
We review scale dependence of hydraulic conductivities and effective porosities for prediction of contaminant transport in four UK karst aquifers. Approaches for obtaining hydraulic parameters include core plug, slug, pumping and pulse tests, calibration of groundwater flow models and spring recession curves. Core plug and slug tests are unsuitable because they do not characterize a large enough volume to include a representative fracture network. Pumping test values match regional-scale hydraulic conductivities from flow modelling for the less intensively karstified aquifers: Magnesian Limestone, Jurassic Limestone and Cretaceous Chalks. Reliable bulk hydraulic conductivities were not available for the intensively karstified Carboniferous Limestone due to dominance of flow through pipe conduits in Mendips. Here, the only hydraulic conductivity value found from spring recession is one order of magnitude higher than that indicated by pumping tests. For all four carbonate aquifers, effective porosities assumed for transport modelling are two orders of magnitude higher than those found from tracer and hydrogeophysical tests. Thus, a combination of low hydraulic conductivities and assumed flowing porosities resulted in underestimated flow velocities. The UK karst aquifers are characterized by a range of hydraulic behaviours that fit those of karst aquifers worldwide. Indeed, underestimation of flow velocity due to inappropriate parameter selection is common to intensively karstified aquifers of southern France, north-western Germany and Italy. Similar issues arise for the Canadian Silurian carbonates where the use of high effective porosities (e.g. 5%) in transport models leads to underestimation of groundwater velocities. We recommend values in the range of 0.01–1% for such aquifers.
It is a challenge to describe the hydrogeological characteristics of karst aquifers due to the complex structure with extremely high heterogeneity. As the response of karst aquifers to rainfall events, spring discharge variations after precipitation can be used to identify the internal structure of karst systems. In this study, responses of spring discharge to different kinds of precipitations are investigated by continuously monitoring precipitation and karst spring flow at a single-conduit karst aquifer in western Hunan province, China. Recession curves were used to analyze hydrodynamic behaviors and separate recession stages. The results show that the shape of the recession curve was changed under different rainfall conditions. Recession processes can be divided in to three recession stages under heavy rain conditions due to water drainage mainly from conduits, fracture, and matrix at each stage, but only one recession stage representing drainage mainly from matrix in the case of light rain. With the change in amount and intensity of precipitation, the calculated recession coefficient at each stage changes in an order of magnitude. The influence of precipitation on the recharge coefficient and the discharge composition at each recession are discussed, and then the conceptual model diagram of water filling and releasing in the single-conduit karst aquifers is concluded. The findings provide more insight understand on hydraulic behaviors of karst spring under different types of rainfall events and provide support for water resource management in karst regions.
Clean groundwater is essential for water supply in many regions of the world. Fast flow to the groundwater through enlarged cracks and fissures, which is known to transmit short-lived pollutants into the groundwater, is often neglected in large-scale studies. We quantify the rapid transport of pollutants by fast flow into the carbonate rock groundwater storage in Europe, North Africa, and the Middle East. We show that, through rapid transport, up to 50% of infiltrating pollutants may still reach the groundwater table, which is substantially more than estimated when fast transport is neglected. These results imply that the contamination risk of usable groundwater storages is much larger than expected where fast flow to the groundwater occurs.
In this study, a synthetic modeling approach is proposed to quantify the effect of the amount and direction of the exchange flow on the karstic spring discharge fluctuations under different hydrologic conditions corresponding to high and low flow conditions. We hypothesis that the spring discharge fluctuations constitute a valuable proxy to understand the internal processes of the karst system. An ensemble of spring hydrographs was synthetically produced to highlight the effect of exchange flow by exploring the plausible range of variability of coefficients of exchange flow, conduit diameter, and matrix hydraulic conductivity. Moreover, the change of the rate of point recharge through the karst conduit allows for the quantifying of the sensibility of the spring hydrograph to the directions of exchange flow. We show that increasing the point recharge lies to a remarkable linear recession coefficient (β) as an indication of the conduit flow regime. However, a reduction in and/or lack of the point recharge caused the recession coefficient to change to exponential (α) due to the dominant effect of the matrix restrained flow regime and/or conduit-influenced flow regime. The simulations highlight that the exchange flow process from the conduit to the matrix occurred in a short period and over a restricted part of the conduit flow regime (CFR). Conversely, the exchange flow dumped from the matrix to the conduit occurs as a long-term process. A conceptual model is introduced to compare spring hydrographs’ characteristics (i.e., the peak discharge, the volume of baseflow, and the slope of the recession curve) under the various flow conditions with the directions of the exchange flow between the conduit and the matrix.
Interactions between surface water (SW) and groundwater (GW) have been a focus of watershed hydrology research for a long time. A holistic perspective on integrated SW–GW modeling approach is necessary to understand the hydrological and biogeochemical processes of these two interconnected systems within the watershed. This paper reviewed the progress and coupling strategy of one important SW model (Soil and Water Assessment Tool, SWAT) and GW model (Modular Finite Difference Groundwater Flow, MODFLOW) since 1999. Three main stages of development of coupled SWAT–MODFLOW model are reflected by the high citation of publications by three pioneer studies, which are Sophocleous et al., 1999, Kim et al., 2008, Bailey et al., 2016. Currently, the research scope of coupled SWAT–MODFLOW models is focused on hydrologic processes, solute transport and the effects of climate change and human activity on water resources. Major uncertainties of SWAT–MODFLOW from model structure, database and parameterization are discussed. In an era of big data, the coupled SWAT–MODFLOW model has great potential to improve understanding of hydro–biogeochemical processes and support sustainable water and ecological management in the watershed.
Dye tracing is an efficient method for spring watershed delineation, but is also used in surface waters to assess pollution migration over several kilometers. The aim of this study is to develop a simple and parsimonious approach that accounts for a linear relationship between dispersivity and scale that could be used for the simulation of large-scale transport processes in aquifers. The analysis of 583 tracer recoveries is used to validate an inverse relationship between arrival time and peak concentration, which is shown to be a consequence of the linear relationship between dispersivity and scale. These results show that the tracer displacement through a given tracing system can be characterized at a large scale by a constant Peclet number. This interpretation is used to propose a new approach for tracer test design based on the analytical expression of the peak/time factor. It is also used for Peclet number assessment and simulation of the whole tracer residence-time distribution using a new method based on the ratio between the mode of the residence time distribution (hmod) and the corresponding time from injection (tmod), which is called the hmod/tmod method. This methodology is applied to two tracer tests carried out in a karst aquifer over 13 km between the same injection and detection points under distinct hydrological conditions. These results found practical applications in generalizing tracer test results to various flow conditions, or guiding the parameterization of physically-based vulnerability mapping methods.
Tracer methods have been widely used in many fields of environmental and natural sciences, and also in human health sciences. In particular, tracers are used in the study of karst hydrogeology, typically focusing on phenomena such as sinkholes, sinking rivers and large karst springs. It is known that tracers have been used since antiquity. The aim of tracer tests has been to investigate underground flow paths, transport processes and water–rock interactions, and to get an insight into the functioning of a karst aquifer. In karst hydrogeology, tracer methods are the most important investigation tools beside conventional hydrological methods. In early times, tracer methods were applied only to investigate underground flow-paths. Later they were also used to elucidate transport processes associated with water flow, and today they are often the basis, together with detailed hydrological information, of groundwater protection investigations and aquifer modelling. Many substances (spores, microspheres, bacteriophages, salt tracers, fluorescent dyes, radioactive substances) have been investigated for their properties and potential usage in environmental investigations, in particular the often unknown and inaccessible underground systems of karst areas. A great number of analytical techniques is available. This includes instrumentation for laboratory applications and direct online, on-site or in-situ field measurements. Modern instruments have a high capability for data acquisition, storage and transmission in short intervals, as a basis for quantitative evaluation and modelling. This enables research on the hydrological and hydrochemical dynamics of aquifers and their response to different natural or anthropogenic impacts.
The tailing of breakthrough curves (BTCs) commonly observed in the field is often associated with the existence of pools in karst conduits. The transient storage model was always used to simulate the BTC tailing caused by the storage zone, i.e. pool, however, the relationship between parameters in this model and pool volume is not clear, which is very useful for the understanding of the internal geometry of the conduit using this model or prediction of the contaminant transport. In this paper, we performed a series of indoor tracer experiments in a pool-pipe system involving two different pool structures, symmetrical pool (SP) and asymmetrical pool (ASP) with different positions, sizes, and numbers, to explore the relationship between model parameters and the pool volume. Several experiments were conducted in the ASP of 20 cm with different discharges so as to investigate the effect of turbulent flow condition on model parameters. The experimental results indicate that the duration time of the BTC, and the mean residence time of the solute increase linearly with the pool volume (pool size or pool number). For the model parameters, the cross-sectional area of the storage zone (As) or the calculated volume (Vs) of the storage zone using As shows a good linear relationship with the pool volume (Vp) (As=0.0078Vp+3E-06; Vs=0.8195Vp+0.0003, R²=0.95) indicating a potential to use this parameter, which is almost constant with the discharge, to roughly estimate the pool volume. The cross-sectional area (A) of the main channel does not change with the pool volume and changes little with the discharge; it has a close value to the actual cross-sectional area. The remaining parameters, dispersion coefficient (D) and exchange coefficient (α), have strong connections with the pool type, pool distribution or flow condition, which can hardly be used to deduce the internal conduit structure directly. The relationships between model parameters and the pool volume are helpful to use the model parameters to identify the conduit geometry roughly.
Solute storage and release in groundwater are key processes in solute transport for groundwater remediation and protection. In karst areas where concentrated recharge conditions exist, pollution incidents can easily occur in springs that are hydraulically connected to densely inhabited karst depressions. The intrinsic heterogeneity common in karst media makes modeling solute transport very difficult with great uncertainty. Meanwhile, it is noteworthy that solute storage and release within subsurface conduits and fissures exhibit strong controlling function on pollutant attenuation during underground floods. Consequently, in this paper, we identified and estimated the solute storage and release processes in karst water systems under concentrated recharge conditions. The methodology uses the advection-dispersion method and field tracer tests to characterize solute transport in different flow paths. Two solute transport pathways were established (i.e., linear pathway (direct transport through karst conduits) and dynamic pathway (flow through fissures)). Advection-dispersion equations were used to fit the breakthrough curves in conduit flow, while the volume of solute storage in fissures were calculated by segmenting the best fitting curves from the total breakthrough curves. The results show that, greater recharge flow or stronger dynamic conditions leads to lower solute storage rate, with the storage rate values less than 10% at high water level conditions. In addition, longer residence time was recorded for solute exchange between conduits and fissures at the low water level condition, thereby contributing to a higher solute storage rate of 26% in the dynamic pathway.
To investigate the effects of flow rate variation on solute transport in a karst conduit, three pipe structures of a constant diameter pipe, the pipe connected to a symmetrical pool and an asymmetrical pool respectively were chosen, and several tracer experiments were conducted separately in each of the three pipe structures at nine flow rates. Experimental results show that the peak of the breakthrough curve (BTC) increased and the tailing decreased with increasing discharge. Three models, the advection–dispersion equation (ADE), the two-region nonequilibrium model (TRNM) and the transient storage model (TSM), were used to simulate BTCs and explore the change of transport parameters with increasing flow rate. Simulations show that ADE was capable of replicating the almost symmetrical BTCs of the single pipe but incapable of fitting the appreciable BTC tails for the pools. Nevertheless, TRNM and TSM can reproduce all BTCs of single pipe and pipe with a pool very well. The research demonstrates the significant effect of the pool on solute transport. The parameters in the two models (TRNM and TSM) exhibited similar trends with increasing discharge in either pool. In the TRNM, a clear positive correlation with discharge emerged for the partition coefficient and mass transfer coefficient. Meanwhile, the main channel cross-sectional area and exchange coefficient in TSM increased gradually with discharge. The storage zone area decreased generally with increasing flow rate. The relationship between solute transport and the flow rate is more complex in the asymmetrical pool than in the symmetrical pool.
OM-MADE (One-dimensional Model for Multiple Advection, Dispersion, and storage in Exchanging zones) is an open-source python code for simulating one-dimensional solute transport in multiple exchanging conduits and storage zones in steady-state flow conditions. It aims at helping the interpretation of multi-peaked skewed breakthrough curves (BTCs) that can be observed in tracer tests conducted in karstic systems. OM-MADE is based on the resolution of classical mass conservation equations. In OM-MADE, all parallel and exchanging flow zones are divided along the direction of flow into reaches, in which all model parameters are kept constant. The total flowrate may be modified through lateral in and outflows. The solute may also be affected by decay processes either in mobile or immobile zones. Each reach is subdivided into discrete segments of equal length. The partial differential equations can be solved using two second order schemes, one based on an operator-split approach, the other on Crank-Nicholson pondered scheme. A verification is performed against analytical solutions, OTIS software (Runkel, 1998), and the Dual-Advection Dispersion Equation (DADE) proposed by Field and Leij (2012). An application to a tracer test carried out in the karstic area of Furfooz (Belgium) is then performed to reproduce the double-peaked positively skewed BTC that has been observed. It constitutes a demonstration of the software capacities in the case of two reaches and three exchanging zones, among which two are mobile ones and one represents a storage zone. It thus permits to verify numerically the consistency of the conceptual interpretation of the observed BTC.
Solute transport characteristics and groundwater connection structures are mainly studied in this paper by adopting the numerical simulation and field tracer test. Firstly, the simulation method of solute transport is proposed. Then two kinds of representative geological models of tracer test are built, and the process of solute transport is simulated. The variation characteristics of solute transport under the conditions of straight pipeline and branch pipeline are summarized by analysis of the simulation results. The effects of the pipeline width, flow velocity and path difference of branch pipelines on the tracer curve are discussed. General laws of groundwater connection tracer curve are obtained. Finally, based on the field test results, and combined with engineering geological conditions, characteristics of karst groundwater systems, the groundwater connection structures are analysed and speculated in detail by the flow velocity and local monitoring curve. The results show that: (i) As the flow velocity increases, the peak of the tracer curve decreases gradually, as well as the time to peak; (ii) The peak of the tracer curve gradually decreases with the increasing pipeline width; (iii) Under the different path difference of branch pipeline, the tracer curves all present obvious ‘trailing’ phenomenon; (iv) The analysis process of groundwater connection structures can provide references for the same type of hydrological geological problems.
Karstic aquifers are important groundwater reservoirs that supply water to many communities in different countries, such as Iran. Therefore, studying the discharge volume and its fluctuations in addition to identify the hydraulic behavior of the karstic springs is very important. This knowledge can be realized by monitoring and analyzing of spring hydrograph in the simplest and effective way, especially by multi-year hydrograph. The main objective of this study is to compare the discharge regimes of Barme-Jamal and Poto karstic springs, formed in Kuhe-Safid anticline, south-east of Khuzestan Province, using multi-year hydrograph analysis. The discharge-stage data have been analyzed for several years to determine the recession coefficients, understand the flow regime in the aquifers and description the hydraulic discharge behavior of each spring. Accordingly, the various scenarios have been investigated based on low and high precipitated water-years. The results show that each of two springs reveals a specific hydraulic behavior based on discharge quantity and recession regimes that indicate its aquifer charactristices. The dominance of diffuse-matrix flow over the conduit flow controls the Barme-Jamal Springs discharge regime and consequently it reveals a normal recession behavior, while for the Poto spring the conduit flow with a threshold is dominant which is a scarce and complex hydraulic behavior.
The spatial and temporal complexities of flooding in karst terrains pose unique challenges in flood risk management. Lowland karst landscapes can be particularly susceptible to groundwater flooding due to a combination of low aquifer storage, high diffusivity and limited or absent surface drainage. Numerous notable groundwater flood events have been recorded in the Republic of Ireland throughout the twentieth century, but flooding during the winters of 2009 and 2015 was the most severe on record, causing widespread and prolonged disruption and damage to property and infrastructure. Effective flood risk management requires an understanding of the recharge, storage and transport mechanisms governing water movement across the landscape during flood conditions. Using information gathered from recent events, the main hydrological and geomorphological factors influencing flooding in these complex lowland karst groundwater systems are elucidated. Observed flood mechanisms included backwater flooding of sinks, high water levels in ephemerally flooded basins (turloughs), overtopping of depressions, and discharges from springs and resurgences. This paper addresses the need to improve our understanding of groundwater flooding in karst terrains to ensure efficient flood prevention and mitigation in the future, and thus helps to achieve the aims of the European Union Floods Directive.
The existence of double-peaked breakthrough curves (BTC), which are the result of the transport of a dye tracer through underground lakes, is reported. Investigations were undertaken on the Furfooz karst system in southern Belgium. In this system, the River Lesse sinks partially into a swallow hole. The water follows a solitary conduit leading to an underground lake that is directly connected to a second underground lake. Double-peaked BTCs were detected in the resurgent water, downstream of this second lake. The report first describes field data (tracer tests in various hydrologic conditions) which point towards the double peak being linked to a nonlinear process that originates within the lakes. Complementary investigations within the lakes show a complex behavior of the dye tracer related to a specific hydrodynamic feature that leads to the separation of the solute plume. A conceptual model of the solute transport within the lakes is proposed. This model emphasizes the physical effect of the lakes on the dye flow-through process.
Approximately 33% of China is karstic. The most extensive karst areas are in southwestern China and cover approximately 540,000 km 2. Southwestern China hosts some of the most typical karst landforms in the world and has important high-quality karst water resources. Due to the rapid development of China, karst waters are threatened by various types of contamination. Detail field and laboratory investigations in five provinces including several cities in southwestern China were conducted in 2008 and 2009. Eighty-three springs and underground rivers were surveyed and water samples collected from each for laboratory analyses for major ions. Four main types of karst aquifer contamination were identified based on contaminant sources: Rural and agricultural pollution, pollution from urban development and industry, pollution from mining, and accidental groundwater pollution. Several representative instances for each type of contamination and their impacts on the environment are discussed in more detail. Contamination countermeasures of karst waters and a framework for overall management of karst water resources in southwestern China are provided.
In this study, we established 7 sampling points to collect water samples from the Qingshuiquan underground river system in December 2011, which was during the dry season. By analyzing those samples, we determined the present quality of underground water of the Qingshuiquan underground river system, the characteristics of its contamination situation, the source of pollutants, and the reasons for the pollution. Despite some light-compound pollution caused by PAHs and HCHs, the overall quality of the underground water of the Qingshuiquan underground river system is good. PAHs are due to coal burning; HCHs are from the use of Lindane. 'Skylights' (roof collapse features) and sinkholes can lead pollutants to the underground water intermittently and to pollute the underground water. (C) 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of Organizing and Scientific Committee of WRI 14-2013
A karstic aquifer typically has significant secondary porosity
consisting of an interconnected system of caves or conduits.
Conduit-borne contaminants can enter the contiguous limestone matrix,
remain inside for a longer time than in the conduit, and subsequently be
flushed out. This retention or sequestration can significantly influence
the fate of contaminants within the aquifer and alter the shape of the
breakthrough curve. The mechanisms involved in sequestration have been
identified and quantified by analysis of the breakthrough curves
generated by a set of laboratory experiments in which a conduit, porous
limestone matrix, and conservative contaminant were simulated by a
porous-walled pipe, chamber of closely packed glass beads, and salt,
respectively. Experiments were conducted with both active and passive
transfer of water between conduit and matrix, simulating differing
hydrogeologic regimes. In active transfer the primary control parameter
is the volume of water transferred; sequestration is primarily due to
advection with the effects of diffusion and dispersion being minimal. In
passive transfer the control parameters are the conduit Reynolds number
and the duration that contaminant resides in the conduit; sequestration
is caused by the combined effects of the conduit pressure drop, pressure
variation due to bedform, and diffusion. Active and passive transfer can
be unified by analyzing the ratio of the scale of pressure variation to
the conduit length. In accordance with the resolved mechanisms a variety
of models have been constructed to recover solute distributions in the
matrix and to regenerate breakthrough curves. These analyses and models
provide a potential approach to investigate contaminant migration in
This review focuses on the challenges that need to be addressed to increase knowledge on the distribution, transport and biogeochemical reactions of contaminants in carbonate media. In order to do so, it has been necessary to review recent studies on contaminants that due to their emerging interest and unawareness of their threat to human have not yet been deeply analyzed in carbonate aquifers (e.g.: Dense Non-aqueous Phase Liquids (DNAPLs), light non-aqueous phase liquids (LNAPLs), Volatile Organic Compounds (VOCs), Pathogens, Contaminants of Emerging Concern (CECs), Micro- and Nanoplastics and Nanoparticles). To this end, it seems necessary to progress along two lines: (1) a methodological line, in which the advance must be achieved in the field of distribution of heterogeneous elements in carbonate formations and in early detection and warning systems; and (2) a more scientific line, in which specific studies of emerging pollutants might produce an advance in the development of flow and transport models in heterogeneous and anisotropic media.
Identification and location of contamination sources is crucial for water resource
protection — especially in karst aquifers which provide 25% of the world´s population
with water but are highly vulnerable to contamination. Transport-based source tracking
is proposed and verified here as a complementary approach to microbial and chemical
source tracking in karst aquifers for identifying and locating such sources of
contamination and for avoiding ambiguities that might arise from using one method
alone. The transport distance is inversely modelled from contaminant breakthrough
curves (BTC), based on analytical solutions of the 1D two-region non-equilibrium
advection dispersion equation using GNU Octave. Besides the BTC, the model
requires reliable estimates of transport velocity and input time. The model is shown to
be robust, allows scripted based, automated 2D sensitivity analyses (interplay of two
parameters), and can be favorable when distributed numerical models are
inappropriate due to insufficient data. Sensitivity analyses illustrate that the model is
highly sensitive to the input time, the flow velocity, and the fraction of the mobile fluid
region. A conclusive verification approach was performed by applying the method to
synthetic data, tracer tests, and event-based field data. Transport distances were
correctly modelled for a set of artificial tracer tests using a discharge-velocity
relationship that could be established for the respective karst catchment. For the first
time such an approach was shown to be applicable to estimate the maximum distance
to the contamination source for coliform bacteria in karst spring water combined with
microbial source tracking. However, prediction intervals for the transport distance can
be large even in well-studied karst catchments mainly related to uncertainties in the
flow velocity and the input time. Using a maximum transport distance is proposed to
account for less permeable, “slower” pathways. In general, transport-based source
tracking might be used wherever transport can be described by the 1D two-region non-
equilibrium model, e.g. rivers and fractured or porous aquifers.
Colloidal particles are an important vector for the transport of contaminants in karst aquifers, characterized by a high degree of hydrologic variability. Understanding this heterogeneity and hydrogeological functioning is of particular importance for water management strategies. Until now, transport parameters for particles were mostly determined by injection of solutes or surrogates for natural sediments. But suspended particles are easier to analyze, less expensive, and better represent natural conditions. Therefore, tracer tests with sediments represent natural transport and a conservative solute dye were conducted simultaneously in an active and partly accessible cave system in Vietnam during constant high-flow and low-flow conditions. Breakthrough curves (BTCs) for 10 different particle sizes classes ranging from 1 μm to 15 μm and the solute were recorded in situ at the two main resurgences of the cave stream at high temporal resolution.
This study gives new insights into the transport processes of suspended particles and the highly dynamic exchange between mobile and immobile regions. Major findings include: (1) at low-flow conditions, inflow from the surrounding aquifer matrix and the velocity distribution inside the karst conduits themselves lead to hydrodynamic focusing, mainly affecting particles; (2) this highly advective, preferential transport of particles in the center of the karst conduits results in narrow BTC. The more dispersive transport of solutes results in a longer tailing; (3) at high-flow conditions a more homogeneous distribution of the particles indicates a reversal of the conduit-matrix interaction; (4) by comparing the results at different hydraulic conditions, the activation of additional flow paths with increasing discharge could be identified.
This study also presents the first tracer test in the field resulting in pronounced double peaks for the particulate tracer but only a single, right-skewed BTC for the simultaneously injected solute, including a conceptual model for particle transport.
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Challenged by rapidly changing climate in combination with an increase in anthropogenic pressures, karst groundwater resources in the Old Town of Lijiang (OTLJ), SW China, are diminishing. Higher frequency and longer duration of dried-up periods have been observed at the Heilongtan Park (HP) Springs in recent years. Thus, there is an urgent need for an artificial recharge scheme, aimed at replenishing groundwater in the aquifer and increasing the outflow of the springs to ensure effective water resources management. Evaluation of the scheme feasibility, prior to its implementation, is important. In this study, tracer tests were conducted between the recharge area and receiving springs in order to gain insight into the transport mechanisms of karst groundwater and the structural characteristics of the aquifer. Multiple underground flow paths, exhibiting high conductivity between the recharge area and HP Springs, were revealed by the interpretation of tracer breakthrough curves. Three springs considered as the leakages of the scheme were identified. Moreover, the outflow of springs at HP and OTLJ were predicted to be increased by the artificially recharged water after 9.2 and 12.5 days, respectively. Quantitative analysis of tracer recoveries demonstrates that the springs to be recharged and the springs considered as leakages, respectively, share 45 and 55% of the increased outflow. The feasibility of the scheme has been confirmed by the tracer tests. This report provides references for the evaluation of artificial groundwater recharge and protection strategies, particularly in large and poorly investigated karst spring fields.
Breakthrough curves (BTCs) are known as adequate evidence to characterize karst aquifers which mainly specified by dual flow systems (matrix and conduit) as well as some degree of connection between dual media. Conduit Flow Process version 2 (CFPv2) is a discrete conduit continuum model applying which it is possible to simulate such characteristics. In this research, first CFPv2 was employed to evaluate the effect of several factors including hydraulic conductivity, exchange flow, tracing distance, hydraulic gradient, and conduit diameter and roughness on BTCs in a synthetic karst aquifer. Then, 253 BTCs from 24 dye tracing tests, conducted in karstic aquifers of Iran, were analyzed to evaluate the effect of hydraulic gradient and tracing distance on the BTCs with taking into account the effect of medium type (matrix or conduit) and karst development. These tests were conducted under either natural or elevated man-made hydraulic gradient conditions. The results revealed that peak concentration decreased with enlarging conduit diameter because of considerable flow rate. Likewise, it decreases over longer distances specifically in the elevated man-made hydraulic conditions. However, with increasing hydraulic gradient, peak concentration and its timing have upward and downward trends, respectively. When BTCs start to demonstrate tailing and become flattened, it could be an indication of low matrix hydraulic conductivity or lower hydraulic gradient in conduit as a result of less exchange flow from the matrix into conduits. Based on this study a new intermixture of flow and solute transport parameters was engineered to classify karst development. High peak concentration, short peak time, high velocity particularly over long distances and under the natural hydraulic gradient conditions and feasible conduit flow regime whether laminar or turbulent suggested as indication of well-developed karstic system.
This paper focuses on the solute transport characteristics of karst water tracing and its engineering application by numerical simulation and analysis of field tracer curves. First, the computation models and simulation methods of solute transport are proposed. Then, five kinds of representative geological models of karst water tracing are built, and seven cases of solute transport are studied. Solute transport characteristics in the straight pipeline model, bend model, depression model, waterfall model, and branch pipeline model are analyzed. The solute mass fraction affected by the pipeline width, flow velocity, bend radius, depression depth, the number of bends, depressions, and waterfalls are discussed. Finally, the numerical results are applied in actual tracer tests for analyzing the groundwater connection medium. Some preliminary laws and conclusions are obtained as follows: (1) The peak of solute mass fraction decreases gradually with the bend radius, pipeline width, flow velocity, the number of bends, depressions, and waterfalls rising. (2) The influence of depression depth on the tracer curve is little. (3) The time to peak increases gradually with the number of depressions and waterfalls rising, while it decreases gradually with the increase of flow velocity. (4) The distribution of the tracer curve widens gradually with the number of bends, depressions, and waterfalls rising, while it narrows gradually with the increase of flow velocity.
This review focuses on investigations of groundwater flow and solute transport in karst aquifers through laboratory scale models (LSMs). In particular, LSMs have been used to generate new data under different hydraulic and contaminant transport conditions, testing of new approaches for site characterization, and providing new insights into flow and transport processes through complex karst aquifers. Due to the increasing need for LSMs to investigate a wide range of issues, associated with flow and solute migration karst aquifers this review attempts to classify, and introduce a framework for constructing a karst aquifer physical model that is more representative of field conditions. The LSMs are categorized into four groups: sand box, rock block, pipe/fracture network, and pipe-matrix coupling. These groups are compared and their advantages and disadvantages highlighted. The capabilities of such models have been extensively improved by new developments in experimental methods and measurement devices. Newer technologies such as 3-D printing, CT scanning, X-rays, NMR, novel geophysical techniques, and use of nano-materials allow for greater flexibilities in conducting experiments. In order for LSMs to be representative of karst aquifers, a few requirements are introduced: (1) the ability to simulate heterogeneous distributions of karst hydraulic parameters, (2) establish Darcian and non-Darcian flow regimes and exchange between the matrix and conduits, (3) placement of adequate sampling points and intervals, and (4) achieving some degree of geometric, kinematic, and dynamic similitude to represent field conditions. This article is protected by copyright. All rights reserved.
Bimodal transport due to dual-mobile advection or mass exchange between mobile and immobile zones has been widely observed for pollutants moving in heterogeneous media. Existing nonlocal transport models, however, cannot capture either heterogeneity in mobile domains or bi-peak concentration phenomena for breakthrough curves (BTCs) of solute transport in complex media. Therefore, this study proposed a dual heterogeneous domain model (DHDM) framework, conceptualized from the distributed-order time fractional derivative that incorporates a broader spectrum of particle movement in the dual-domain system than the classical single-domain model. A Lagrangian scheme was developed to solve the DHDM using the Bernoulli trial with the transition probability simulating mass exchange between different domains. Phase transition probabilities calculated based on the zeroth spatial moments of dual domain transport equations were used to simulate the particle partitioning between different domains. Applications to transport experiments in silt-clay columns conducted in our laboratory showed that the DHDM yielded significant improvement for simulating solute transport behavior compared to the single-domain anomalous transport model. Parameter analysis of the DHDM further revealed that the velocity disparity of the two domains controlled the BTC rising time and the shape of the second peak in the BTC. When the velocity of the slow domain is extremely small, the second BTC peak develops a ‘shoulder’ characteristic during the late-time tail of the BTC. This study improved our understanding for bimodal transport in a dual domain system and provides a feasible tool for capturing a wide range of bimodal transport in complex media.
We conducted lab-scale experiments to investigate the mechanism of dual-peaked breakthrough curves (BTCs) in karst tracer tests. Three groups of dual conduit structures were constructed by varying: (1) the length ratio between the two conduits for a fixed length of the shorter conduit, (2) the total length of the conduits for a fixed length ratio, (3) the connection angle between the conduits. The BTCs generated by the tracing experiments were fitted by a Dual-Region Advection Dispersion (DRAD) model to derive effective transport parameters.
Our results confirm that the dual conduit structure triggers the double-peaked BTCs. Increasing the conduit length for a fixed length ratio or increasing the length ratio increases peak separation. The connection angles between the two conduits θ1 and θ2 also influence the BTCs: increasing θ1 and decreasing θ2 causes the first peak to get smaller and the second peak to get larger. The DRAD model can reproduce the dual-peaked BTCs while its application to the case of single-peaked BTCs may be problematic due to strong interaction between model parameters. A method is proposed for estimating underground karstic conduit lengths from experimental dual-peaked BTCs.
Fissures and conduits are main void of karstic aquifer. Karst aquifers are highly vulnerable to contamination as water exchange between fissure system and conduit system greatly influences the karstification of limestone terrains, heterogeneity of karstic aquifer, and spring flow regime. A two-dimensional tank model was constructed, and a corresponding numerical model (MODFLOW-2005 Conduit Flow Process Mode 1, CFPM1) was developed to explore the effects of rainfall intensity and intrinsic structure of karst system (hydraulic conductivity of fissures and conduit diameter) on conduit-matrix water exchange. Nine different scenarios were designed to quantitatively simulate the spatial and temporal variability of water exchange between the fissures and conduits. The study reproduced the area of fissure water recharging to conduit (recharge zone), the area of conduit water discharging to fissure water (discharge zone) and the area of fissure water moving laterally (transport zone) in both physical model and numerical model. The results from rainfall scenarios show a relatively stable ratio of exchange zone (recharge zone and discharge zone) with increasing rainfall intensity, indicating that conduit-matrix water exchange is not sensitive to rainfall intensity, and conduit water discharging to fissure water is just a short-term phenomenon. It is also found that an increase in hydraulic conductivity of fissure system enhances extensiveness of water transport, but restrains conduit-matrix water exchange. However, the enlargement of conduit can prompt the extent of conduit-matrix water exchange. A positive feedback between the extent of conduit-matrix water exchanges and karstification can be confirmed. These findings might be essential for better understanding groundwater flow dynamics in karst aquifers.
Karst aquifers are important drinking water resources, but highly vulnerable to contamination. Contaminants can be transported rapidly through a network of fractures and conduits, with only limited sorption or degradation, which usually leads to a fast and strong response at karst springs. During migration, contaminants can also enter less mobile zones, such as pools or water in intra-karstic sediments, or advance from conduits into the adjacent fractured rock matrix. As contaminant concentrations in the main flow path(s) decrease, contaminants may migrate back into the main flow path and reach the karst springs at low (but significant) concentrations over a long time span. This is the conventional interpretation for the oft-observed steep rising limb and the long-tailed falling limb of tracer breakthrough curves in karst systems. Here, field measurements are examined from an alpine karst system in Austria where a series of distinctive, long-tailed breakthrough curves (BTCs) of conservative tracers were observed over distances up to 7400 m. Recognizing that the conventional advection-dispersion equation (ADE) cannot usually quantify such behavior, two other modeling approaches are considered, namely the two-region non-equilibrium (2RNE) model, which explicitly includes mobile and immobile zones, and a continuous time random walk (CTRW) model, which is based on a physically-based, probabilistic approach that describes anomalous (or non-Fickian) transport behavior characteristic of heterogeneous systems such as karst. In most cases, the ADE and 2RNE models do not quantify the low concentrations at longer travel times. The CTRW, in contrast, accounts for the long-tailed breakthrough behavior found in this karst system.
Colloids and particles act as vectors for contaminant transport. In karst aquifers, particle transport is particularly efficient and plays critical roles in soil erosion and in the process of karstification. However, available techniques for particle tracing are either expensive or not representative for the transport of natural colloids and particles. We developed a new method for particle tracing, using natural sediments as artificial tracers, and first applied this method at a karst experimental site in the Alps. Suspended particles were injected into a swallow hole together with a conservative solute tracer for comparison. Breakthrough curves for 32 different particle size classes between 0.8 and 450 μm were recorded at a karst spring 230 m away using a mobile particle counter that allows quantitative detection at high temporal resolution. Results show that (i) sediments can be used as efficient particle tracers in karst groundwater; (ii) recoveries are similar for particles and solutes; (iii) mean velocity increases with increasing particle size; (iv) dispersion decreases with increasing particle size; (v) these observations point to exclusion processes. As a conclusion, this new experimental technique allows new insights into the transport and fate of colloids and particles in groundwater at affordable costs.
This work adopts a multicriteria approach, combining diverse karst hydrogeology techniques, to investigate the functioning of evaporite-karst systems, which are normally less studied due to the poor quality of their water resources. The applicability of using several methods was also verified. In view of previous experience at a pilot site within southern Spain, the natural responses of the main discharge point of the system of study (Lower Anzur spring) were controlled by continuous monitoring of the discharge rate, water temperature, and electrical conductivity. Water samples were collected for chemical analysis and for the determination of intrinsic fluorescence, water stable isotopes (δ18O,δ2H) and tritium (3H) concentrations. Physico-chemical data were statistically examined applying principal component analysis (PCA) and trend analysis tests (Mann-Kendall and Sen’s slope). The system shows a clear karstic functioning, highly dependent on concentrated recharge, and influenced by a limited drainage capacity of the conduit network. The piston-flow effects at the beginning of floods indicate a contribution to the drainage of deep ascending flows through the saturated zone. Subsequent drops in temperature and mineralization values in the water drained by the spring, as well as a recent infiltration deduced from the signature of intrinsic fluorescence and stable isotopes, point to a good hydrogeological connection from the recharge areas to the spring. Later restoration of the initial values of mineralization —in a much shorter time than those of temperature— reveals a faster geochemical evolution of the recently infiltrated groundwater, which may be a distinctive trait of evaporite karst aquifers. Lastly, the high solute content of the spring water in depletion conditions, with ascending trends of mineralization and temperature observed during several dry
years as a consequence of lower recharge contribution, together with the tritium activity of selected samples, suggest the existence of an old component partly responsible for the spring base flow. Thus, groundwater would be linked to gravity-driven regional flows, but also to recent conduit flows. All the results obtained led us to devise a hydrogeological conceptual model to precisely reflect the functioning of the studied system. The results presented here demonstrate the advantages of using diverse techniques when investigating evaporite karst aquifers.
Methods of modelling non-reactive solute transport based on artificial tracer tests have been widely developed in the past few decades. The dependence of solute transport parameters on boundary conditions has been investigated across different hydrological settings (low and high water level) but still not investigated at the short-term scale (i.e. hourly and daily scale). In this study, a campaign of several tracer tests was performed over a few days to investigate the short-term variations of tracer-test responses in a conduit-dominated karst system (Baget watershed, in the Pyrenees Mountains, France) during a recession without the influence of rainfall. Also, an improved method of interpreting artificial tracer test results, using a process engineering tool, is introduced, consisting of a Laplace-transform transfer function approach with respect to the residence-time distribution curve. Considering the karstic system as a chemical reactor, the introduction of a transfer function approach appears to be an efficient way to describe the solute transport. Moreover, the transfer function is parametrized depending on the spring discharge. The model is extended for testing source pollution scenarios.
Investigation of groundwater pollution has been carried out on multiple typical karst underground river systems in recent years in order to reveal the characteristics and mechanism of underground river pollution. Through the summary of the survey results, five characteristics of underground river pollution have been revealed, including directionality, weak hysteresis, easier for reversibility, linear and intermittent (or seasonal); combining with karst geomorphic types, pollution sources (primordial, secondary pollution and compound), pollution ways and pollution receptors, three basic pollution patterns of the underground river were depicted: Recharge area pollution, runoff area pollution and discharge area pollution. At the same time, the study discloses the reality of the underground river pollution is the three basic patterns and their subgenera in combination with super imposition on the space and time. And from the angle of the hydrogeological model, the study expounds the process of underground river pollution. Finally, the study gives naming rules of pollution pattern of the underground river and sets up PISAB method of pattern recognition. The results provide scientific basis for pollution repair and management of underground river in the southwest karst region.
Amid surging population and industrial growth, there is a need to assess the vulnerability of groundwater to pollution. In this study, the case of Xingshan County in South China where karst groundwater is a major water source is presented. Field survey revealed hazardous landuse practices in highly karstified areas. To delineate areas of high vulnerability, an intrinsic vulnerability map (IVM) was created based on the EPIK (Epikarst, Protective cover, Infiltration condition, Karst network) method. Results show that 10.1, 25.8, 58.7 and 5.3% of the area are of low, medium, high and very high vulnerability. Although IVMs are powerful tools since they highlight areas of high “intrinsic” susceptibility to pollution, they do not answer the important question, “what sources are at immediate risk.” With this limitation, tracer technique was coupled with IVM to establish a link to resources/sources. Rhodamine and uranine were injected into two sinkholes in high vulnerability zones near hazardous activities. Both were detected 57.3 and 5.65 h after injection at a major spring (Bailongquan). This established link validates the IVM and highlights specific sources at risk. This study demonstrates the need to couple IVMs with tracer test in order to prioritize areas to probe in order to determine the extent of their impact by establishing active karstic groundwater flow paths. The case here is an example of source/resource vulnerability assessment, hazard/risk assessment and validation of an IVM. Our findings present a cost-effective approach to vulnerability assessment by emphasizing the reliability of EPIK method as it requires less data.
Karst aquifers account for up to 20% of the world land area and are a source of drinking water for much of the world. Despite the critical value of these aquifers as a drinking water source, there are a growing number of incidences of karst aquifer contamination worldwide, including inadvertent spills, dumping, industrial discharges, or sewage seepage events. Given the porous nature of carbonate rocks, the hydrogeology of karstic aquifers is extremely complex, making it difficult to predict movement of contamination in these aquifers and to identify exposure risks. These contamination events—together with emerging issues such as climate change, exposures to infectious agents, as well as the increase in informal mining practices—indicate the need to explore linkages between karst groundwater, contamination, and health. Accordingly, the issue of karst groundwater contamination presents a unique global public health challenge requiring a multidisciplinary problem-solving approach. The National Institute of Environmental Health Sciences (NIEHS) Superfund Research Program’s (SRP) multidisciplinary approach serves as a model for integrating expertise across health, engineering, geological, and community-based approaches to solve problems. Using examples relevant to karst contamination, NIEHS SRP grantees are engaged in research endeavors to address issues of drinking water safety—from remediation to well-testing best practices. It is recommended that continued research addresses karst contamination, with particular attention given to identifying people at risk of exposures and to developing proactive means to prevent further exposures. This is particularly important in the USA, where two-fifths of the population’s drinking water comes from karst aquifers. Furthermore, over 40 million US citizens are on private well water for drinking, yet testing for contamination in these wells is often not required. Given the challenges predicting contaminant transport in karst and the lack of uniform private well water testing regulations, there is a need to promote awareness of risks for people living in karst areas among public health, hydrogeology, and government officials, and to use community-based approaches as models for intervention and exposure prevention.
The Karst Waters Institute sponsored a conference on karst groundwater contamination and its impacts on public health. The objective was to facilitate communication between hydrogeologists and the biomedical community, especially those dealing with public health issues. This volume contains the papers presented at the conference.
Turbulent flow has a different hydraulic response compared to laminar flow and so it is important to be able to identify its occurrence in an aquifer, and to predict where it is likely to be found. Turbulent flow is associated with large apertures and rapid velocities, and these occur most frequently in carbonate aquifers. Methods for identifying turbulent flow include correlating spring discharge with head variation, calculating Reynolds numbers from spring discharge and tracer velocity, and plotting the spatial variation of head differences between high flow and low flow. The probability of turbulent flow increases as a function of permeability and of spring discharge, and the probability increases in a downgradient direction in an aquifer. Spring discharge is a key parameter for evaluating the presence of turbulent flow, which is likely to occur where a spring with a discharge >1 L/s is fed by a single channel. Turbulent flow appears to be a major contributing factor to the occurrence of groundwater flooding in carbonate aquifers.
Water exchanges between a karstic conduit and the surrounding aquifer are driven by hydraulic head gradient at the interface between these two domains. The case-study presented in this paper investigates the impact of the geometry and interface conditions around a conduit on the spatial distribution of these exchanges. Isotopic (δ¹⁸O and δD), discharge and water head measurements were conducted at the resurgences of a karst system with a strong allogenic recharge component (Val d’Orléans, France), to estimate the amounts of water exchanged and the mixings between a saturated karstic conduit and the surrounding aquifer. The spatio-temporal variability of the observed exchanges was explored using a 2D coupled continuum-conduit flow model under saturated conditions (Feflow®).
Basic Hydrogeological Concepts, Terms and Definitions Controls on the Development of Karst Hydrological Systems Energy Supply and Flow-Network Development Development of the Water Table and Phreatic Zones Development of the Vadose Zone Classification and Characteristics of Karst Aquifers Applicability of Darcy's Law to Karst Freshwater–Saltwater Interface
This paper presents the modelling results of several tracer-tests performed in the cave system of Han-sur-Lesse (South Belgium). In Han-sur-Lesse, solute flows along accessible underground river stretches and through flooded areas that are rather unknown in terms of geometry. This paper focus on the impact of those flooded areas on solute transport and their dimensioning. The program used (One-dimensional Transport with Inflow and Storage: OTIS) is based on the two-region non equilibrium model that supposes the existence of an immobile water zone along the main flow zone in which solute can be caught. The simulations aim to replicate experimental breakthrough curves (BTCs) by adapting the main transport and geometric parameters that govern solute transport in karst conduits. Furthermore, OTIS allows a discretization of the investigated system, which is particularly interesting in systems presenting heterogeneous geometries. Simulation results show that transient storage is a major process in flooded areas and that the crossing of these has a major effect on the BTCs shape. This influence is however rather complex and very dependent of the flooded areas geometry and transport parameters. Sensibility tests performed in this paper aim to validate the model and show the impact of the parametrization on the BTCs shape. Those tests demonstrate that transient storage is not necessarily transformed in retardation. Indeed, significant tailing effect is only observed in specific conditions (depending on the system geometry and/or the flow) that allow residence time in the storage area to be longer than restitution time. This study ends with a comparison of solute transport in river stretches and in flooded areas.
The Large-Eddy Simulation (LES) module of the Virtual StreamLab (VSL3D) model is applied to simulate the flow and transport of a conservative tracer in a headwater stream in Minnesota, located in the south Twin Cities metropolitan area. The detailed geometry of the stream reach, which is ∼135m long, ∼2.5m wide, and ∼0.15m deep was surveyed and used as input to the computational model. The detailed geometry and location of large woody debris and bed roughness elements up to ∼0.1m in size were also surveyed and incorporated in the numerical simulation using the Curvilinear Immersed Boundary (CURVIB) approach employed in VSL3D. The resolution of the simulation, which employs up to a total of 25 million grid nodes to discretize the flow domain, is sufficiently fine to directly account for the effect of large woody debris and small cobbles (on the stream bed) on the flow patterns and transport processes of conservative solutes. Two tracer injection conditions, a pulse and a plateau release, and two cross-sections of measured velocity were used to validate the LES results. The computed results are shown to be in good agreement with the field measurements and tracer concentration time series. To our knowledge, the present study is the first attempt to simulate via high-resolution LES solute transport in a natural stream environment taking into account a range of roughness length-scales spanning an order of magnitude: from small cobbles on the stream bed (∼0.1m in diameter) to large woody debris up to ∼3m long.
7 sampling points are set to collect water samples from the Qingshuiquan underground river system in December 2012, which was the dry season of it. By detecting those samples, we figured out the present quality of underground water of the Qingshuiquan underground river system, the characteristics of its contamination situation, the source of pollutants and the reason of pollution. Despite some light compound pollution caused by PAHs and HCHs, the integral quality of the underground water of Qingshuiquan underground river system is good. PAHs are origin from coal burning; HCHs are from the use of Lindane. These skylights and avens of the Lava-water system can lead pollutants to the underground water intermittently, which resulted the pollution.
Karst aquifers differ from other types of hydrogeological systems because of their complex behavior, which originates from strong heterogeneity. A karst spring carries an imprint of hydrologic information for the karst aquifer. The shape of the outflow hydrograph recorded at a spring is a unique reflection of the aquifer's response. A karstic aquifer consisting of a fracture network and a conduit was proposed, and the spring recession curves generated from the designed karst aquifer were analyzed in this study. The purpose of the study was to discuss the influence of the main spring conduit diameter and the saturated thickness of the aquifer on spring recession curves. A combination of 7 groups of outlet pipes with different diameters and 7 initial water levels were used to simulate changes in diameter and saturated thickness, respectively. Thus, a total of 49 experimental tests were carried out in the laboratory karst aquifer. The results indicate that spring recession curves can be separated into two flow segments, and that the main spring conduit diameter has a remarkable influence on the initial flow of recession, recession duration, and recession coefficient. The results also show that the saturated thickness of the aquifer has a great effect on the initial flow of the recession and recession duration, but that its effect on the recession coefficient and the shape of the recession curve is slight.
We analyzed the behavior of convergent ﬂow tracer tests performed in a 3-D heterogeneous
sandbox in presence of connected gravel channels under laboratory-controlled conditions. We focused on the evaluation of connectivity metrics based on characteristic times calculated from experimental breakthrough curves (BTCs), and the selection of upscaling model parameters related to connectivity. A conservative compound was injected from several piezometers in the box, and depth-integrated BTCs were measured at the central pumping well. Results show that transport was largely affected by the presence of gravel channels, which generate anomalous transport behavior such as BTC tailing and double peaks. Connectivity indicators based on BTC peak times provided better information about the presence of connected gravel channels in the box. One of these indicators, b, was deﬁned as the relative temporal separation of the BTCs peaks from the BTCs centers of mass. The mathematical equivalence between b and the capacity coefﬁcient adopted in mass transfer-based formulations suggests how connectivity metrics could be directly embedded in mass transfer formulations. This ﬁnding is in line with previous theoretical studies and was corroborated by reproducing a few representative experimental BTCs using a 1-D semianalytical bimodal solution embedding a mass transfer term. Model results show a good agreement with experimental BTCs when the capacity coefﬁcient was constrained by measured b. Models that do not embed adequate connectivity metrics or do not adequately reproduce connectivity showed poor matching with observed BTCs.