Q1. We get so much water from underground rocks. Are there natural streams flowing underground?

Not really! Groundwater moves through porous rock formations similar to the way water flows through a sponge with inter-connected pores. In nature, no space remains empty. Therefore, the pore space within the underground rock formations, no matter how small, remains filled either with air or water (sometimes oil and gas in deeper formations). Given a continuous supply, water enters a porous rock formation replacing the air and gradually saturates all the pore spaces. As the process continues, excess water tends to move through the saturated formation under gravitational force. Water can even seep through poorly cemented house walls and concrete basements, particularly during rainy season.

Therefore, when a storage space such as a dug well is constructed within a saturated rock formation, a part of the water from within the formation flows out as free water (specific yield) and gradually accumulates to fill in the well till it reaches to a level equal to the water level in the formation. Water also flows through joints, fractures and contact zones between two hard rock formations. Sometimes, one can even see water flowing out through a fracture or a contact zone in a dug well constructed in hard rock formation tapping such zones giving credence to the erroneous notion that groundwater moves underground as a subterranean stream.

Carbonate rocks are also known to develop due to the action of water, large sized cavities and inter-connected solution channels which contain and transmit large quantities of groundwater. Sometimes, in coal mines, groundwater accumulated in large quantity in spaces created due to removal of coal over the years can cause accidents thus creating the effect of a flowing underground river. Coal mines can also get flooded due to direct inflow of water from surface water bodies.  

Q2. Which rock formations are good for transmitting groundwater?

From the hydrogeology point of view, rock formations are categorized conveniently as unconsolidated (loose), consolidated (hard) and semi-consolidated. Recent and older alluviums are unconsolidated sedimentary formations, which occur usually as alternate beds of sand and clay (or shale) with varying thickness and proportion. Sand formation is a natural carrier of water; coarser the grain size and lesser the compaction of the sand, better the water content and its flow. Clay and shale on the other hand being impervious are natural barriers to groundwater flow.

Water is also unable to pass through compact rocks like granite, basalt, quartzite etc., which are usually devoid of any primary porosity. However, in course of time the top portions of these hard rocks when exposed to extensive weathering can develop numerous fractures and get weathered into loose formation with granular consistency. Such weathered and fractured portions can transmit groundwater commensurate to the degree of secondary porosity developed due to weathering. 

Although, formations like sandstone are compact in nature, they are prone to quick weathering and develop extensive joints and fractures and turn out to be good aquifers. Similarly, limestone is prone to develop cavities and solution channels. Both sandstone and limestone yield moderate to good quantity of groundwater and may be referred to as semi-consolidated formations. Read More

(Source: Dr M K Maitra)