Crystalline (igneous and metamorphic) rocksare mostly of negligible primary porosity (usually less than 1%) and permeability and are considered impervious. However, owing the influence of tectonic processes, secondary porosity and permeability are developed along fractures and fissures and other discontinuities. The origin, extent, orientation and widths of the fissures and fractures depend on the intensity and directions of various tectonic movements and stresses that occurred over the geo -logical history of the earth. The study of the nature of tectonic and structural events is therefore fundamental to a better understanding of groundwater flow systems in hard rocks. The orientation of foliation in metamorphic rocks also influence rock permeability. Generally, with increasing depth and temperature, the fissures close, the rocks become less permeable and groundwater flow intensity decreases. However, active groundwater circulation has been observed in deep fractures and folds of crystalline rocks hundreds of meters below ground (Singhal and Gupta, 1999). Groundwater circulation has been observed also in plutonic rocks at depths of more than a thousand meters along deep tectonic structures (lineaments) and may be indicated by the occurrence of thermal springs. The age of fissures also controls rock permeability. Fissures of post-orogenic age are mostly more permeable than old fractures consolidated by orogenic phases and indurated by secondary minerals. The contact between intrusive plutonic bodies and the host permeable sedimentary rocks define boundary conditions for the aquifer. There are also differences in hydraulic properties and groundwater flow between plutonic rocks, e.g.
granite (higher transmissivity, yields and groundwater flux rates in weathered and fractured zones) and metamorphic rocks. Foliation, a typical property of metamorphic rocks (e.g. gneiss, schist) has signi -ficant influence on groundwater movement. Dissolution (chemical weathering) of siliceous rocks may be observed in the increase of the width of fractures and thus in the increase of fracture permeability.
Volcanic rocks(e.g. basalts, rhyolites, andesites, dacites). Porosity and permeability vary and depend mainly on the age and chemical composition (basic or acidic) of the magma, its viscosity, nature of eruption, gas loss during cooling producing pore spaces, amount of enclosed ash and pyroclastics and susceptibility to weathering. Plateau basalts, composed of a number of low viscosity lava flows covering areas of thousands of square kilometres and attaining a thickness of hundreds of metres contain valuable sources of groundwater due to intensive fracturing, high primary and secondary permeability and horizontal hydraulic conductivity and groundwater flow parallel to the lava flows (David, 1969). Multi-layered aquifers may be formed in permeable volcanic sediments trapped between lava flows as well as in other fractured volcanic rocks, particularly in unconsolidated pyroclastic deposits. Low hydraulic conductivity is observed in dense basalts with fracture porosity less than 2%
and in welded tuffs with porosity less than 20 %. It has been reported from many parts of the world that porosity and hydraulic conductivity of volcanic rocks tends to decrease with increasing age.
Through chemical and mechanical weathering, alumino-silicate and quartz-rich rocks produce permeable and sandy elluvia of high porosity (up to 45%) often tens of metres thick, which facilitate groundwater recharge and may constitute locally important aquifers. Basic rocks disintegrate into minerals with high clay content through chemical weathering and form less permeable soils, clog the fissures in the underlying bedrock and thus reduce recharge potential and rock permeability. Generally, porosity decreases with depth in zones of weathering. However, permeability and hydraulic conductivity may increase with depth as the rate of weathering and clay mineral reduction declines as one approaches the rock basement.
Groundwater aquifers which occur in granite and young volcanic rocks are a valuable source of drinking water in many parts of the world. Groundwater sources in both types of rocks located away from flood plains and the influence of tsunami and flooding through storm events may be considered as a safe source of water for emergency situations, provided that they are protected from ongoing pollution. Springs discharging from volcanic rocks are also important sources of emergency water supplies.
Sedimentary rocksvary considerably in their composition. They exhibit all types of interstices (pores,
fissures and karst cavities), all contributing to their bulk permeability. Double, or dual (e.g. primary plus secondary) porosity has to be considered when evaluating ground water flow and the behaviour of pollutants and tracers in such a rock medium. The most prominent aquifers are developed in sandstones and carbonate rocks, particularly limestones. In claystones (mudstones) and siltstones groundwater movement is possible only in layers which contain a proportion of sand and/or open fractures where these are developed. Though groundwater resources in these rocks are often too small to be used for public water emergency supply, they might constitute a valuable domestic drinking water source in areas of water scarcity and also in emergency situations.
The most productive aquifers, aquifer systems and groundwater basins in sedimentary rocks world-wide are developed in coarse porous and fractured sandstones. Sedimentary sandstone basins often attain a thickness of several hundreds or even thousands of metres. Changes in basin size may have occurred through transgression and regression or changes in palaeographic setting affecting sedimentary conditions and are reflected in the considerable variability in a complex sedimentary lithology. Examples are the formation of lithofacies represented by sandstones of variable grain size, with layers of marlstones, siltstones, and basal conglomerates of terrestrial, marine or lacustrine origin – and in groundwater occurrence and the formation of aquifers both confined and unconfined, and aquitards. The tectonic structure of sedimentary basins controls aquifer boundary conditions and groundwater pathways. Further tectonic features are fold structures defining groundwater basins with distinctive groundwater circulation and block tectonics that limit individual groundwater structures and may juxtapose aquifers. Aquifers in sandstones containing large groundwater resources, usually exhibit appreciable dual porosity and permeability and are usually characterised by a number of laterally interconnected groundwater flows forming multi-aquifer systems. However, vertical hydraulic inter-aquifer connection commonly also exists. In young sandstones primary porosity dominates; in older sandstones the primary porosity is controlled by the degree of cementation. Cementing minerals, mainly quartz and calcite, reduce sandstone porosity and hydraulic conductivity and compaction increases with depth and sandstone age. Sandstone porosity in the range 30 – 35% and hydraulic conductivity in the order 10-4– 10-6m/s is observed in many parts of the world.
Large groundwater storage is typical for aquifers in sedimentary basins, implying lower pollution impact that becomes negligible in deep confined aquifers. Many large aquifers in sedimentary basins are shared by two or more countries and referred to as transboundary aquifers. Cooperation among countries is needed to eliminate potential water conflicts by coordinating groundwater extraction and protection based on integrated, sustainable groundwater resources policy and management. Some deep sedimentary basins contain extensive and thick aquifers with limited current replenishment and enormous stored groundwater resources (called non-renewable or fossil), largely from the past hydrological cycles. The absence of significant groundwater renewal is usually the consequence of very low rainfall in the unconfined part of the aquifer. It may also result from hydraulic inaccessibility of some confined aquifers (Foster and Loucks, 2006). In many parts of the world groundwater (both renewable and non-renewable) stored in sedimentary basins is the most important and safe source of drinking water in disaster-prone areas in emergency situations.
Carbonate rocks(limestones and dolomites) are widely developed in many parts of the world. Their primary porosity is low. Groundwater motion and storage occurs in fractures and horizontal bedding planes. Both are enlarged through rock dissolution by chemically aggressive groundwater to form karst cavities and karst landscape features and conduits with groundwater circulation. Folded carbonate rocks, particularly in anticlines, present high secondary permeability owing to their tectonic exposure and enlargement of fractures by weathering and dissolution. High infiltration rates (40% or more of the annual rainfall), rapid groundwater flow (up to several thousands of meters per day) in conduits (channels, caverns), large open fissures and openings, considerable irregularity in karstification and related yield variability are typical for karst aquifers.
Springs, often with substantial discharge, are another typical feature of karst hydrogeology. Terrestrial
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G R O U N D W A T E R R E S O U R C E S F O R E M E R G E N C Y S I T U A T I O N S - A M e t h o d o l o g i c a l G u i d esprings and submarine springs in coastal and off-shore karst regions with discharges up to hundreds of m3/s are known worldwide. Seasonal groundwater level fluctuations in karst regions are high, often in order of tens metres. This, along with highly variable spring discharge reflects both rapid infiltration of seasonal rains and rapid response of the groundwater system.
The hydraulic conductivity of karstic rocks is highly variable due to their heterogeneity and anisotropy and the intensity of fracturing and solubility of the rock mass. Hydraulic conductivity differs by orders of magnitude between unfractured (10-9–10-13 m/s) and fractured (10-1–10-6 m/s) carbonate rocks.
Generally, hydraulic conductivity decreases with increasing rock age and depth, along with porosity.
Productive aquifers are also developed in many coastal areas in young and recent poorly cemented limestones with high primary porosity. Differences in karstification between of dolomites and limestones have been noted and reflect in their hydraulic properties. Although lower well productivity is common in dolomite environments, high-yielding dolomite supplies have been developed e.g. in southern Africa.
Chalks, limestones rich in shell fossils and typically white in colour, present different hydraulic characteristics. Groundwater systems in these rocks were comprehensively studied in England with respect to diffuse nitrate groundwater pollution. Chalk aquifers exhibit considerable variations in porosity and (preferential) groundwater flow, mostly confined to fissures.
Groundwater vulnerability in carbonate rocks and particularly in karst regions is high due to an often thin and permeable soil cover, resulting in rapid infiltration of rainfall, surface streams and pollutants into the aquifer and generally low pollutant attenuation in the unsaturated zone. Groundwater in karst aquifers is not considered a safe source of water for emergency situations because of low resistance to events such as floods and storms and particularly high vulnerability to human impacts. However, deeper aquifers in carbonate rocks and karst springs used for drinking water supplies may serve as suitable emergency sources of drinking water.
Unconsolidated and incoherent sediments of Quaternary and recent age include various kinds of gravel, sand and clay, sometimes containing organic matter. They occur as alluvia; fluvial, lacustrine, marine and delta sediments; sediments of elluvial cones of inter-montane depressions; and glaciofluvial and glaciolacustrine sediments washed out from moraines. Incoherent and thick sediments are prone to compression and to subsidence of the surface, mainly if the pore pressure is lowered by groundwater level decline through pumping. Shallow water table aquifers in coarse grained sediments are characterised by high hydraulic conductivity, interconnected groundwater flow patterns, and interaction with surface water and in coastal aquifers with salt sea water.
Unconsolidated sediments in fluvial depositsof flood plains, deltas of big rivers in coastal areas and in river terraces store very large volumes of groundwater widely used for public and domestic water supplies, irrigation and other purposes. Often a hydraulic relationship between a stream and shallow aquifer is observed. Heterogeneous fluvial deposits in deltas of big rivers may be hundreds of metres thick. Productive, usually water table, aquifers with high hydraulic conductivity (in the order of 10-3– 10-4m/s) occur particularly in coarse grained porous and permeable fluvial sands and gravels with thicknesses varying from a few meters to hundreds of meters. The palaeo-morphology and geology in the development of river valleys (shifting, meandering, eroding) have to be studied to identify buried paleochannels and relicts of past river drainage networks (see chapter 4.6). Palaeo-channels filled by thick coarse-grained sands and gravels can serve as reservoirs of natural and artificially recharged water in arid and semi-arid regions and constitute a safe emergency source of drinking water in areas affected by drought.
Productive aquifers occur also in unconsolidated sediments such as glacial deposits composed of glacial tills and glaciofluvial and glaciolacustrine sediments. Glacial till is poorly sorted, unstratified and thick material extensively deposited during the Pleistocene period mainly in the northern regions
of America, Asia and Europe. Owing to their spatial heterogeneity aquifers and aquitards often alternate in the profile of the till deposits. Porosity of glacial tills lies in the range of 25–45%, hydraulic conductivity is low (10-9–10-10m/s) however, due to deposits weathering and fracturing may acquire 10-6–10-9m/s (Singhal and Gupta, 1999) . Hydraulic conductivity of fine-grained glacial till (sand and silt) and glaciolacustrine deposits is very low, in the order 10-10–10-12m/s (Freeze and Cherry, 1979).
Networks of mostly vertical fractures in fine grained glacial deposits of different origin (glacial till, glacio-lacustrine clay) facilitate hydraulic connection between groundwater flows and aquifers (Cherry, 1989). The most important aquifers occur in buried valleys eroded in the bedrock of glacial deposits and filled by glaciofluvial coarse grained sediments (sands and gravels). Such aquifers are from tens up to hundreds meters thick, up to tens of kilometres wide and drain and store large amounts of groundwater, usually of good quality. Groundwater in buried valleys of glacial origin is a valuable source of drinking water which may be used for emergency situations.
Homogenous and isotropic aeolian depositsoccur mostly as local aquifers in loess composed of fine rounded sand and silt grains, with porosity of 40–50% and hydraulic conductivity on the order of 10-5–10-7 m/s. On the other hand thick (up to 300 m) loess deposits are widely distributed in China and contain valuable groundwater resources.
Aquifers in unconsolidated sediments are mostly unconfined, often overlain by sandy soils and with a groundwater table usually close to the surface. Groundwater vulnerability of such aquifers is generally high. Particularly aquifers in fluvial deposits in flood plains are by their nature highly vulnerable to human impacts (pollution) and natural disasters, such as floods, tsunami and storms. However, aquifers in paleo-channels and buried valleys can serve as a safe source of water in emergency situations.