The properties of rocks which define water bearing capacity is porosity and permeability. To yield water easily a ground water bearing formation should not only hold water in sufficiently large quality, but also be capable to yield this water. Rocks have large variety of water bearing properties of different rocks are stated below:
1. Metamorphic Rocks:
Metamorphic rocks becomes good bearing formations when it is, to some external, weathered resulting considerable increment in their porosity and pore size. Gneiss and quartzite are very massive in nature and almost impervious. But presence of interconnected joints makes them previous and water retaining having a good yield.
Slates results good water bearing formations as they retain large amount of water along bedding planes and cleavage planes.
2. Sedimentary rocks:
The sedimentary rocks have the best water bearing formations as their porosity and permeability are yielding and retaining friendly.
The arrangement of sedimentary rocks in successive order of decreasing retaining and yielding capacity are as below:
The shales are porous but they are impervious in nature. The pore soak water by capillary action, but the pores are so minute that any yield is not possible.
3. Igneous Rocks:
Most of the igneous rocks have no pore resulting impermeability in nature. Only when interconnected joints and fissures present in such rocks circulation of water are generated providing a quick yield of water. Some times interconnected cavities may remain in some basalts and other volcanic rocks, which may retain large quantities of water.
Thus, in ground water exploration work, weathered metamorphic rocks are more promising than the fresh ones. In case of sedimentary rock loose and coarse formation are most promising ones. In massive igneous rock formations their joint patterns defines the accumulation and yielding of water.