Applied Hydrogeology of Fractured Rocks

More than half of the surface area of the continents is covered with hard rocks of low permeability. These rocks may acquire moderate to good permeability on account of fracturing and hence are broadly grouped under the term fractured rocks, in the context of hydrogeology.

From the hydrogeological point of view, fractures and discontinuities are amongst the most important of geological structures. Most rocks possess fractures and other discontinuities (Fig. 2.1) which facilitate storage and movement of fluids through them. On the other hand, some discontinuities, e.g. faults and dykes may also act as barriers to water flow. Porosity, permeability and groundwater flow characteristics of fractured rocks, particularly their quantitative aspects, are rather poorly understood. Main flow paths in fractured rocks are along joints, fractures, shear zones, faults and other discontinuities.

The purpose of most hydrogeological investigations is to locate potential sites for development of adequate quantity of reasonably good quality groundwater for a particular use: domestic, irrigation or industrial etc. The quantity and quality criteria would depend on the local needs and the socio-economic conditions of the people. Alternative possible sources of water supply, e.g. import of surface water or groundwater from adjoining areas may also be considered. Further, hydrogeological investigations in fractured rocks are also taken up for the selection of suitable sites for waste disposal including radioactive waste, tapping of geothermal power and in several other geotechnical problems, viz. tunnelling, mining, hill slope stability etc. These include geological, geomorphological, geohydrological studies given in this chapter. Besides, remote sensing and geophysical studies are described in Chaps. 4 and 5, hydraulic properties and methods of aquifer characterisation are discussed in Chaps. 8–9, and tracer techniques are described in Chap. 10.

Remote sensing, encompassing the study of satellite data and aerial photographs, is an extremely powerful technique for earth resources exploration, mapping and management. It involves measurements of electromagnetic (EM) radiation in the wavelength range of about 0.4 µm–1 m, from sensors flying on aerial or space platforms to characterize and infer properties of the terrain. Remote sensing has evolved primarily from the methods of aerial photography and photo-interpretation used extensively in 1950s–1960s. The technique has grown rapidly during the last four to five decades. In the context of groundwater studies, remote sensing is of great value as a very first reconnaissance tool, the usual sequence of investigations being: satellite images—aerial photographs—geophysical survey—drilling.

Following the investigations by aerial photographic and satellite remote sensing techniques, geophysical survey is carried out to ascertain the subsurface geological and hydrogeological conditions and aquifer characteristics. Various petro-physical properties utilized in geophysical exploration include electrical resistivity, electrical conductivity, density and elasticity (influencing seismic velocity), electrical permittivity (dielectricity), magnetic susceptibility, and radioactivity. Geophysical methods have the potential to predict distribution and flow of groundwater including sites of hazardous substances in a cost-effective manner.

Geographical Information System, also called Geobased Information System (GIS), is a relatively new technology. It is a very powerful tool for processing, analyzing and integrating spatial data sets (e.g. Star and Estes 1990; Lo and Yeung 2006; Chang 2008; Harvey 2008). A GIS deals with information on locational patterns of features and their attributes (characteristics). It can be considered as a higher-order computer-coded map which permits storage, selective dedicated manipulation, display and output of spatial information. GIS software provides the functions and tools needed to store, analyze, and display information about geographical locations. The key components of GIS software include: (a) a database management system (DBMS); (b) tools that create intelligent digital maps that one can analyze, query for more information, or print for presentation; and (c) an easy-to-use graphical user interface (GUI).

Although mechanism of groundwater flow through porous media is well understood but many uncertainties exist about the flow of water and transport of solutes in fractured rocks. This has acquired greater interest as low permeability formations form potential repositories for high level waste.

Hydraulic properties of water bearing formations are important as they govern their groundwater storage and transmitting characteristics. These are described below.

Hydraulic properties of rock materials can be estimated by several techniques in the laboratory and in the field. The values obtained in the laboratory are not truly representative of the formation. However, the advantage of laboratory methods is that they are much less expensive and less time consuming.

Tracers are defined as chemical substances (inorganic or organic molecules, including isotopes), present naturally or introduced in the environment. A variety of tracers are used in geohydrological investigations for estimating the rate and direction of groundwater movement, groundwater recharge and its residence time. Tracers are also used for the study of origin of groundwaters including saline and geothermal waters, contaminant transport including site characterisation of repositories for nuclear waste, interconnection between surface water and groundwater, stream discharge measurement and sediment transport. There are reports of the use of tracers viz. chloride, fluorescein and bacteria in karst aquifers in Europe even in the late 1800s and early 1900s.

The quality of water is as important as its available quantity. Rain and snow are the purest form of water which undergo many complex chemical changes after coming in contact with soil and other rock materials. Man’s activities also have a considerable influence on water quality.

The term contamination is used for addition of any solute into the hydrological system as a result of man’s activity while the term pollution is restricted to a situation when the contamination attains levels that are considered to be objectionable (Freeze and Cherry 1979). There could also be deterioration in water quality due to natural reasons namely dissolution of rock material. Contamination of groundwater can take place from either a wider source like percolation from agricultural fields on account of application of fertilizers and pesticides or from a point source like waste disposal sites. Atmospheric composition will also affect the composition of precipitation and thereby causes water pollution. In a polluted atmosphere, many oxidizing compounds (e.g. ozone), acid-forming gases (oxides of nitrogen and sulfur) and particulate material will be higher than in an unpolluted atmosphere. This will be area specific depending on the anthropogenic activities. For example, combustion of fossil fuels results in acid rains containing H2SO4 and HNO3, which reduces the pH of source water. This induces greater solubility of aquifer material (see effect of Climate Change in Sect. 20.10). Overexploitation of groundwater, especially in coastal areas is also responsible for contaminating fresh water aquifers due to sea-water intrusion. This aspect is discussed in Sect. 20.7.

Crystalline rocks include plutonic igneous rocks (granites, diorites, etc.) and metamorphic rocks (gneisses, granulites, quartzites, marbles, schists and phyllites, etc.). The plutonic igneous rocks, viz. granites, usually occur as large size intrusive bodies (plutons) while some other rocks, viz. dolerites and pegmatites, are of comparatively small size in the form of dykes and veins. The hydrogeological characters of dykes are described in Chap. 14 as they are more commonly found in volcanic rock terrains. Like other hard rocks, the crystalline rocks are characterized by negligible primary porosity and permeability. However, weathering and fracturing can impart significant secondary porosity and permeability which is highly variable.

Volcanic rocks are formed by the solidification of magma at or near the ground surface. The most common volcanic rocks are basalts, which are of basic composition. Acidic and intermediate rock types such as rhyolites and andesites have comparatively very limited occurrences. Basalts are formed due to the eruption of lava either on the ground surface (subaerial eruption) or on the sea floor (submarine eruption). Eruption on the land surface could be of fissure type (plateau basalts), covering large areas on the continents or of central type, which is of limited distribution mostly forming volcanic cones (Fig. 14.1).

Carbonate rocks are sedimentary type of rocks containing more than 50% carbonate minerals which are mainly calcite, CaCO₃, and dolomite, CaMg(CO₃)₂. The carbonate minerals could be a result of chemical precipitation, organic processes or may occur as detrital material. Some of the dolomites may be a result of diagenesis known as dolomitization which involves replacement of calcite by dolomite to a varying extent. The term limestone is used for those rocks which contain more than 90% carbonates. If the rock contains more than 50% but less than 90% carbonates, it is termed as arenaceous limestone or an argillaceous limestone, depending upon the relative amounts of quartz and clay minerals. Chalk is a type of limestone which is soft and white in colour and is rich in shell fragments. The carbonate rocks occupy about 10% of the Earth’ surface and supply nearly a quarter of the world’s population with water. Locally, their cumulative thickness may be 10 000 m (UNESCO 1984b). In the geological past, most limestones and dolomites were deposited during the intermediate phases of Caledonian, Hercynian and Alpine tectonic cycles and their ages are mainly Palaeozoic and Mesozoic. One of the largest carbonate aquifers in the world is the Floridan aquifer system of Palaeocene to Miocene age in the southeast United States consisting of gently dipping thick sequences of carbonate sediments separated by less permeable clastic sediments.

Under clastic formations, we have included both unconsolidated and consolidated sediments. Unconsolidated sediments include various admixtures of boulder, sand, silt and clay deposits. These on consolidation form clastic sedimentary rocks, e.g. sandstone, siltstone and shale.

Water-wells are vertical shafts or holes used by mankind since time immemorial for obtaining groundwater for drinking purposes and other uses. The type of well to be constructed depends on the nature of the geological formation, depth to water-table, yield requirement and economic consideration. Sometimes horizontal or sub-horizontal wells or galleries are also constructed under favourable geohydrological conditions to tap subsurface water. Wells serve other purposes also, such as subsurface exploration, artificial recharge and waste disposal.

Most geothermal reservoirs and hot dry rock (HDR) systems are located in fractured low porosity geological formations. In such settings, fractures provide conduits for fluid flow through such rocks. Therefore, fractures either natural or those artificially created are important for the successful operation of geothermal reservoirs and HDR systems.

Groundwater modeling has become a very useful and established tool for studying problems of groundwater quality and management. Modeling techniques have been used as an aid for: (a) prediction of the effect of groundwater stresses (groundwater recharge and discharge are treated as stresses), (b) groundwater management for sustainable exploitation of groundwater, and (c) to estimate effect of contaminant injection and transport in space and time.

Assessment and management of groundwater resources is important for their optimum utilization and to avoid any adverse effects. First of all, it is necessary to have a prior knowledge of the distribution as well as hydraulic and geochemical characteristics of aquifers as discussed in earlier chapters. It is also necessary to know the amount of natural recharge and groundwater abstraction. Management of groundwater resources is simple when natural recharge is more than the abstraction. However, when the aquifer approaches full development, the management problems increase significantly. For example, in the past half a century, there has been an explosion in groundwater development for irrigation, domestic and industrial uses, particularly in India and China. Abstraction of water more than natural recharge leads to decline in groundwater levels, which causes uneconomic pumping lifts, land subsidence, deterioration of water quality, sea water encroachment in coastal aquifers and so on. In areas where an aquifer is hydraulically connected with the river, lowering of water table will reduce the groundwater discharge into the stream and may result in induced recharge from the river into the aquifer. On the other hand, extensive use of surface water for irrigation may cause water logging and salinization of water and soil. These problems may arise both in granular rock formations and fractured rocks. Groundwater modelling is a useful tool to study various problems related to effect of various stresses on groundwater regime (Chap. 19). The various technical aspects of groundwater assessment and management are outlined in this chapter. Most of the examples cited are from granular formations as such studies in fractured rocks are limited but the general approach is the same.