Subsurface Conduits Research Articles

Two-dimensional high-resolution numerical investigation of eddy effect in artificial rough conduits with different shapes

Subsurface conduit flow is a critical element of investigating groundwater movement in karst regions, and it is still poorly understood due to complexity of subsurface conduit geometries and the nonlinearity of the conduit flux-hydraulic gradient relationship. A significant nature of the flow nonlinearity is the occurrence of different eddies in conduits. To reveal the nonlinear nature of conduit flow, this study investigates the influence of the eddy effect on the hydraulic gradient of different rough conduits based on extensive high-fidelity numerical simulations. Overall, the hydraulic gradients of all types of rough conduits show quadratic relations with an increase in the specific discharge. In addition, the triangular rough conduit has the largest hydraulic gradient, which gradually decreases as the shape coefficient (which is used to quantify rough elements) increases. We have identified the development and evolution of eddies in different types of rough conduits in detail. This study shows that a larger specific discharge leads to a larger eddy area, and the hydraulic gradient changes exponentially with the eddy area ratio within a limited range. Finally, the relationships among specific discharges, eddy area ratio, and hydraulic gradients are constructed and summarized. The results of this investigation are helpful for developing realistic karst groundwater flow and transport models in which subsurface conduit plays a dominant role for conducting groundwater flux and mass transport.

Deciphering Martian Flood Infiltration Processes at Hebrus Valles: Insights From Laboratory Experiments and Remote Sensing Observations

AbstractIn evaluating the unique geomorphology of Hebrus Valles on Mars, an outflow channel southeast of Utopia Planitia, we investigated the role of fluvial dynamics in forming subsurface conduits. Unlike typical Martian channels that terminate under younger sediment rocks, Hebrus Valles retains its downstream features, thus offering a window into surface and subsurface interactions between liquid water and regolith. Employing physical lab simulations, we utilized buried polygonal tunneling configurations in a sandy matrix to replicate the incision features observed at the channel's terminus. This study provides the first lab‐based evidence demonstrating the role of conduits in conveying large floods in the Hebrus Valles. Our results lend support to the interpretation that Hebrus Valles conveyed high‐volume floods by demonstrating that conduits are necessary to form the features observed on the surface of Hebrus Valles. The experiments also highlight the role of sinkholes as floodwater capture points, which suggests the formation of a regional interconnected network of subsurface channels and caverns. These findings affirm that fluvial dynamics in Hebrus Valles could have expanded subsurface discontinuities into functioning conduit systems, potentially directing floodwaters through an interconnected subterranean network.

Effects of global and climate change on the freshwater-seawater interface movement in a Mediterranean karst aquifer of Mallorca Island

Karst aquifers are globally prized freshwater sources, posing a significant preservation challenge. These aquifers typically exhibit dual or even triple porosities, encompassing matrix, fractures-fissures and conduits, rendering them highly responsive to variations in chemical characteristics and hydraulic head. In coastal regions, these aquifers often possess extensive subsurface conduit networks intricately linked to the rock matrix, facilitating groundwater discharge into the sea. Therefore, they display acute sensitivity to seawater intrusion, swiftly reacting to changes in precipitation and pumping regimes. This makes them exceptionally vulnerable to short-term meteorological fluctuations and long-term climate change. Their high heterogeneity leads to uneven penetration of the freshwater-seawater interface, causing rapid seawater intrusion inland over significant distances. The Mediterranean region, characterized by water deficit and water stress, faces strong impacts from climate change, featuring a warming atmospheric trend exceeding the global average, along with diminished rainfall exacerbating water scarcity. Increasing water demands for agriculture, urban development, and the growing tourism industry, because of global change, are worsening water stress. Our primary research objectives were analyzing the environmental consequences of global and climate change on seawater intrusion in Mediterranean coastal karst aquifers, with a focus on the role of the double-flow model, thus contributing to the understanding of the processes involved. To achieve this, we selected a study region on Mallorca Island in the western Mediterranean, where a karst aquifer system discharges into the sea. We employed various study methods, notably hydrochemical techniques and multi-isotopic analysis, encompassing the examination of 2H and 18O isotopes in water, 87Sr/86Sr ratio, Sr and B concentrations, and δ11B in water. A key finding is the rebound effect, wherein aquifers recontaminate due to solute molecular back-diffusion following cessation of extractions and the retreat of marine intrusion, providing insight into the impact of climate and global change on Mediterranean karst aquifers.

Microbial biogeography of the eastern Yucatán carbonate aquifer.

The extensive Yucatán carbonate aquifer, located primarily in southeastern Mexico, is pockmarked by numerous sinkholes (cenotes) that lead to a complex web of underwater caves. The aquifer hosts a diverse yet understudied microbiome throughout its highly stratified water column, which is marked by a meteoric lens floating on intruding seawater owing to the coastal proximity and high permeability of the Yucatán carbonate platform. Here, we present a biogeographic survey of bacterial and archaeal communities from the eastern Yucatán aquifer. We apply a novel network analysis software that models ecological niche space from microbial taxonomic abundance data. Our analysis reveals that the aquifer community is composed of several distinct niches that follow broader regional and hydrological patterns. This work lays the groundwork for future investigations to characterize the biogeochemical potential of the entire aquifer with other systems biology approaches.

Pendugaan Aliran Conduit Sistem Akuifer Kawasan Karst Gua Pindul Gunung Kidul Menggunakan Metode Geolistrik Konfigurasi Wenner

Karst areas contain limestone lithology that is easily dissolved by water, so rainwater can accelerate the process of dissolving rocks below the surface and forming cavities. The hydrological system of the Pindul area has a conduit flow karst aquifer type. The components of conduit flow are open channels and pipe-like passages that have various shapes. The distribution and direction of conduit flow control the Pindul Cave karst aquifer system in Gunung Kidul. The wenner configuration resistivity geoelectric method can determine the presence of a subsurface—conduit flow in karst areas by knowing the difference in rock-specific resistance values. Resistivity geoelectric measurements were carried out using 8 tracks with each track length of 200 meters. The results of data processing and interpretation of the resistivity cross-section show the presence of conduit flow in 5 passes with a resistivity value of 0.174-25 Ωm. The results of the study can provide an overview to determine the subsurface river area.

Glacier structure influence on Himalayan ice‐front morphology

AbstractWe investigate the role of glacier structures in controlling ice‐front morphology and dynamics of four Himalayan lake‐terminating glaciers over a 20‐year period. At Imja, Trakarding, Lumdin and Dang Pu glaciers, lake area was mapped between 2000 and 2020 using Landsat 5/7/8 and Sentinel‐2 imagery. Discrete glacier flow units were identified, with glacier structures (e.g., open crevasses, transverse structures, longitudinal structures) digitised using the finest resolution panchromatic bands in each year (30, 15, or 10 m). Mapping revealed a distinct pattern of transverse structures towards the terminus of each glacier that influence ice‐front position and morphology and are then exploited via iceberg calving events. Our structural analysis also illustrates the role of subsurface conduits in calving events. During subsurface conduit collapse, glacier recession is enhanced, leading to calving along adjacent transverse structures. Furthermore, our analysis shows that ice‐front morphology influences the pattern of glacier recession. Ice fronts with distinct ice aprons undergo slower periods of recession than ice fronts with ice cliffs. We conclude that glacier structures are important in determining ice‐front morphologies at lake‐terminating Himalayan glaciers, and therefore, structural analysis is vital when assessing future ice‐front positions and behaviour, as well as rates of glacier recession.

Urban DAS Data Processing and Its Preliminary Application to City Traffic Monitoring.

Distributed acoustic sensing (DAS) is an emerging technology for recording vibration signals via the optical fibers buried in subsurface conduits. Its relatively easy-to-deploy and high spatial and temporal sampling characteristics make DAS an appealing tool to record seismic wavefields at higher quantity and quality than traditional geophones. Considering that the usage of optical fibers in the urban environment has drawn relatively less attention aside from its functionality as a telecommunication cable, we examine its ability to record seismic signals and investigate its preliminary application in city traffic monitoring. To solve the problems that DAS signals are prone to a variety of environmental noise and are generally of weak amplitude compared to noise, we propose a fast workflow for real-time DAS data processing, which can enhance the detection of regular car signals and suppress the other components. We conduct a DAS experiment in Hangzhou, China, a typical metropolitan area that can provide us with a rich data library to validate our DAS data-processing workflow. The well-processed data enable us to extract their slope and coherency attributes that can provide an estimate of real traffic situations. The one-minute (with video validations) and 24 h statistics of these attributes show that the speed and volume of car flow are well correlated demonstrates the robustness of the proposed data processing workflow and great potential of DAS for city traffic monitoring with high precision and convenience. However, challenges also exist in view that all the attributes are statistically analyzed based on the behaviors of a large number of cars, which is meaningful but lacking in precision. Therefore, we suggest developing more quantitative processing and analyzing methods to provide precise information on individual cars in future works.

On an Extensive Late Hydrologic Event in Gale Crater as Indicated by Water‐Rich Fracture Halos

AbstractWe analyze spatially pervasive, light‐toned “halos” associated with fractures in a sedimentary unit (Stimson) of Gale crater, Mars, and report a similar network of halos discovered in a separate geologic group (Bradbury). Through a dedicated active neutron measurement campaign, we provide independent confirmation of the water‐rich nature of these features. Together with mineralogical and geochemical data, these features are consistent with abundant hydrated amorphous silica (opal‐A). We suggest that the mineral and amorphous assemblages are indicative of formation under low‐temperature and predominantly low‐pH conditions (passive silica enrichment) with minor contribution of silica (active silica enrichment) from adjacent units. We show that there is significant amorphous silica in the array of sedimentary rocks in Gale crater, allowing them to play a role in an active silica enrichment phase of halo formation. We suggest that the involved alteration event was short lived and our finding of vast halo networks in a distant, older unit implies a more vast network of hydrologic subsurface conduits than previously known. This relatively recent subsurface hydrologic system was present long after the transition from a warm and wet to a cold and dry Martian environment, extending the habitability conditions on Mars to an epoch that is generally considered not favorable for life on the surface. Finally, our bulk H quantification of these features, which ranges from ∼3–6 wt% H2O‐equivalent‐H, suggests that the amorphous material in halos hosts ample supplies of readily released water, making them a considerable resource at the otherwise dry Martian equator.

Potential Caves: Inventory of Subsurface Access Points on the Surface of Titan

AbstractWe propose that cycling hydrocarbon rains on Saturn's moon Titan are likely to create subsurface conduits through dissolution geology of the extensive organic and largely‐soluble substrate. In this work we inventory different terrains and locations on Titan that could host potential subsurface access points or caves. Using Cassini data, we identified and enumerated a cryovolcanic construct, filled and empty lake basins that may have formed through karstic processes, estimates of closed valleys in organic plateaux, and enigmatic equatorial pits. Our preliminary survey identified over 21,000 estimated locations for future exploration to identify potential caves and cave forming processes on that world. We speculate that the exploration of these caves could teach us about the cave forming processes, as well as the past climate history and perhaps even chemical deposition history on Titan.

Development of Shalbatana Vallis (Mars) by dry volcanic processes

Shalbatana Vallis is a large Martian outflow system that extends into Chryse Planitia from areas of topographic disturbance located north of Ganges Chasma. Development of this system has previously been attributed to processes including: 1) the expulsion of groundwater generated by interactions between magmatic intrusions and ground ice; and 2) the northward conveyance of water along subsurface conduits connected to a deep lake impounded within Ganges Chasma. However, aqueous interpretations of Shalbatana Vallis suffer from weaknesses such as the implausible nature of hypothesized outburst processes, discrepancies between the magnitudes of subsidence that characterize parts of the system and realistic volumes of excess ice that might have once existed in the subsurface, and contradictions between the nature of hypothesized aqueous processes and the mineralogy of associated geological materials. The attributes of Shalbatana Vallis are instead consistent with dry development as a result of the rapid and voluminous effusion of low-viscosity lavas of mafic or ultramafic composition. Volcanic development of the main channel of Shalbatana Vallis is estimated to have required a minimum effusion of ∼360,000 ​km3 of lava, though a total of several times this volume is more likely to have been necessary since channel development cannot be assumed to have taken place with full efficiency. Involved lavas are hypothesized to have been fed by a deeply-rooted igneous plumbing system, and terrain collapse and disturbance located at the head of the system is expected to be partly related to the collapse of intrusions during the latter stages of igneous events. A volcanic origin for Shalbatana Vallis further highlights the critical role played by igneous processes in channel development on rocky bodies of the inner solar system.

Dynamics of hydrological and geomorphological processes in evaporite karst at the eastern Dead Sea – a multidisciplinary study

Abstract. Karst groundwater systems are characterized by the presence of multiple porosity types. Of these, subsurface conduits that facilitate concentrated, heterogeneous flow are challenging to resolve geologically and geophysically. This is especially the case in evaporite karst systems, such as those present on the shores of the Dead Sea, where rapid geomorphological changes are linked to a fall in base level by over 35 m since 1967. Here we combine field observations, remote-sensing analysis, and multiple geophysical surveying methods (shear wave reflection seismics, electrical resistivity tomography, ERT, self-potential, SP, and ground-penetrating radar, GPR) to investigate the nature of subsurface groundwater flow and its interaction with hypersaline Dead Sea water on the rapidly retreating eastern shoreline, near Ghor Al-Haditha in Jordan. Remote-sensing data highlight links between the evolution of surface stream channels fed by groundwater springs and the development of surface subsidence patterns over a 25-year period. ERT and SP data from the head of one groundwater-fed channel adjacent to the former lakeshore show anomalies that point to concentrated, multidirectional water flow in conduits located in the shallow subsurface (< 25 m depth). ERT surveys further inland show anomalies that are coincident with the axis of a major depression and that we interpret as representing subsurface water flow. Low-frequency GPR surveys reveal the limit between unsaturated and saturated zones (< 30 m depth) surrounding the main depression area. Shear wave seismic reflection data nearly 1 km further inland reveal buried paleochannels within alluvial fan deposits, which we interpret as pathways for groundwater flow from the main wadi in the area towards the springs feeding the surface streams. Finally, simulations of density-driven flow of hypersaline and undersaturated groundwaters in response to base-level fall perform realistically if they include the generation of karst conduits near the shoreline. The combined approaches lead to a refined conceptual model of the hydrological and geomorphological processes developed at this part of the Dead Sea, whereby matrix flow through the superficial aquifer inland transitions to conduit flow nearer the shore where evaporite deposits are encountered. These conduits play a key role in the development of springs, stream channels and subsidence across the study area.

Characterization of karstic aquifer complexity using fractal dimensions

The present study analyzes fractal dimensions for the daily discharge data series of 12 karstic springs registered over two decades in Northeast Hungary. Fluctuation in the observed data is frequent and irregular, producing rough time series. The level of roughness is measured by the fractal dimension defined in different ways and corresponds to the intensity of fluctuation. That, in turn, results from the structure of the karstic aquifer, its conduits’ geometry, and the water migration in them. In the given case of springs, p-variogram based fractal dimensions reflect the karstification level primarily. On the other hand, box-count and information dimensions are associated with mixing karstic and hydrothermal components when the latter is present. Therefore, the analysis of fractal dimensions of spring discharges may provide a way to obtain information on the complexity of the hidden subsurface conduits and the water flows in them in an exploratory and comprehensive way.

Decadal exploration of karst hydrogeology in the Woodville Karst Plain (WKP): A review of field investigation and modeling development

This paper provides a review of over thirty years of research and investigation in the Woodville Karst Plain (WKP) in northern Florida, which is representative of the Floridan Aquifer system, a karst aquifer in the southeastern United States. The WKP is a well-developed karst region where over 40 km of subsurface conduits are mapped by divers and tracer tests, and connected to Wakulla Spring, one of the largest freshwater springs in the U.S. Furthermore, the conduit networks expand and interact with seawater via the connection to a submarine spring, and impact groundwater flow cycle in the aquifer. Numerical models, based on multiple field investigations, have been developed to quantify the understanding of hydrogeologic processes in the WKP. Novel modeling approaches with capabilities were developed to simulate seawater and groundwater interaction in this dual-permeability aquifer. The WKP site has served as a valuable natural observatory for a series of studies documented in this paper to understand groundwater flow, contaminant transport and seawater intrusion in the well-developed karst subsurface system.

Identification and Estimation of Solute Storage and Release in Karst Water Systems, South China.

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.

Identifying and estimating soil erosion and sedimentation in small karst watersheds using a composite fingerprint technique

The highly heterogeneous surface environment and subsurface conduit network in karst areas make estimating soil erosion extremely complicated. Sediment fingerprint techniques have been successfully applied in soil erosion research in karst areas as a new way to more effectively collect information on soil loss. However, few sediment fingerprint factors, most of which are based on radioactive fallout tracers, such as 137Cs and/or 210Pb, have been used in existing studies. The soil loss rate estimated by these fingerprint factors varies greatly among different watersheds. To clearly identify the sedimentary sequence, to estimate the changes in soil erosion in recent years, and to relate the soil erosion process to rocky desertification, a composite sediment fingerprint technique is used in this study. Two typical watersheds with peak clusters and depressions and with different degrees of rock desertification were selected, and sediment cores were extracted from the depressions to measure changes in 137Cs, magnetic susceptibility, soil organic carbon (SOC) and sediment particle composition at different depths. The results indicated that the soil loss rates in the Liaojiawodang (LJWD) and Dajutang (DJT) watersheds showed a fluctuating decrease from 1954 to July 2018. The severe soil erosion in the periods of 1954–1963，1963–1970 and 1976–1986 was mainly caused by large-scale reclamation and deforestation. In addition, heavy rainfall in 1983 and 1991 might have played a significant role in the soil loss in the DJT watershed in 1978–1986 and in the LJWD watershed in 1986–2000, respectively. The construction of agricultural terraces several decades ago and the implementation of the Closing Mountains for Forest Cultivation, Grain for Green and Poverty Alleviation projects in recent years to improve the ecological environment have effectively alleviated the soil erosion in the studied watersheds. Over the past 60 years, soil loss in the LJWD and DJT watersheds decreased by approximately 89 % and 87 %, respectively. In karst areas, rocky desertification and soil erosion do not exhibit identical trends. Because of severe rocky desertification, the soil erosion in the LJWD watershed was significantly less than that in the DJT watershed. This study provides important information on detailed sediment dating and demonstrates the potential of using 137Cs, magnetic susceptibility, SOC and sediment particle composition to identify changes in erosion in a watershed.

Quantification of nitrate fate in a karst conduit using stable isotopes and numerical modeling

Nitrate (NO3⁻) fate estimates in turbulent karst pathways are lacking due, in part, to the difficulty of accessing remote subsurface environments. To address this knowledge and methodological gap, we collected NO3⁻, δ15NNO3, and δ18ONO3 data for 65 consecutive days, during a low-flow period, from within a phreatic conduit and its terminal end-point, a spring used for drinking water. To simulate nitrogen (N) fate within the karst conduit, the authors developed a numerical model of NO3⁻ isotope dynamics. During low-flow, data show an increase in NO3⁻ (from 1.78 to 1.87 mg N L−1; p < 10−4) coincident with a decrease in δ15NNO3 (from 7.7 to 6.8‰; p < 10−3) as material flows from within the conduit to the spring. Modeling results indicate that the nitrification of isotopically-lighter ammonium (δ15NNH4) acts as a mechanism for an increase in NO3⁻ that coincides with a decrease in δ15NNO3. Further, numerical modeling assists with quantifying isotopic overprinting of nitrification on denitrification (i.e., coincident NO3⁻ production during removal) by constraining the rates of the two processes. Modeled denitrification fluxes within the karst conduit (67.0 ± 19.0 mg N m−2 d−1) are an order-of-magnitude greater than laminar ground water pathways (1–10 mg N m−2 d−1) and an order-of-magnitude less than surface water systems (100–1000 mg N m−2 d−1). In this way, karst conduits are a unique interface of the processes and gradients that control both surface and ground water end-points. This study shows the efficacy of ambient N stable isotope data to reflect N transformations in subsurface karst and highlights the usefulness of stable isotopes to assist with water quality numerical modeling in karst. Lastly, we provide a rare, if not unique, estimate of N fate in subsurface conduits and provide a counterpoint to the paradigm that karst conduits are conservative source-to-sink conveyors.

Origin and development of true karst valleys in response to late Holocene sea‐level change, the Transverse Glades of southeast Florida, USA

AbstractThe Miami Limestone is an oolite depositional body that is used as an analog model for geological interpretation of the rock record. Barrier‐bar complex, oolite banks, extensive bryozoan flats and tidal creeks, referred to as transverse glades, have been described. High‐resolution LiDAR data are used to produce unprecedented, detailed topographic maps of the transverse glades in the southern Atlantic Coastal Ridge. These maps were originally used to calculate historic discharge from the Everglades but revealed features inconsistent with the prevailing theory that the topography is of a depositional origin. Field observations verified an epikarst terrain truncated by collapsed subsurface conduits creating valleys, previously described as palaeotidal creeks. Observations of the 12 southern tidal creeks or transverse glades provided a sequence of six stages in the development of these karst valleys. After Late Pleistocene deposition of the Miami Limestone, sea‐level dropped producing conditions of: (a) epikarst which reduced the surface elevation preferentially, forming the southern Everglades Basin and modifying Biscayne Bay and Florida Bay precursor basins and (b) downward water movement producing vertical solution features. As Holocene sea‐level approached the South Florida carbonate platform margin a freshwater lens formed and groundwater movement became horizontal. Two sets of cavernous zones developed during the Middle and Late Holocene. Solution pipes and dolines provide a connection between the surface and groundwater along alignments which delineate subsurface conduits. Stages of valley formation are associated with the collapse of these conduits first forming a series of pocket valleys followed by narrow blind valleys and half blind or through valleys linking the Everglades to the coastal plain. Valleys then expanded in width until: (a) completion of cavernous zone collapse, (b) most boulder fields and valley margins reduced by weathering and (c) the valleys filled with wetland soils.

Soil volume restrictions and urban soil design for trees in confined planting sites

Urban trees often suffer from poor soil conditions, compromising their health and safety and raising management issues. In compact development areas, planting sites are beset by critical physical soil constraints, especially confined soil volume on narrow roadsides. These include internal (micro-scale) restrictions due to compaction and excessive coarse fractions such as gravels, and external (macro-scale) restrictions due to limited soil depth and width. Soil area provision (SAP) is assessed based on the moisture need of trees. A creative soil design scheme is proposed to expand soil volume, based on the principle of soil volume sharing, subsurface soil conduits, and subsurface soil corridors supplementing the routine open soil corridor. SAP could extend from the narrow footpath to the adjoining carriageway as a soil peninsula and green space as offset soil volume. The study aimed at knowledge exchange among science, policies, and practices, with implications for the sustainable management of many roadside trees in cities.

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

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

A study on landslides and subsurface piping, facilitated by dykes, using vertical electrical sounding and δO18 and δH2 stable isotopes

A combination of vertical electrical sounding (VES) and δO18 and δH2 stable isotope geochemistry is used in this study to trace out the extension of a dyke and for deciphering the subsurface piping phenomena in a landslide-affected hamlet, Pasukadavu, in the Western Ghats of Kerala, India. VES was successful in extracting three to four different subsurface layers characterized by differing resistivity. Two VES sections were prepared from 24 different VES locations, each one for understanding the dyke extension and for delineating subsurface conduits. The dyke was characterized by high resistivity of 800–5000 Ω.m and shows varying thickness. In the second profile, the void zone, which is characteristic of piping, is delineated through a low resistivity zone (75 to 350 Ω.m). δO18 and δH2 stable isotopes collected along the second VES profile show the same chemistry, indicating that it is the same water which flows all along the VES profile 2. A four-stage conceptual model was developed to illustrate and narrate the sequence of development of the piping phenomena and landslide activity.