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.

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).

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.

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.

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.