Conduit Network Research Articles

Numerical study of groundwater flow cycling controlled by seawater/freshwater interaction in Woodville Karst Plain

Seawater intrusion due to sea level rise and climate change could significantly contaminate coastal groundwater resources, particularly in Florida, the flat low-land state in the United States. Based on the field investigation and hydrological measurements, a three-dimensional SEAWAT model is developed to evaluate the groundwater flow cycling and seawater intrusion to freshwater system in the Woodville Karst Plain (WKP), a typical karst groundwater system in the Floridan aquifer. The karst conduit network in the aquifer acts as fast flow pathway for groundwater flow and solute transport, so seawater could deeply intrude into the aquifer. Wakulla Spring, an inland spring 17 km from the coast and a coastal submarine spring, Spring Creek Spring Complex are connected through the conduit network. The flow direction between the two springs switches under various rainfall conditions in this region, thus the discharges at two karst springs are used to estimate the location of seawater/freshwater mixing interface. The SEAWAT modeling results indicate that the mixing interface, defined as 2 PSU (Practical Salinity Unit), intrudes 3 to 5 km through the subsurface karst conduit during the dry season and severely contaminates nearly 1 km width of groundwater around the conduit. The salinity distribution and the distance of seawater intrusion through the conduit system are very sensitive to precipitation variation and the sea level boundary condition. Furthermore, predictions are made for seawater intrusion to the aquifer under various sea level rise, precipitation scenarios and water pumping. The results show that the seawater intrusion could reach and contaminate inland freshwater systems if sea level rises 1.0 m or during a long-term no-precipitation season. This study provides insights for modeling and predicting the vulnerability of a coastal karst aquifer through the simulation of variable-density flow.

Simplified physically based models for free-surface flow in karst systems

Most karst aquifers are characterized as a dual-flow system comprised of a highly conductive conduit network embedded in a low porosity matrix. The conduits are hydraulically connected to the matrix and behave either as a drain or a source depending on the recharge conditions. Simplified physically based models are herein employed to simulate the spring outflow for such aquifer systems. The processes consist of a free-surface flow in the conduit that is interacting laterally with a laminar groundwater flow in the surrounding matrix. The conduit is subject to a concentrated recharge at its upstream end while the groundwater aquifer is subject to a diffuse recharge over its contributing surface area. The flow system is described by a coupled system of partial differential equations: the conduit flow is approximated by the kinematic wave equation and the groundwater flow by the linearized Boussinesq equation. The governing equations are solved using the Laplace transform method after an appropriate linearization of the nonlinear coefficient. The derived spring discharge models are a function of three dimensionless parameters: the time lag parameter ξ, the lumped conduit parameter λ, and the aquifer parameter η. The simulation results highlight the contrast between pressure-driven and gravity-driven flows and the importance of the conduit-matrix interaction on the response of the karst system. Application of the models on real karst aquifers demonstrates their effectiveness in simulating the observed spring hydrograph using lumped physical parameters of the karst system.

Karstic aquifer structure from geoelectrical modeling in the Ring of Sinkholes, Mexico

Groundwater models based on terrain resistivity can show the diverse properties and processes occurring between fractures, boreholes and conduits. Using electrical resistivity tomography, a conceptual model was created of the aquifer within the Ring of Cenotes, which surrounds the Chicxulub Crater on the Yucatan Peninsula, Mexico. Determining the hydrogeological parameters of an aquifer is vital since they determine flux and hydrodynamic patterns. In karstic environments, conventional techniques are not effective at recording heterogeneities such as fracture zones and conduits. Six transects were investigated among areas near cenotes (sinkholes) and aguadas (small surface-water bodies), as well as areas lacking these formations. The resulting profiles reached up to 30 m deep and showed an aquifer consisting of a fractured matrix saturated with freshwater. The matrix exhibiting resistivity values from 3 to 250 Ωm, lying under a vadose zone with values from 300 to 25,000 Ωm. Aguadas and cenotes are surface manifestations of the aquifer’s internal structure, which is a network of caves and conduits. In the Ring of Cenotes, the presence of these surface formations indicates increased hydraulic conductivity in their vicinity, with smaller conduits or fractures providing lesser conductivity in areas further away.

Effect of Different Conduit-Network Recharge Ways on Karst Spring Simulation

AbstractTwo recharge ways, point inflow and lateral inflow, exist on the conduit network in karst aquifers. Point inflow is commonly included in pipe-flow models, but lateral inflow along the condu...

Physics-based fine-scale numerical model of a karst system (Milandre Cave, Switzerland)

In karst aquifers, groundwater flow is often turbulent and occurs in variably saturated conduits. This implies a nonlinear response to recharge events which cannot be reproduced with the tools commonly used for groundwater flow modeling. Recent studies have shown the usefulness of using conduit flow models to simulate flow in karst systems. However existing models are built on abstract representations of the actual conduit network or at a rather coarse resolution. Such models cannot provide information on local flow conditions in the conduits or be used to simulate mass transport. In the catchment of the Milandre Cave, in the Jura Mountains of Switzerland, a good portion of the active drainage network is accessible by caving, which opens the possibility for a distributed fine-scale numerical model. This report presents the development of a hydraulic model of the downstream part of this system (1.5 km × 0.5 km) using EPA SWMM 5. The network geometry is based on a detailed cave survey. Calibration is achieved by fitting the observed hydraulic head vs. flow-rate curves. The model performs well when compared against the hydraulic heads that were monitored throughout the system. Simulated transit times are also in line with tracer test results. This confirms that the model reproduces well the physics of flow in this karst system, while being spatially distributed at a fine scale (median element size of 6 m). It can thus provide information on local flow conditions in the conduits at various water stages and be used to address mass transport problems.

Links between karst hydrogeological properties and statistical characteristics of spring discharge time series: a theoretical study

Physics-based modeling of karst systems remains almost impossible without enough accurate information about the inner physical characteristics. Usually, the only available hydrodynamic information is the spring discharge at the karst outlet. Numerous works in the past decades have used and proven the usefulness of time-series analysis applied to spring discharge, precipitations or even physico-chemical parameters, for interpreting karst hydrological functioning. The main objective of this work is to provide additional insights of to what extent the informative content of the hydrodynamic signal at karst springs is sensitive to karst aquifers internal physical properties. To address this issue, we undertake an empirical approach based on the use of both distributed and physics-based models and on synthetic systems responses. The link between karst hydraulic and physical properties was studied. For this purpose, forward modeling of flow through several simple, constrained and synthetic cases in response to precipitations is undertaken. It allows us to quantify how the statistical characteristics of flow at the outlet are sensitive to changes (1) one hydraulic parameter of the model and (2) in conduit network geometry. The matrix/conduit exchange parameter appeared clearly as a determinant model parameter in the spring discharge simulation. The auto-and cross-correlation functions seem to be of particular interest for the understanding of the karst inner physics. Indeed, these functions are always different, despite not so pronounced configuration differences. This would highlight that there is an informative content within the spring discharge time series and the usefulness of such analysis methods.

Characterising the role of parametric functions in the van Genuchten empirical model on CO2 storage performance

In the application of two-phase flow in porous media within the context of CO2 sequestration, a non-wetting phase is used to displace a wetting phase residing in-situ to the maximum extent through a network of pore conduits. The storage performance of this physical process can be assessed through numerical simulations where transport properties are usually described using the Brooks & Corey (BC) or van Genuchten (vG) model. The empirical constant, namely the pore geometry index, is a primary parameter in both of these models and experimental evidence shows a variation in the value of this empirical constant. It is, therefore, essential to cast this empirical constant into a ternary diagram for all types of clastic porous media to demarcate the efficiency of two-phase flow processes in terms of the pore geometry index (PGI). In doing so, this approach can be used as a tool for designing more efficient processes, as well as for the normative characterisation of two-phase flow, taking into consideration the predominance of capillary pressure or relative permeability effects. This concept is based on the existence of a PGI estimation for clastic sediments, for which the value for 12 sediment mixtures fall between 1.01 and 3.00. Statistical data obtained from soil physics is used for developing and validating numerical models where a good match is observed in numerical simulations. In this context, a new methodology for the effective characterisation of PGI for different clastic rocks is proposed. This paper presents theoretical observations and continuum-scale numerical simulation results of a PGI characterisation for the prediction of the hydraulic properties of clastic reservoir rocks. The effect of key parameters in the vG empirical model, such as the pressure strength coefficient and the PGI, is incorporated into the simulation analysis. In particular, the model is used to investigate the effects of parameter representation on CO2 storage performance in a saline aquifer. Subsequent analysis shows that the PGI is a very important parameter for defining the flow characteristics of simulation models. It can also be flexibly changed for each rock type and this approach may thus be practical when simulating the evolution of CO2 plume in reservoirs with sedimentary heterogeneities, such as intra-aquifer aquitard layers or graded beds. The use of the realistic PGI boundaries promises a more precise description of the hydraulic behaviour in sandstones and shale when using either the BC or vG model.

Combining hydrodynamics, hydrochemistry, and environmental isotopes to understand the hydrogeological functioning of evaporite-karst springs. An example from southern Spain

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.

Paleo-Hydrothermal Predecessor to Perennial Spring Activity in Thick Permafrost in the Canadian High Arctic, and Its Relation to Deep Salt Structures: Expedition Fiord, Axel Heiberg Island, Nunavut

Paleo-Hydrothermal Predecessor to Perennial Spring Activity in Thick Permafrost in the Canadian High Arctic, and Its Relation to Deep Salt Structures: Expedition Fiord, Axel Heiberg Island, Nunavut

Detecting and characterising an englacial conduit network within a temperate Swiss glacier using active seismic, ground penetrating radar and borehole analysis

ABSTRACTEnglacial hydrology plays an important role in routing surface water to the glacier's bed and it consequently affects the glacier's dynamics. However, it is often difficult to observe englacial conduit conditions on temperate glaciers because of their short-lived nature. We acquired repeated active surface seismic data over the Rhone Glacier, Switzerland to monitor and characterise englacial conduit conditions. Amplitude-versus-angle analysis suggested that the englacial conduit is water filled and between 0.5 and 4 m thick. A grid of GPR profiles, acquired during the 2018 melt season, showed the englacial conduit network persisting and covering ~ 14,000 m2. In late summer 2018, several boreholes were drilled into the conduit network. We observed generally stable water pressure, but there were also short sudden increases. A borehole camera provided images of a fast flowing englacial stream transporting sediment through the conduit. From these observations, we infer that the englacial conduit network is fed by surface meltwater and morainal streams. The surface and morainal streams merge together, enter the glacier subglacially and flow through subglacial channels along the flank. These subglacial channels flow into highly efficient englacial conduits traversing the up-glacier section of the overdeepening before connecting with the subglacial drainage system.

Short Overview of Early Developments of the Hardy Cross Type Methods for Computation of Flow Distribution in Pipe Networks

Hardy Cross originally proposed a method for analysis of flow in networks of conduits or conductors in 1936. His method was the first really useful engineering method in the field of pipe network calculation. Only electrical analogs of hydraulic networks were used before the Hardy Cross method. A problem with flow resistance versus electrical resistance makes these electrical analog methods obsolete. The method by Hardy Cross is taught extensively at faculties, and it remains an important tool for the analysis of looped pipe systems. Engineers today mostly use a modified Hardy Cross method that considers the whole looped network of pipes simultaneously (use of these methods without computers is practically impossible). A method from a Russian practice published during the 1930s, which is similar to the Hardy Cross method, is described, too. Some notes from the work of Hardy Cross are also presented. Finally, an improved version of the Hardy Cross method, which significantly reduces the number of iterations, is presented and discussed. We also tested multi-point iterative methods, which can be used as a substitution for the Newton–Raphson approach used by Hardy Cross, but in this case this approach did not reduce the number of iterations. Although many new models have been developed since the time of Hardy Cross, the main purpose of this paper is to illustrate the very beginning of modeling of gas and water pipe networks and ventilation systems. As a novelty, a new multi-point iterative solver is introduced and compared with the standard Newton–Raphson iterative method.

A holistic approach to groundwater protection and ecosystem services in karst terrains

A holistic conceptual approach to groundwater and natural resources protection, surface and subsurface biodiversity conservation and ecosystem services in karst terrains is presented. Karst landscapes and aquifers consist of carbonate rock in which a part of the fractures has been enlarged by chemical dissolution. They are characterized by unique geomorphological and hydrogeological characteristics, such as rapid infiltration of rainwater, lack of surface waters, and turbulent flow in a network of fractures, conduits and caves. Karst terrains contain valuable but vulnerable resources, such as water, soil and vegetation, and they provide a great variety of habitats to many species, both at the surface and underground, including many rare and endemic species. Karst terrains deliver valuable ecosystem services and act as natural sinks for carbon dioxide (CO2) thus helping to mitigate climate change. It is demonstrated that all these resources and ecosystem services cannot be considered in an isolated way, but are intensely interconnected. Because of these complex feedback mechanisms, impacts on isolated elements of the karst ecosystem can have unexpected impacts on other elements or even on the entire ecosystem. Therefore, the protection of natural resources, biodiversity and ecosystem services in karst requires a holistic approach.

Groundwater temperature as a natural tracer to characterize hydraulic behaviour and geometry of carbonate aquifers: Mt. Nerone karst system, central Italy

Temperature variations in groundwater discharge from the Giordano karst system (northern Apennines) have been observed continuously for six months (from January 2016 to July 2016), to determine the flow patterns and geometrical properties of aquifer. The karst system discharge about 32.2 L/s on average during over the hydrological year and it’s characterized by two outlets: a basalcontinuousspring (BCS) with a mean discharge of 8.9 L/s and an upper-intermittent spring (UIS) with a mean discharge of 23.3 L/s, located about 60 m above the previous one.Results show significant difference in the two spring outlets between the time lags as well as non-simultaneous and not analogue responses of temperature (T) to the same recharge events: temperature of UIS range from 9.7 to 10.7°C whereas temperature of BCS remain rather stable, ranging from 9.8 to 9.9°C. This data suggests a stratification of the water along the aquifer probably associated by different residence times and linked to the structural organization of karst aquifer (conduit and fracture networks): deeper and oldest water in the BCS and youngest water in the UIS.

Seismic imaging of complex geometry: Forward modeling of sandstone intrusions

Sandstone intrusions form large bedding-discordant sandstones that intruded into finer grained, less permeable host strata. They form naturally sand-propped hydraulic fractures that constitute a connected network of permeable conduits through which fluids escape to the Earth's surface. Saucer-shaped sandstone intrusions are among the largest volume intrusions and are commonly resolved on seismic data. Outcrop analogues of seismically-resolved saucers-shaped intrusions reveal that many attendant intrusions, in particular dikes, are undetected in seismic data. Seismic forward modeling of a detailed outcrop description of a saucer-shaped intrusion demonstrates that intrusions steeper than 45° are undetected and that up to 40% of the entire volume of sandstone intrusions is not seismically imaged. Wedge geometry – associated with discordant contacts between different lithologies – causes constructive and destructive amplitude interference, creating imaging artefacts of sandstone thickness and geometry. Comparison of the outcrop seismic models with 3D seismic data from Volund oilfield demonstrate both the similarity of the saucer-shaped intrusions and the distribution and quantity of dikes that may be undetected (ca. 78%) using subsurface data. Lack of detection of dikes has direct implications on the valuation of upward migration of fluids and an overestimation of seal capacity. This therefore has major implications when using seismic data to evaluate waste sequestration or to execute hydrocarbon or groundwater exploration and production.

Conduit Integrity is Compromised During Acute Lymph Node Expansion

Lymph nodes (LNs) act as filters, constantly sampling peripheral cues. This is facilitated by the conduit network, a tubular structure of aligned extracellular matrix (ECM) fibrils ensheathed by fibroblastic reticular cells (FRCs). LNs undergo 3-5-fold expansion during each adaptive immune response but these ECM-rich structures are not permanently damaged. Whether conduit integrity and filtering function is affected during LN expansion is unknown. Here we show that conduits are disrupted during acute LN expansion but FRC-FRC contacts remain connected. We reveal that polarised FRCs deposit ECM basolaterally utilising LL5-β, and that ECM production is regulated at transcriptional and secretory levels by the C-type lectin CLEC-2, expressed by dendritic cells. Inflamed LNs maintain conduit size-exclusion, but flow becomes leaky, allowing soluble molecules increased access to antigen-presenting cells. We show how dynamic communication between peripheral tissues and LNs changes during immune responses, and describe a mechanism enabling LNs to prevent inflammation-induced fibrosis.

Improving accuracy and efficiency in discrete-continuum karst models

Discrete-continuum or hybrid models couple one-dimensional conduit flow with the surrounding three-dimensional flow in the fractured rock matrix. The applicability of these models to real karst aquifers is somewhat limited not only because they require detailed information about the conduit network, but also these models typically require considerable computational efforts. Moreover, in some models the simulated conduit-matrix exchange fluxes critically depend on a lumped exchange parameter and as such the accuracy of discrete-continuum models can be questionable. This study presents some ideas to improve the accuracy and efficiency of discrete-continuum models. It is shown that a well-index should be used instead of a lumped exchange parameter. It is also illustrated that local grid refinement around the conduits as well as parallel solvers based on domain decomposition can be useful to increase the computational efficiency of discrete-continuum models.

A review of fractals in karst

Many features of a karst massif can either be modelled using fractal geometry or have a fractal distribution. For the exokarst, typical examples include the geometry of the landscape and the spatial location and size-distribution of karst depressions. Typical examples for the endokarst are the geometry of the three-dimensional network of karst conduits and the length-distribution of caves. In addition, the hydrogeological parameters of the karst massif, such as hydraulic conductivity, and karst spring hydrographs may also exhibit fractal behaviour. In this work we review the karst features that exhibit fractal behaviour, we review the literature in which they are described, and we propose hypotheses and conjectures about the origin of such behaviour. From the review and analysis, we conclude that fractal behaviour is exhibited at all scales in karst systems.

Modelling spring discharge and solute transport in conduits by coupling CFPv2 to an epikarst reservoir for a karst aquifer

Karst aquifers differ from unconsolidated granular aquifers with respect to their groundwater flow behavior because of the dual-permeability characteristics (discrete conduit network coupled to a fissured matrix), the duality of recharge (point vs. diffuse recharge) as well as the duality of flow (laminar vs. turbulent state). In many karst aquifers, the epikarst frequently plays an important role in redistributing precipitation as diffuse recharge into the fissured matrix and as point recharge into the conduit network as well as for the temporal distribution of recharge input. In this paper, a coupled groundwater model CE (CFPv2 + ERCH) is developed, consisting of a combination of a simple reservoir model generating the recharge source function assuming the concept of an epikarst horizon (ERCH) with CFPv2, a discrete conduit-continuum groundwater flow model of the phreatic karst aquifer. The CE model is applied to simulate spring discharge and a tracer breakthrough in a karst aquifer near Guilin City in southwest China. In order to examine the effect of an added epikarst reservoir model, simulations are carried out with CFPv2 but without including ERCH. Simulation results of both, spring hydrograph and tracer breakthrough, indicate that CFPv2 with ERCH performs much better. A sensitivity analysis of the CE reveals the relative importance of parameters characterizing the epikarst reservoir. Point recharge is a main factor determining the large variation in spring discharge and its proportion to total recharge can reach 58%. These simulation results highlight the importance of the temporal and spatial distribution of recharge to karst spring discharge response. The coupled model provides an effective approach to include the effect of an epikarst zone in CFPv2 and it can easily be applied for the simulation of karst aquifers with similar characteristics as the study site.

Water and air dynamics within a deep vadose zone of a karst massif: Observations from the Lukina jama–Trojama cave system (−1,431 m) in Dinaric karst (Croatia)

AbstractExtreme heterogeneity of karst systems makes them very challenging to study. Various processes within the system affect its global response, usually measured at karst springs. Research conducted in caves provides a unique opportunity for in situ analysis of separate processes in karst underground. The aim of the present study was to research the water and air dynamics within a deep karst system. Air and water basic physical parameters across the Lukina jama–Trojama cave system (−1,431 m) were continuously monitored during a 1‐year period. Recorded hydrograph of the siphon lake at the bottom of the cave was used to interpret the characteristics of an unexplored phreatic/epiphreatic conduit network. Water origin in the siphon was determined based on temperature and electrical conductivity. Air temperature and humidity monitoring revealed a strong inflow of air of sub‐zero temperature into the upper portion of the cave during winter. Cave passage morphology was interpreted as the main determinant of air dynamics, which caused ice to accumulate extensively in the upper portions of the cave and caused the temperature on the top of the homothermic zone to be significantly below the mean outside temperature. Air dynamics also lowered the temperature of water flowing through the cave vadose zone and feeding the phreatic zone of the massif. The pronounced temperature difference between the phreatic zone and the top of the homothermic zone probably contributed to the thermal gradient observed in the cave, which is steeper than in ice‐free caves in the area. Our results enabled the development of a conceptual model that describes coupling between air and water dynamics in the cave system and its surroundings.

Crustal Deformation and Seismicity Modulated by Groundwater Recharge of Karst Aquifers

AbstractTriggered seismicity in karst regions has been explained assuming the existence of a hydraulically connected fracture system and downward diffusion of surface pore pressures. Karst systems are, in fact, able to swiftly channel large amount of rainfall through networks of conduits increasing the hydraulic head loading upon the fluid‐saturated, poroelastic crust. Here we use Global Positioning System and hydrological and seismicity data to show that poroelastic strain in the shallow crust (0–3.5 km) controls seasonal and multiannual modulation of seismicity along the Irpinia Fault Zone (Southern Italy) without requiring a hydraulically connected fracture system from the surface to hypocentral depths. We suggest that groundwater recharge of karst aquifers along the Irpinia Fault Zone produces stress perturbations large enough to modulate strain accumulation and seismicity and temporarily modify the probability of nucleation of seismic events such as the 1980 Irpinia, MS 6.9, earthquake.