About Karst Hydrogeology
What is karst?
Karst is a type of landscape, and also an aquifer type. Karst areas consist of solid but chemically soluble rock such as limestone (most important) and dolomite, but also gypsum, anhydrite and several other soluble rocks. The chemical reaction describing limestone dissolution is:
CaCO 3 + CO 2 + H 2O = Ca 2+ + 2HCO 3 -
Karst landscapes show characteristic landforms caused by chemical dissolution, such as karren (crevices and channels, tens of cm wide), dolines and sinkholes (closed depressions, tens of m in diameter) and poljes (large depressions with flat floor, several km 2 or more). Streams and rivers sinking underground via swallow holes are also frequent.
Picture 1: swallow hole:
Surface stream sinking underground via a swallow hole, Sierra de Libar , Spain (photo: N. Goldscheider)
Karst aquifers are characterised by a network of conduits and caves formed by chemical dissolution, allowing for rapid and often turbulent water flow.
Picture 2: water cave:
Turbulent water flow in a cave, Grotte de Vallorbe , Switzerland (photo: R. Wenger, ISSKA)
A karst aquifer may be present even when there are no discernible karst landforms at the land surface, and even when there are no known and accessible caves.
Why are karst aquifers important?
Hundreds of millions of people worldwide live in karst areas and are supplied by drinking water from karst aquifers. These aquifers include valuable freshwater resources, but are sometimes difficult to exploit and are almost always vulnerable to contamination, due to their specific hydrogeologic properties. Therefore, karst aquifers require increased protection and application of specific hydrogeologic methods for their investigation. Other problems frequently encountered in karst areas include: soil erosion and rock desertification, leakages of channels and reservoirs, collapse of underground cavities and formation of sinkholes, and flooding. Resolution of these problems requires involvement of karst hydrogeology experts.
Hydrogeologic characteristics of karst aquifers
Evolution : Karst aquifers form by flowing water containing carbon dioxide (CO 2) which dissolves carbonate rocks. Therefore, there is a close relation between aquifer evolution, the formation of caves (speleogenesis) and groundwater flow.
Individuality : Although there are many similarities among different karst systems, every karst system is also a special case and generalisation is difficult.
Heterogeneity : The properties of karst aquifers greatly vary in space. There may be large quantities of water in a cave, but a borehole a few metres away may be completely dry.
Anisotropy : The aquifer hydraulic properties depend on the orientation of geologic fabric elements; for example, the hydraulic conductivity is typically high in the direction of large fractures and conduits, but may be low in other directions.
Duality of recharge : Recharge water may originate from the karst area itself (autogenic recharge) or from adjacent non-karstic areas (allogenic recharge).
Duality of infiltration : Infiltration occurs through the soil and unsaturated zone (diffuse infiltration), and may also be concentrated via swallow holes/sinks (point infiltration).
Duality of porosity and flow : There are two or even three types of porosity in karst aquifers: intergranular pores in the rock matrix, common rock discontinuities such as fractures (fissures) and bedding planes, and solutionally-enlarged voids such as channels and conduits developed from the initial discontinuities. Whereas groundwater flow in the matrix and small fissures is typically slow and laminar, flow in karst conduits (caves) is often fast and turbulent.
Variability : The water table in karst aquifers can sometimes fluctuate 10s or even 100s of metres in short periods of time, and karst springs typically show rapid variations of discharge and water quality.
Picture 3: block diagram:
Block diagram of a heterogeneous karst aquifer illustrating the duality of recharge (allogenic vs. autogenic), infiltration (point vs. diffuse) and porosity/flow (conduits vs. matrix) (Goldscheider & Drew 2007).
Difficulties in using karst groundwater
Vulnerability to contamination : Contaminants can easily enter karst aquifers through thin soils or via swallow holes (sinks). Inside the aquifer, contaminants can quickly spread over large distances, due to rapid and turbulent flow in the conduit network. Natural attenuation processes, such as filtration and retardation, are often less effective than in other aquifers.
Access to water : Due to the high degree of heterogeneity, it is difficult to drill a successful water supply well into a karst aquifer. In mountainous karst regions, the water table is often very deep below land surface, sometimes 100s of metres. Karst springs are typically very large, but also quite rare. Even in humid regions, there are often large areas without any accessible water because surface water runoff and rainfall quickly infiltrate into the karst aquifer and flow to distant springs.
Variability : Water suppliers prefer water sources with stable discharge and water quality, but karst springs often show high variations of both. Periods of excellent water quality may be interrupted by short contamination events.
Methods to study karst aquifer systems
Due to the characteristics described above, conventional hydrologic and hydrogeologic methods often fail when applied to karst; their adaptations and karst-specific methods are therefore required.
Geologic methods : The lithology, stratigraphy, fracturing, fault pattern and fold structures are crucial to understanding groundwater flow in karst aquifers.
Speleology : Conduits and underground channels are crucial for groundwater flow in karst aquifers. Caves make it possible to enter the aquifer and directly observe and study a part of the conduit-channel network.
Hydrologic methods : Due to the high variability of flow rates of sinking streams, cave streams and karst springs, continuous monitoring of water quantity and quality is crucial in karst hydrogeologic studies.
Hydraulic methods : Potentiometric maps and hydraulic tests in boreholes and wells are widely applied in hydrogeology but require specific adaptations when applied to karst.
Isotopic techniques : Stable and radioactive isotopes can help to identify the origin of the water, determine transit times, and characterise mixing processes.
Tracer tests : Tracer tests are the most powerful method to identify point-to-point connections (typically between swallow holes/sinks and springs), to delineate karst spring catchments, and to characterise flow and transport in the conduit networks.
Picture 4: green tracer: Injection of a fluorescent tracer (uranine) into a swallow hole, Austrian Alps (photo: N. Goldscheider)
Geophysical methods : Geophysics can help identify locations for well drilling, investigate subsurface cavities (potential sinkholes) and obtain other information on the aquifer structure.
Modelling : Mathematical models can help to better understand speleogenesis, flow and transport in karst aquifers. However, there are examples where the application of conventional groundwater flow models in karst environments produced catastrophically wrong results and resulted in delineation of grossly inadequate source protection zones, leading to disease outbreaks, all because the specific nature of karst was ignored.
Picture 5: Walkerton tragedy:
Illustration of the hydrogeologic reasons of a waterborne disease outbreak that occurred in May 2000 in Walkerton, Canada. The 30-day capture zone for drinking water well 7 was delineated on the basis of modelling (MODFLOW), ignoring the specific nature of karst. Subsequent tracer tests demonstrated that the protection zones were inadequate (S. Worthington, in Goldscheider & Drew 2007).
Selected books for further reading
Drew D, Hötzl H (1999) Karst Hydrogeology and Human Activities. Impacts, Consequences and Implications. Balkema, Rotterdam, 322 pp.
Ford D, Williams P (2007) Karst Hydrogeology and Geomorphology. Wiley, 576 pp.
Goldscheider N, Drew D (Eds.) (2007) Methods in Karst Hydrogeology. Taylor & Francis, London, 264 pp.
Käss W (1998) Tracing Technique in Geohydrology. Balkema, Rotterdam, 581 pp.
Kresic N (2007). Hydrogeology and Groundwater Modeling, Second Edition. CRC Press/Taylor & Francis, Boca Raton, New York, London, 807 p.
White WB (1988) Geomorphology and Hydrology of Karst Terrains. Oxford University Press, New York, NY, 464 pp.