Dynamic equilibrium in natural aquifers

As discussed earlier, an aquifer acts as a storage tank under the ground. The water enters into ground by the process of infiltration through vadose zone. Once the water reaches the aquifer , it flows in the direction of lower hydraulic head and finally comes out of the aquifer through the discharge zones. Thus the groundwater in an aquifer is in a state of continuous movement from one place to other. The movement of water depends on the nature of the subsurface geology. Many times, the average value of amount of water that enters into the ground is equivalent to the average value of the amount of water coming out of the aquifer and often reach an equilibrium state. This state of equilibrium is known as dynamic equilibrium of a natural aquifer.

Consider a case of pumping from an isolated well. The potential surface will start depleting just after the start of the pumping and will form a cone of depression. The rate of depletion depends on the aquifer storativity. Lesser the value of storativity more will be the spread of the cone of depression. The spread of cone of depression will increase with the continuation of the pumping. With the depletion of the potential surface, more and more water will move towards the well due to the increased gradient. And finally, a state will reach where the radial inflow rate to the well is equal to the pumping rate from the well. At this state, the aquifer will reach the dynamic equilibrium state.

Functions of Aquifer

Aquifer plays an important role in overall development of water resources of a region. The primary and one of the most significant uses of aquifer is the withdrawal of water for various purposes like municipal use, agricultural use, industrial use, etc. For the people living away from the river, stream and other surface water sources, the main source of water is the groundwater withdrawal from unconfined or confined aquifers. Thus one of the main functions of an aquifer is the supply of water for various needs.

Another function of an aquifer is the transmission of water from one place to another. The water withdrawal from a confined aquifer is the water recharged into the ground at a location far away from the location of withdrawal. Thus aquifer also acts as a water transmission conduit.

The groundwater is recharged into the aquifer through the process of infiltration. The rain water falls on the ground or the water carrying through a canal or the water spread over an irrigation field percolates through the unsaturated zone above the aquifer. The unsaturated zone above the aquifer acts as a filtration layer and water is purified while seeps through the zone. Thus aquifer is also acting as a filtration plant.

Aquifer can also be used to store natural gases. One of the potential applications is the storage of CO₂ in the aquifer to reduce CO₂ emission to the atmosphere. The process of storing of CO₂ in a geological formation is known as geo-sequestration.

Groundwater can also be mined similar to the process of mining of natural minerals. In general in a large time scale, the withdrawal from aquifer is nearly equal to the average natural replenishment. However, in certain situation, we may withdraw all the available water in an aquifer just like nonrenewable resources.