Biswarup Dhar Choudhury

 ASSIGNMENT on Groundwater occurerences in different rock types : Igneous, Metamorphic and Sedimentary SUBMITTED BY Biswarup Dhar Choudhury Roll no. 04 M.Sc 3rd Semester Dept. of Earth Science INTRODUCTION The subsurface occurrence of groundwater may be divided into zones of aeration and saturation. The zone of aeration consists of interstices occupied partially by water and partially by air. In the zone of saturation all interstices are filled with water, under hydrostatic pressure. One most of the land masses of the earth, a single zone of aeration overlies a single zone of saturation and extends upward to the ground surface, as shown in FigureA. In the zone of aeration (unsaturated zone), Vadose water occurs. This general zone may be further subdivided into the soil water zone, the intermediate Vadose zone (sub-soil zone), and capillary zone (Figure A). The saturated zone extends from the upper surface of saturation down to underlying impermeable rock. In the absence of overlying impermeable strata, the water table, or phreatic surface, forms the upper surface of the zone of saturation. This is defined as the surface of atmospheric pressure and appears as the level at which water stands in a well penetrating the aquifer. Actually, saturation extends slightly above the water table due to capillary attraction; however, water is held here at less than atmospheric pressure. Water occurring in the zone of saturation is commonly referred to simply as groundwater, but the term phreatic water is also employed. FigureA . A schematic cross-section showing the typical distribution of subsurface waters in a simple “unconfined” aquifer setting, highlighting the three common subdivisions of the unsaturated zone and the saturated zone below the water table. Factors that determine the value of structure or rock type as a potential aquifer Certain major factors that determine a structure or a rock type as a potential aquifer include the following: Climatic conditions of temperature, rainfall intensity and distribution (depending on the locality). Surface conditions, e.g., relief, soil, vegetation Geologic history, e.g., the climatic condition in the past. Structural geology, e.g., folding, taunting, uplifting arc. GEOLOGICAL FORMATIONS AS AQUIFERS Aquifers in Metamorphic and Intrusive Rocks a) Groundwater associated with metamorphic rocks The formation of metamorphic rocks is as a result of intense pressures and high temperatures to sedimentary rocks or igneous rocks. Whatever the origin of the meatamorphic rock is, its mass is made up of solid and dense material, which may be intensively folded. Jointing patterns can be present in these rocks and, in places, the rock may be fractured as well. Porosities of the crystalline metamorphic rocks are in most places very low. They vary from 0 to 0.1. For jointed and fractured formations, common ranges are from 0.01 to 0.03. Less jointed and solid metamorphic formations are also very low and may be near to zero. Weathering in metamorphic rocks can be considerable. In particular, for the fine-grained rocks such as slates and schists, weathering may extend to a depth of over 100 metres in fresh bedrock. Weathering in quartzites and gneisses is much less pronounced or is not present at all. The effect of weathering is that joints and fractures are enlarged and that the minerals that make up the bedrock are (partly) transformed into gravels, sands, clays and laterites. These processes tend to increase the porosity of the rock. As a result of the larger porosity of the weathered rock and the formation of connections between joints and fractures, the permeability of the weathered zone also tends to be higher than in fresh rock. Wells located in deeply weathered zones with sufficient recharge may also be successful. Well yields of up to 300 m3/day have been reported. The metamorphic rocks are generally considered as aquifuges. The overall permeability is usually low and the yield of wells tapping these formations may not be sufficient for exploitation. Where metamorphic rock is well jointed and fractured or is weathered the formation may be classified as a local aquifer. Fig. Folded and fractured metamorphic rock of Precambrian age. While the rock blocks are of low permeability and porosity, the factures have a high permeability and low porosity that facilitates the movement of contaminants over long distances. A folded metamorphic rock is shown in Figure above. It is noteworthy that although the matrix of this rock is of very low porosity and permeability, there are numerous fracture planes associated with the rock fold. The fracture planes provide conduits for rapid movement of water, although the storage capacity of the overall rock mass is small. The consequence of this is that in rocks of this kind contaminants can move large distances rapidly. b) Groundwater associated with intrusive rock   The intrusive rocks belong to the group of the igneous rocks. In intrusive rocks, the fluid magma hardened before it reached the land surface. This resulted in the formation of dense crystalline rock in which the individual mineral grains can usually be observed. In most intrusive rocks, crystallisation of the minerals has not left any open spaces between the minerals. However, openings may be present at joints or at fractures (secondary porosity). These may have formed in the contact zone with the ‘country rock' into which the intrusive rock formed. They may also have formed at the intercalated boundaries where rocks have crystallised in different periods making the contact. Last but not least, joints and fractures may have formed at the later stage due to intense lateral pressure or the development of faults. Common intrusive rocks are granites, gabbro and diorite. Porosities, permeability and well yields for intrusive rocks are much the same as for metamorphic rocks. Although we usually assume that porosity in these rocks is associated with joints and fractures we also have to realise that these may be closed as a result of cementation. Permeability and well yields in intrusive rocks are generally small. Acceptable yields will be struck in wells in the weathered part of the rock or wells penetrating open joints or tapping from well developed fault zones. In metamorphic and intrusive rocks, porosity and permeability tend to decrease with depth. This phenomenon can be explained by the decrease in weathering with increasing depth, and the tendency of joints and faults to close as a result of the weight of the overburden. Granitic Rocks Potential ground water accumulation are restricted to basin where three fractures sets are developed, interconnected and numerous at shallow depth but with increasing depth they  more widely spaced and less and less open. The small basin within granitic rocks requires a closure at the downstream.  The residual soil is sandy-salty, becoming more clayey downwards.  Under humid conditions, the residual soil shows a plastic subsoil, low percolating rates. Under arid conditions, the soil will be thinner and courser and groundwater supply may be favourable where fractures are intersecting. In summary, intersecting fractures and joints in granitic rocks remain a major factor that induces percolation and accumulation of water forming potential aquifers. In figure 1, several types of groundwater zones are schematically illustrated. These zones indicate areas with different hydrogeological characteristics and which require different approaches with respect to groundwater exploration within the metamorphic and intrusive rocks. Zone 1 comprises most of the hills, where the metamorphic rock is shallow, or where the rocks outcrop. The steep topography and the lack of a weathered zone that can retain the rainwater limits the recharge into the potential groundwater bearing zone. Therefore, the groundwater potential in this part of the zone is seasonal at best. This zone marked by borehole C. At the foot of the outcrop, the runoff water infiltrates into the colluvium. In this part of the zone, seasonal groundwater may be found. In Zone 2 the weathered zone has considerable thickness and may form reasonable aquifers. Either moderately productive boreholes or seasonal wells may be developed in these aquifers. This zone is marked by boreholes G and H. Zone 3 comprises an area with faults and fractures. These features enhance storage and provide more surface area for weathering. This zone is marked by boreholes B and D. Fig. Several types of groundwater zones. Development of Aquifers in Volcanic Rocks  a) Groundwater in lava flows These are igneous rocks that originated from magma that has erupted at earth's surface. The magma flowing over and crystallising at the land's surface is called lava. In many places, the lava flows are manifested as a sequence of horizontal or sub-horizontal layers, which may be several hundred metres thick. The upper part of the individual layers may be weathered and even old land soil profile may be visible. Also, the individual layers or flows are, in many places, separated by alluvial deposits or other materials. Common rock types are rhyolites and basalts. The porosities of lava formation may vary considerably. In some formations, the rock is extremely dense with porosities far less than 0.01. In the lava formations the zones where these rocks are porous consist of horizons with gas holes or vesicles, lava tubes, parts with pronounced joints or fracturing and it includes the weathered zones. Also alluvial sands and gravels or other sediments that are deposited in between the basalt layers have high porosity.  Permeability of lavas covers a wide range. It can range from 10-3 to 103 m/day. Due to the uneven distribution of the openings, permeability may vary significantly within lava flows themselves: the coefficient of permeability may range from almost zero to over 1000 m/day. As a result of the mainly horizontal distribution of the openings, permeability is strongly anistropic. This means that, for a sequence of lava layers, permeability in the vertical direction is much smaller than in the horizontal direction. Wells tapping these permeable zones consisting of vesicles, weathered basalts, or interbedded alluvium will be successful if sufficient recharge and storage capacity is available in the rock. b) Occurrence of groundwater in volcanic rocks  There is more to volcanic rocks than only lava flows. Part of the magma that erupts at the earth's surface is thrown into the air. Clouds of volcanic material may contain volcanic bombs, ash and even smaller sizes of particles, which are subsequently ‘deposited' on the slopes of the volcanoes and areas beyond them. These deposits are usually referred to as pyroclasts. Special forms of pyroclasts are, for example, tuffs, pumice and volcanic ash beds. Pyroclatic deposits may vary significantly in terms of porosity and permeability. Unwelded tuffs usually consist of loose angular and unsorted fragments. Porosities of these tuffs may be moderate and even high but as a result of the poor sorting permeabilities are usually low. In welded tuffs, the fragments are molten together and the porosities and permeability are usually lower than in the unwelded tuffs. Pumice and volcanic ash usually have porosities and permeability which are higher than in the tuffs; in pumice, a very light rock, the porosity may be as high as 0.85. In volcanic deposits, we may have sequences of lava flows, sequences of lava flows interbedded with alluvial or pyroclastic deposits, or there may be just thick deposits of pyroclasts. A thick sequence of lava flows or interbedded lava may consist of an alteration of aquiferous layers and aquifuges: the aquifers being subhorizontal zones with vesicles, the weathered profiles, fractured and jointed zones or the interbedded series of alluvial material or pyrolastic pumice and volcanic ash beds. The aquifuges are the mainly sub-horizontal layers of dense lava rock which may also have columnar appearance. Tuffs interbedded in these volcanic aquifer systems may play a dual role: in most cases they act as aquitards or even aquicludes. In other places, for example where they are sandwiched between impermeable columnar basalts and tuffs, they may take role as aquifer. The alteration of the aquifers, aquifuges, and aquitards in volcanic aquifer system also has consequence for groundwater development.  In many places, the impermeable aquifuges ‘force' water to discharge out of the system. Springs will then emerge at the land surface. Aquifers in Sedimentary Rocks  a) Groundwater occurrence in fine grained sediments  When we inspect consolidated fine-grained sediments under the microscope we will note that the openings around the grains are significant. Open spaces in these rocks are rather found at joints, contact between the individual beds, and at zones of fracturing. Values for secondary porosity in fine-grained shales are in the order of 0.01 to 0.25. Values for the coefficient of permeability of many fine-grained sediments are usually less than 10-4 m/day. Porosities and permeability of fine-grained sediments also decrease with an increase in the weight of the overburden on the sediments, which may cause compaction. Nevertheless, where the sediments are jointed and fractured or openings exist between the beds, the permeability may be high enough to supply small amounts of water to the wells.   b) Coarse-grained sediments and groundwater   Through the action of pressure and temperature, coarse sands and gravel turn into sandstones and conglomerates. Sandstones, referred to as consolidated sedimentary rocks, are composed of angular or rounded quartz grains, and other minerals including feldspars or micas. In sandstones, the pores around the grains may be open (primary porosity) or cemented to a certain degree. The cement may be in the form of calcite, silica or clay. In general, sandstones have porosities similar to those of fine-grained shales and siltstones. However, the permeability of coarse-grained rocks is usually one or two magnitudes higher than that for shales and siltstones. Success rates of drilling productive boreholes in sandstones, where secondary porosity is dominant in the form of openings at joints, fractured zones and bedding planes are much smaller. In these formations, more extensive investigations are needed to locate sites for wells with sufficient yields. Fig1. Secondary permeability observed in limestone in western Kentucky. Secondary permeability is illustrated in Figure1, where it is evident that water percolating along fractures in the consolidated sedimentary rock has dissolved the rock and produced enormous secondary permeability. In Figure2 we see how secondary permeability facilitates the transmission of large quantities of water significant distances in relatively short times. Because flow due to secondary permeability tends to be directional in nature, there is often significant anisotropy, that is, preferential flow direction. The importance of secondary permeability also gives rise to the concept of a double porosity conceptual model of the aquifer system. In such a conceptual model there are two sets of permeabilities and two sets of porosities, one associated with the secondary permeability features, such as the fractures, and the other associated with the primary permeability features, namely, the host rock. Fig2. Spring emanating from secondary permeability in a limestone formation in Kentucky Carbonate rocks The value of these rocks as potential groundwater conduits and reservoirs depend on secondary pore space for initial solution of the rocks.  The carbonate rock reacts with acidic solution creating the reservoir spaces that enhance both storage and transmission of groundwater. Fractures and solution openings are more abundant along crests of anticlines and syncline troughs than on the flanks.  Therefore, secondary folding plays a big role in creating necessary conditions for aquifer formation in these rocks. Intersecting fractures enlarge openings thus increasing the possibility of cave forming and, as a surface indication, may increase the probability of successful drilling. Basal conglomerates Basal conglomerates have quartzitic features and are quite impervious when fresh.  These are pebbles of varying sizes and some of them as big as an inch cube, are all completely cemented together.  The thickness of the basal conglomerates is not frequently more than 2 to 3 feet.  Due to its mode of bedding, it has a potential of being a good aquifer after some kind of secondary disturbance, i.e., graded bedding takes place resulting in course grains being settled first followed by thin fine grains which, after erosion, any water in it settles in lower parts of the basin. The conglomerates are susceptible to weakening and eventually the conglomerates may weaken and be disintegrated into sandy mass with its original fragments separated.  This type of condition makes this formation highly potential aquifer. Alluvial Deposits Probably 90% of all developed aquifers consist of unconsolidated rocks, chiefly, gravel and sand. These aquifers may be divided into four categories based on the manner of occurrence: water courses, abandoned or buried valleys, plains and intermontane valleys. Aquifers developed along watercourses, i.e., river valleys consists of the alluvium that forms and underlies stream channels, as well as forming adjacent to flood plains. Wells located in highly permeable strata bordering streams produce large quantities of water, as infiltration from the streams augments groundwater supplies. Abandoned or buried valleys are valleys that are not any longer occupied by streams that formed them. Although such valleys may resemble watercourses in permeability and quantity of groundwater storage, their recharge and perennial yield are usually less. Extensive plains underlain by unconsolidated sediments are important aquifers. They are normally formed of sands and gravels. These plains flank highlands or other features that served as the source of the sedimentary deposits. The aquifers are recharged chiefly in areas accessible to downward percolation of water from precipitation and from occasional streams. Intermontane valleys are underlain by tremendous volumes of unconsolidated rock materials derived by erosion of bordering mountain. The sand and gravel beds of these aquifers produce large quantities of water, most of which is replenished by seepage from streams into alluvial fans mouths of mountain canyons. REFERENCE http://ecoursesonline.iasri.res.in/mod/page/view.php?id=1812 Google – occurrence of groundwater Subsurface hydrology by GEORGE F. pinder and MICHAEL A. celia