Understanding Ground Water

What is ground water?

Any water that is below the ground surface is ground water. Unlike water in rivers and reservoirs, we cannot see this water but it a very important source of water for drinking, irrigation, industries and the environment.

The water that fills the pore spaces in rocks (fractures and other such openings of rocks and other geological materials), sediment, and soil deep beneath the surface is called ground water. In other words, ground water is contained in any pore spaces underneath the surface of the ground.

The water drawn from the ground water is derived from rainfall and infiltration within the normal water cycle.

Aquifer

Aquifers are geological formations composed of permeable sand and gravel (in alluvial systems) or fractured rocks (in hard rock systems) that are capable of storing water and allowing it to flow, in sufficient quantities, to wells and springs. Aquifers perform two functions- firstly that of “storing” water and secondly that of transmitting it from one location to another.

The transmissivity of an aquifer (that is how easily water flows through the aquifer) depends on the pore size and connectivity of pores or fractures.

Water table

The water table is the level at which the ground water is found. If you dig a well in the ground, and it reaches an aquifer, the well will fill up with water to a certain depth or level which is known as the water table. Knowing the depth of the water table is very important – periodic measurements can indicate whether the depth of water table is falling or rising.

In a year of a good monsoon, the water table depth is expected to rise and it will fall when there is drought. In India, the water table year after year is going down (ground water getting deeper), meaning the current amount of pumping cannot be sustained. For sustainable supplies, less water needs to be pumped out and ground water recharge needs to be increased.

An explanation of water table and wells

When water is withdrawn from a well, the level of water in the well gets lower, and water flows from the aquifer into the well. This causes the water level to drop in the aquifer with the biggest fall being at the edge of the well, and progressively smaller falls further from the well. This new shape of the water table is called a drawdown cone that is centred on the pumping well.

When a drawdown cone from one well impinges on another well, it will reduce the amount of water that can be pumped from the second well, and if the water table is lowered below the base of the second well that well will run dry, regardless of whether water is pumped out or not. The owner of a new or a deeper tube well, assuming it is not a dry well, will get a new supply of water, and in so doing will create a drawdown cone that can influence how much water, if any, can be pumped from the adjoining dug wells.

The influence of pumping will spread with time. Water that would have been recovered from dug wells now goes to the tube well instead. The same or smaller total volume of water can be recovered by the village, but instead of it coming from all dug wells it will come from a smaller number of tube wells. 

When there are multiple tube wells in a village, the drawdown cone of one tube well can impinge on another. Because most tube wells are only cased near the surface, water can run down the tube well from upper water bearing layers to lower layers due to the absence of an extensive confining layer between them.

The only circumstances where building a new tube well will produce more water for the village is when there is no connection between upper and lower aquifer layers. 

How important is ground water in India?

India is the largest user of ground water in the world. It uses an estimated 248 cubic kilometres of ground water per year - over a quarter of the global total.

Ground water is a major source of irrigation and industrial uses in India. In India, about 75% of irrigation water comes from ground water sources. About 85% of rural water demand and about 50% of urban water demand in India is met by ground water.

The demand for ground water is even greater in times of drought. Ground water accounts for one-third of the total water used by humans worldwide. Ground water is also important for the environment. Ground water helps water flow in streams and rivers during dry periods and maintains wetlands and lakes.

Many people in cities nowadays also rely on ground water for their daily needs due to inadequate supplies of water from traditional water supply sources. Farmers in India, often rely on ground water to provide water for their crops when rains do not fall during the cropping season.

Ground water is commonly used as a source of drinking water supplies in India. It is often a more convenient source of water, and it is often considered less vulnerable to pollution than surface water. However, once polluted, ground water is more difficult to clean up than rivers and lakes. Increasingly treated surface water is being supplied to people in India.

Overexploitation of ground water in India

Ground water, although a huge resource, is not infinite. Its overexploitation in many parts of India is leading to a decline in water table and even the drying up of shallow wells. Therefore, many people do not have enough ground water for drinking purposes and for irrigating their crops.

Water levels in more than half of the all wells in India are becoming deeper year after year. Some farmers are resorting to tube wells to access water from deeper aquifers. Generally, it is not possible or economical for a small farmer to extract deeper ground water. Due to widespread pumping through tube wells, these aquifers are also being exhausted now. Further, the quality of ground water from deeper aquifers is sometimes poor due to the presence of salts, heavy metals and other substances that are harmful to people, crops and soils.

As mentioned earlier, ground water in India is a critical resource. An increasing number of cities, towns and villages across the country are running out of ground water supplies and the ground water has become unsustainable. Punjab, Haryana, Gujarat, Rajasthan, Tamil Nadu are some of the states that are experiencing the greatest decline in ground water levels. If we do not take suitable action now (see module 10), it will seriously limit drinking and irrigation water supplies leading to serious social, economic and environmental consequences.

Situation in Gujarat and Rajasthan

The map on the left shows that the ground water sources in many states of India, including Rajasthan and Gujarat, are under stress. It is clear from the map that ground water in the country is over-used. The Government of India have now categorised many assessment Units (Block, Mandal, Taluka, Watershed or District) throughout India as over-exploited, critical and semi-critical to ensure that appropriate measures are taken to improve the situation.

Why ground water levels are dropping?

• Over exploitation – too many users and too many wells.
• Limited regulation – permission required for digging a well only for industries, infrastructure units and mining projects as of now - no restrictions on ground water use for agriculture.
• Insufficient ground water recharge from rainfall due vagaries of monsoon and largescale urbanisation.
• Other reasons such as cultivation of high water consuming crops, inefficient irrigation practices etc.

What may happen if the ground water level keeps falling?

• Shallow wells become dry during dry season.
• Not enough water for growing crops and therefore people’s livelihood is affected.
• Not enough water for drinking and other uses.
• People have to fetch water from long distances, leaving little time for other work and this can affect income.
• Water quality may become poorer and people’s health may be affected.
• As the water table goes deeper, power costs of pumping water will increase considerably.

Why ground water is becoming saline?

• Not enough recharge is occurring.
• When the water level in wells goes down in the coastal areas, seawater may intrude and make the ground water saline.
• Shallow wells have water, which is a recharge from rain in recent years. But deeper ground water is older water that is sometimes saline.

What can we do to fix the problem?

There are two basic steps involving the local communities with help from government and non-government organisations: (i) Enhance ground water recharge (ii) Manage ground water extraction.

Communities can help replenish the ground water resources by allowing greater infiltration of rainwater into the ground and by creating ponds and other water storage structures on land surfaces. Such actions can increase natural recharge significantly. At the same time, we can reduce the demand on the ground water by choosing crops that require less water, improve irrigation efficiency as well as recycling and reusing water. 

Source : Jaldoot Resource Book