remaining 4% fresh liquid water is available. It will be clear from this figure that the development of worlds economy must rely on fresh water resources. The storage of fresh water on earth consists mainly of groundwater. Even if we consider only the most active groundwater storage, 4 million km instead of 60 million km, groundwat still accounts for 95%, the surface water in rivers and lakes accounts only 3. 5 of the total quantity and is most vulnerable to pollution. On the other hands, groundwater more homogeneous and stable spatial and temporal distribution and good quality. In the future, people will rely more and more on groundwater for water supply 1.2 Aquifer systems 1. 2.1 Aquifer, aquitard, and aquiclude Groundwater is stored in geological formations. According to the volume of storage and capacity of transmitting water, geological formations are distinguished relatively as aquifer aquitard and aquiclude An aquifer is defined as a saturated permeable geological unit that stores a big quantity of water and is permeable enough to yield economic quantities of water to wells. The most common aquifers are unconsolidated sand and gravel, but permeable sedimentary rocks such as sandstone and limestone and heavily fractured can also be classified as aquifers An aquitard is a geological unit that is permeable enough to transmit water in significant quantities when viewed over large areas and long periods, but its permeability is not sufficient to justify production wells being placed in it. Clays, loams and shale are typical aquitards An aquiclude is an impermeable geological unit that dose not transmit water at all. Dense unfractured igneous or metamorphic rocks are typical aquicludes. In nature, truly permeable geological units seldom occur; all of them leak to some extent, and must therefore be classified as aquitards. In practice, however, geological units can be classified as aquicludes when their permeability is several orders of magnitude lower than that of an overlying or underlying aquifer 1.2.2 Aquifer types There are three main types of aquifer: confined, unconfined and semi-confined aquifers (Figure 1. 2)remaining 4% fresh liquid water is available. It will be clear from this figure that the development of world's economy must rely on fresh water resources. The storage of fresh water on earth consists mainly of groundwater. Even if we consider only the most active groundwater storage, 4 million km3 instead of 60 million km3 , groundwater still accounts for 95%, the surface water in rivers and lakes accounts only 3.5 % of the total quantity and is most vulnerable to pollution. On the other hands, groundwater is of more homogeneous and stable spatial and temporal distribution and good quality. In the future, people will rely more and more on groundwater for water supply. 1.2 Aquifer systems 1.2.1 Aquifer, aquitard, and aquiclude Groundwater is stored in geological formations. According to the volume of storage and capacity of transmitting water, geological formations are distinguished relatively as aquifer, aquitard and aquiclude. An aquifer is defined as a saturated permeable geological unit that stores a big quantity of water and is permeable enough to yield economic quantities of water to wells. The most common aquifers are unconsolidated sand and gravel, but permeable sedimentary rocks such as sandstone and limestone, and heavily fractured or weathered volcanic and crystalline rocks can also be classified as aquifers. An aquitard is a geological unit that is permeable enough to transmit water in significant quantities when viewed over large areas and long periods, but its permeability is not sufficient to justify production wells being placed in it. Clays, loams and shale are typical aquitards. An aquiclude is an impermeable geological unit that dose not transmit water at all. Dense unfractured igneous or metamorphic rocks are typical aquicludes. In nature, truly impermeable geological units seldom occur; all of them leak to some extent, and must therefore be classified as aquitards. In practice, however, geological units can be classified as aquicludes when their permeability is several orders of magnitude lower than that of an overlying or underlying aquifer. 1.2.2 Aquifer types There are three main types of aquifer: confined, unconfined, and semi-confined aquifers (Figure 1.2). 3