Groundwater banking is a water management mechanism designed to increase water supply reliability. Groundwater can be created by using dewatered aquifer space to store water during the years when there is abundant rainfall. It can then be pumped and used during years that do not have a surplus of water. People can manage the use of groundwater to benefit society through the purchasing and selling of these groundwater rights. The surface water should be used first, and then the groundwater will be used when there is not enough surface water to meet demand. The groundwater will reduce the risk of relying on surface water and will maximize expected income. There are regulatory storage-type aquifer recovery and storage systems which when water is injected into it gives the right to withdraw the water later on. Groundwater banking has been implemented into semi-arid and arid southwestern United States because this is where there is the most need for extra water. The overall goal is to transfer water from low-value to high-value uses by bringing buyers and sellers together.
Groundwater storage concepts The bank is an aquifer used as an underground storage tank, and the recharge of water causes an increase in the volume of water stored in the aquifer to have increasing water levels. In the case of a withdrawal there would be a decrease in water levels. The amount of water depends also on a couple of other factors including groundwater pumping by other users, leakage, and natural recharge. The recharge of water by land application or injection increases the volume of water, and then some of the water will be used at a future time. It can be looked at as inputs of water minus outputs is equal to the change in water storage. Another aspect is the hydrology which is the difference between dynamic and static response to recharge and abstraction. The water levels will rise or fall in the well during recharge and recovery. Once recharge and recovery stops the water levels return to background levels, and one of the main issues is the change in static water levels after the dynamic response from recharge or recovery disappears. There can be some technical issues with the aquifer response to manage recharge and recovery. If the aquifer is hydraulically connected to body of water on the surface there are increases in the water table elevation as the result of managed recharge. This could increase the rate of discharge or decrease the amount of induced recharge, and both of these cause water to leave the basin. The recharge and recovery could also affect the lateral and vertical groundwater flow into the aquifer. There is not always a one-to-one correspondence between the volume of water and the change in storage.
Methods Groundwater banking is accomplished in two ways: through in-lieu and direct recharge. In-lieu recharge is storing water by utilizing surface water "in-lieu" of pumping groundwater, thereby storing an equal amount in the groundwater basin. In-lieu recharge is the renewable surface water used to irrigate the farmland in place of using regular groundwater. This is helping to save more groundwater because the water stays in the aquifer to be used later. Direct recharge is storing water by allowing it to percolate directly to storage in the groundwater basin. With direct recharge it floods an area so that water seeps through the ground to get to the aquifers. The water is then pumped out when there is more of a demand with the use of recovery wells.
Pros and cons There are some disadvantages to retrieving this groundwater. As groundwater is withdrawn from below the surface, the ground above settles. This settling of land, known as subsidence, can fracture roads and building foundations and can burst water, sewer, and gas lines. This method hasn't been well tested yet, so there could be some negative impacts on the environment. Water is a public resource and this could make water become a private industry. Groundwater banking can be compared and contrasted to the use of surface reservoirs. Groundwater banking has many advantages over the use of surface reservoirs. The projects do not cost as much to construct and store the same amount, if not more, than the surface reservoirs. The bank will have less of an impact on the environment than a surface reservoir. The water that is in the bank will no longer be exposed to evaporation, but several feet per year of water is lost in the reservoirs. It is more reliable to use when the climate is changing and can respond to seasonal changes better to manage the water than the surface reservoirs. There are also some disadvantages to groundwater banking over surface reservoirs. There are energy costs to recovering the water and these costs are usually more than the reservoirs. There is also a pumping capacity and when the demands change during the year the productiveness can be limited. Not all groundwater is used when sold. Some groundwater is being studied for its benefits. Groundwater banking and aquifer storage systems are being explored to control flooding during times of high precipitation. The groundwater is being traded in many regions. There are trades even in the United States. The city of San Antonio, Texas is the largest city in the United States that relies solely on groundwater for its municipal supply.
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