Question

Groundwater is a valuable resource both in the United States and throughout the world. Where surface water, such as lakes and rivers, are scarce or inaccessible, groundwater supplies many of the hydrologic needs of people everywhere. In the United States, it is the source of drinking water for about half the total population and nearly all of the rural population, and it provides over 50 billion gallons per day for agricultural needs. Groundwater depletion, a term often defined as long-term water-level declines caused by sustained groundwater pumping, is a key issue associated with groundwater use. Many areas of the United States are experiencing groundwater depletion.

Excessive pumping can overdraw the groundwater “bank account”
The water stored in the ground can be compared to money kept in a bank account. If you withdraw money at a faster rate than you deposit new money you will eventually start having account-supply problems. Pumping water out of the ground faster than it is replenished over the long-term causes similar problems. The volume of groundwater in storage is decreasing in many areas of the United States in response to pumping. Groundwater depletion is primarily caused by sustained groundwater pumping. Some of the negative effects of groundwater depletion:

drying up of wells
reduction of water in streams and lakes
deterioration of water quality
increased pumping costs
land subsidence
A pipe pumping water into a field.
A tube well pumping ground water.
What are some effects of groundwater depletion?
Pumping groundwater at a faster rate than it can be recharged can have some negative effects of the environment and the people who make use of the water:

Lowering of the water table

The most severe consequence of excessive groundwater pumping is that the water table, below which the ground is saturated with water, can be lowered. For water to be withdrawn from the ground, water must be pumped from a well that reaches below the water table. If groundwater levels decline too far, then the well owner might have to deepen the well, drill a new well, or, at least, attempt to lower the pump. Also, as water levels decline, the rate of water the well can yield may decline.

Increased costs for the user

As the depth to water increases, the water must be lifted higher to reach the land surface. If pumps are used to lift the water (as opposed to artesian wells), more energy is required to drive the pump. Using the well can become prohibitively expensive.

Reduction of water in streams and lakes

There is more of an interaction between the water in lakes and rivers and groundwater than most people think. Some, and often a great deal, of the water flowing in rivers comes from seepage of groundwater into the streambed. Groundwater contributes to streams in most physiographic and climatic settings. The proportion of stream water that comes from groundwater inflow varies according to a region's geography, geology, and climate.

Groundwater pumping can alter how water moves between an aquifer and a stream, lake, or wetland by either intercepting groundwater flow that discharges into the surface-water body under natural conditions, or by increasing the rate of water movement from the surface-water body into an aquifer. A related effect of groundwater pumping is the lowering of groundwater levels below the depth that streamside or wetland vegetation needs to survive. The overall effect is a loss of riparian vegetation and wildlife habitat.

Land subsidence

The basic cause of land subsidence is a loss of support below ground. In other words, sometimes when water is taken out of the soil, the soil collapses, compacts, and drops. This depends on a number of factors, such as the type of soil and rock below the surface. Land subsidence is most often caused by human activities, mainly from the removal of subsurface water.

Deterioration of water quality

One water-quality threat to fresh groundwater supplies is contamination from saltwater intrusion. All of the water in the ground is not fresh water; much of the very deep groundwater and water below oceans is saline. In fact, an estimated 3.1 million cubic miles (12.9 cubic kilometers) of saline groundwater exists compared to about 2.6 million cubic miles (10.5 million cubic kilometers) of fresh groundwater (Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp. 817-823). Under natural conditions the boundary between the freshwater and saltwater tends to be relatively stable, but pumping can cause saltwater to migrate inland and upward, resulting in saltwater contamination of the water supply. question ; How can groundwater depletion affect streams and water quality?

Answer 1

Groundwater depletion affects streams and water quality in several connected ways:

Effects on streams and surface water
- Reduced baseflow: Lower groundwater levels reduce the natural seepage (baseflow) that keeps many streams, rivers and lakes flowing between storms. This can make perennial streams intermittent or dry them up.
- Altered flow direction: Pumping can reverse the natural flow between aquifer and stream, causing stream water to be drawn into the aquifer rather than the aquifer feeding the stream, further lowering surface flows.
- Loss of wetlands and riparian vegetation: Lower water tables can dry wetlands and shallow-rooted riparian plants, degrading habitat for fish and wildlife.
- Increased stream temperature and habitat stress: Smaller flows warm more quickly and provide less habitat, harming aquatic life.

Effects on water quality
- Saltwater intrusion: In coastal aquifers, lowering freshwater heads allows seawater to move inland and upward, contaminating wells and aquifers with saline water.
- Concentration of pollutants: Less groundwater and reduced flow mean contaminants (natural salts, nutrients, sewage, agricultural chemicals) are less diluted, so concentrations rise.
- Mobilization of geogenic contaminants: Changes in redox conditions or pressure from drawdown can release naturally occurring contaminants (e.g., arsenic, manganese, iron) from sediments into groundwater.
- Ingress of poor-quality surface water: Increased infiltration of runoff or surface-water contaminants into depleted aquifers can degrade groundwater quality.
- Long-term contamination and treatment costs: Degraded aquifers may become unusable without expensive treatment or deeper wells.

In short: overpumping can both reduce and dry up surface waters and degrade the chemical quality of remaining groundwater and surface water, with ecological, health and economic consequences.