Balancing Recharge and Discharge

When recharge and discharge are balanced, the reservoirs of groundwater and the water table remain constant in natural situations even though water is continually flowing through the aquifer. For recharge to balance discharge, rainfall must be frequent enough to equal the sum of the runoff from rivers and the outflow from springs and wells.

But recharge and discharge will not always be equal, because rainfall varies from season to season. Typically, the water table drops in drier season and rises during wet periods. A decrease in recharge, such as during a prolonged drought, will be followed by a longer term imbalance and lowering of water table.

An increase in discharge, usually from increased well pumping, can produce the same imbalance. Shallow wells may end up in the unsaturated zone and dry up. When a well pumps water out of an aquifer faster than recharge can replenish it, the water level in the aquifer is lowered in a cone-shaped area around the well, called a cone of depression (Figure 11.6). The water level in the well is lowered to the depressed level of the water table. If the cone of depression extends below the bottom of the well, that well goes dry. If the bottom of the well is above the base of the aquifer, extending the well deeper into the aquifer may allow more water to be withdrawn, even at continued high pumping rates. If the well is deepened so much that the entire aquifer is tapped and the rate of pumping is maintained, however, the cone of depression can reach the bottom of the aquifer and deplete it. The aquifer will recover only if the pumping rate is reduced enough to give it time fore recharge.

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Figure 11.6 Excessive pumping in relation to recharge draws down the water table into a cone-shaped depression around a well. The water level in the well is lowered to the depressed level of the water table.

The extreme withdrawal of water not only can deplete the aquifer but may also cause another undesirable environmental effect: as the pressure of the water in pore space falls, material formerly overlying the aquifer may be lowered, creating sink-like depressions (Figure 11. 7).

Figure 11.7 In Antelope Valley, California, overpumping of groundwater has led to fissures and sinklike depressions on Rogers Lakebed at Edwards Air Force Base, the landing site for the space shuttle. This fissure, formed in January 1991, is about 625 m long[1].

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As water in some sediments is removed, the sediments undergo additional compaction; the loss of volume is reflected in the lowering of the surface. Subsidence attributable to this effect has occurred in Mexico City and Venice, Italy, as well as in numerous other regions of heavy pumping, such as San Joaquin Valley in California. In these places, the rate of subsidence of the surface has reached almost 1 m every three years. Although a few experiments have attempted to reverse the subsidence by pumping water back into the groundwater system, they have not been very successful because most compacted materials do not expand easily to their former state. The best that has been done is to halt further subsidence by restricting pumping.