Tracer tests in karst hydrogeology and speleology (original) (raw)
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Spatially resolved information on karst conduit flow from in-cave dye tracing
Hydrology and Earth System Sciences, 2014
Artificial tracers are powerful tools for investigating karst systems. Tracers are commonly injected into sinking streams or dolines, while springs serve as monitoring sites. The obtained flow and transport parameters represent mixed information from the vadose, epiphreatic and phreatic zones (that is, the aquifer remains a black box). Accessible active caves constitute valuable but underexploited natural laboratories to gain detailed insights into the hydrologic functioning of the aquifer. Two multi-tracer tests in the catchment of a major karst spring (Blautopf, Germany) with injections and monitoring in two associated water caves aimed at obtaining spatially and temporally resolved information on groundwater flow in different compartments of the system. Two tracers were injected into the caves to characterize the hydraulic connections between them and with the spring. Two injections at the land surface, far from the spring, aimed at resolving the aquifer's internal drainage structure. Tracer breakthrough curves were monitored by field fluorimeters in caves and at the spring. Results demonstrate the dendritic drainage structure of the aquifer. It was possible to obtain relevant flow and transport parameters for different sections of this system. The highest mean flow velocities (275 m h −1 ) were observed in the near-spring epiphreatic section (openchannel flow), while velocities in the phreatic zone (pressurized flow) were one order of magnitude lower. Determined conduit water volumes confirm results of water balances and hydrograph analyses. In conclusion, experiments and monitoring in caves can deliver spatially resolved information on karst aquifer heterogeneity and dynamics that cannot be obtained by traditional investigative methods.
Hydrochemistry, a tool for understanding karst groundwater flows
2014
Karst is a highly heterogeneous medium (i.e. properties vary from one point to another one), and can be studied by variations of water chemical composition, which reflect in space and time differences in origin (runoff on impervious media, diffuse seepage, inflows from evaporites...) or residence time throughout the various aquifer compartments (soil, epikarst, vadose and saturated zones. This natural background is supplemented by anthropogenic inputs (domestic, urban or industrial wastes, leachates from agricultural soils and livestock by-products...). Any natural (cave, chiasm) or artificial (tunnel, mine) penetrable gallery is a means of observation and sampling, at least of infiltration water, and sometimes of water from the saturated zone. Several cave sites in Europe provide examples for understanding the behavior of karst flows, both through the vadose and the saturated zones.
Hydrological Processes, 2012
A structure model was used to analyse solute-transport parameter estimates based on tracer breakthrough curves. In the model system, groundwater flow is envisioned to be organised in a complex conduit network providing a variety of short circuits with relative small carrying capacities along different erosion levels. The discharge through the fully filled conduits is limited owing to void geometries and turbulent flow; thus, a hierarchic overflow system evolves where conduits are (re-)activated or dried up depending on the flow condition. Exemplified on the Lurbach-Tanneben karst aquifer, the applicability of the model approach was tested. Information derived from multi-tracer experiments performed at different volumetric flow rates enabled to develop a structural model of the karst network, under constraint of the geomorphological and hydrological evolution of the site. Depending on the flow rate, groundwater is divided into up to eight flow paths. The spatial hierarchy of flow paths controls the sequence of flow path activation. Conduits of the topmost level are strongly influenced by reversible alteration processes. Sedimentation or blocking causes an overflow of water to the next higher conduit. Flow path specific dissolutional denudation rates were estimated using the temporal development of the partial discharge.
Hydrogeology Journal
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 crosssectional 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.
Geophysical study to characterize input karst water circulation in Saulge's cave (Mayenne, France)
Geophysical measurements were carried out around Saulges caves that sought to highlight the local karstic morphologies and the impact on preservation ofarchaeologicalmaterialwithinthe caves.Electricalresistivity tomographies (ERTs) and apparent conductivity mapping detected a soil cover on the plateausthat ends abruptly over fractured limestone or over a bowl-shaped structure filled with clay soil. Moreover, there is at least one zone of soil accumulation with a basin form with almost no soil cover around this structure. The ERT and seismic refraction tomography (SRT) detected an important anomaly in the valley. Many clues indicate that this anomaly is a karstic conduit filled with water orclay.Therefore, somekarstic dissolutionzoneshavebeenfoundandonlygeophysicalmethodsareableto detect such features. Detection of preferential pathways could help to protect prehistoric art within the caves and new karstic morphologies help to better understand this karstic system.
Separation of groundwater-flow components in a karstified aquifer using environmental tracers
Applied Geochemistry, 1999
Groundwater discharges from the intensively karstified Taurus Mountains to the Mediterranean Sea, either along the contact zone between the mountains and the Travertine Plateau (the Kirkgozler Springs, 15 m3/s), or through the travertine (e.g. the Dudenbasi Spring, 18 m3/s) and underneath it (unnamed submarine springs, unknown discharges). In an attempt to identify the hydraulic connections between the various outlet points,
Chapter 4 Overview of Methods Applied in Karst Hydrogeology
2015
Karst aquifers have a different hydraulic structure and behavior than porous media and therefore require specific investigation methods (Goldscheider and Drew 2007). As discussed in Chap. 3, they are characterized by a high degree of heterogeneity and discontinuity, resulting in a duality of recharge, infiltration, porosity, flow, and storage (Bakalowicz 2005; Ford and Williams 2007). Recharge either originates from the karst area itself (autogenic) or from adjacent non-karst areas that drain toward the karst aquifer (allogenic). Infiltration occurs diffusely through soil and epikarst or concentrated via dolines or swallow holes. Karst aquifers show double or triple porosity, consisting of intergranular pores and fractures (often summarized as matrix porosity), and solutional conduits. Flow in the network of conduits and caves is often rapid and turbulent, while flow in the matrix is generally slower and laminar (Fig. 4.1). Storage occurs in the matrix and conduits, but residence ti...
Journal of Hydrology, 1997
Study of the movement of chemical compounds naturally present in the water, or which result from pollution, are examined according to the reservoir structure in karstic aquifers. Structure is represented by a simple geometrical model; slow flow takes place in blocks with a network of lowpermeability cracks. The blocks are separated by highly permeable karstic conduits that allow rapid Row, and these form the aquifer drainage system. The karst studied covers 110 km*. It is fed by an interrupted stream draining a 35 km' non-karstic basin, contaminated at the entry to the karst by effluents from a sewage treatment station. The underground water reappears as a resurgence with an annual average flow of approximately 1 m3 s-', after an apparent underground course of 8 km in the karst. Several local sources of pollution (effluent from septic tanks) contaminate the underground water during its course. Sixteen measurement operations were performed at 12 water points, between the interrupted stream and the spring. Some sampling points were at drains, and others were in the low-permeability fissured blocks. Comparison at each point of the concentrations of 14 chemical compounds gave the following results: when pollutant discharge occurs in a permeable zone, movement is rapid in the drainage network formed by the karstic conduits, and does not reach the less permeable fissured blocks which are thus protected; however, if discharge is in a low-permeability zone, the flow does not allow rapid movement of the polluted water, and this increases the pollutant concentration at the discharge. This simple pattern can be upset by a reversal of the apparent piezometric gradient between a block and a conduit during floods or pumping; this may reverse flow directions and hence modify the movement of contaminants. The study made it possible to site five boreholes whose positions in the karstic structure were unknown, showing the interest of such an approach for the forecasting of the impact of potential pollution. 0 1997 Elsevier Science B.V.