Distinctive Characteristics of Karst Aquifers

With their specific morpho-hydrology, karst terrains are distinct from non-karstic areas. Effective porosity is mostly tertiary in karstic terrains, which gives rise to high anisotropy and heterogeneity, distinctive characteristics of karst aquifers. Flow in a dissolutional (karstic) porosity media is likely rapid and turbulent, which, in most cases, prevents the application of Darcy's law to predict flow. Recharge may occur either as concentrated (not seen in granular aquifers) or dispersed, depending on the existence and type of surficial karst morphological features. Type of recharge is controlled by karst morphological features and development: concentrated where sinkholes and dolines dominate the area, diffuse where non karst features are predominant. On the other hand, storage may be only present in the saturated zone or also in the subcutaneous zone. To a great extent, the types of recharge and storage control the response of the karst aquifer to an input. This response is recorded in terms of variability degree in head (or flow rate) and in water quality. Temporal variations of head (or flow rate) and water chemistry are generally recorded as moderate or extreme in karst aquifers. The variability of water chemistry and the magnitude, timing and duration of response of springs and wells to storms is indicative of the hydrologic behavior of karst aquifers. In many real cases, springs are fed by a mixture of both types of flow [1, 9].

On the other hand, the size of the aquifer has a significant influence on this response. Large aquifers dampen the response [9]. Time of transmission depends on the size and hydraulic conductivity of the aquifer. Geologically, carbonate rock mass can be very thick. But it is the karstification (or erosion) base that controls the depth and thickness of the phreatic (saturated) zone. Three main types of erosion bases can be defined in karst terrains. The contemporary sea level is the main and the ultimate base of karstification. The lithological contact between karstifable carbonate rock mass and non-karstic unities above the contemporary sea level marks the ultimate base of karstification. A former base of karstification can be inundated by the eustatic rise of sea level or land subsidence due to epeirogenic movements, or, on the contrary, karstification base may rise as a consequence of decline of sea level or uplift of land due to epeirogenic movements. In mountainous areas, local base of karst may develop at some lower lands and topographical depressions such as river valleys or inland lakes. In areas where mechanic erosion is faster than karst development, deep valleys are incised and karstification base stands at a level higher than the bottom of the incised valley.

The knowledge of the type and position of the karstification base is important in developing projects to mitigate the adverse affects of climate change on karst groundwater resources. Most karst aquifers are naturally discharged by springs. Character of spring recession curve provides an indication of the type of spring flow. Worthington [10] has classified karst springs in relation to the recession curve. Flow characteristics of a spring can be used to have a preliminary assessment of vulnerability of karst aquifers against the impacts of climate changes. Residence time is another important parameter that might indicate the hydrologic behavior of karst aquifers. Karst aquifers with typical short residence times are generally classified as systems closer to surface waters. This conception stems from the fact that karst aquifers have a rapid flow component and in most cases the rapid flow component dominates the total flow.

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