Pollution in groundwater aquifers has made many of these wells unfit for consumption. Overexploitation of ground water lowers water tables, which may damage wetlands, cause ground subsidence, and induce salt-water intrusion in coastal aquifers.
Rivers and streams have long been used for discharging the waters. Most of the civilization have grown and furnished on the banks of rivers but unfortunately, growth in turn, has been responsible for pollution of the rivers.
Effect of Groundwater Usage:
When groundwater withdrawal is more than its recharge rate, the sediments in the aquifer get compacted, the phenomenon known as ground subsidence. Ground subsidence at the surface can be regarded as ground movement which takes place due to the intensive abstraction of groundwater.
It is attributed to the consolidation of sedimentary deposits in which the groundwater is present, consolidation occurring as a result of increasing effective stress. The total overburden pressure in partially saturated or saturated deposits is borne by their granular structure and the pore water.
It occurs where excessive pumping of groundwater removes ground support, particularly in areas of unconsolidated sediments and sedimentary rocks. Removal of water causes sediment compaction. The weight of buildings can also cause compaction and subsidence.
Lowering of Water Table:
Lowering of water table occurs where groundwater is withdrawn faster than it can be recharged. It can cause wells to dry up; for example: in some areas of the mid-West, 2-10 times ? more water is being withdrawn from the High Plains aquifer than is being recharged, it is not advisable to do excessive mining as it would cause a sharp decline in future agricultural production, due to lowering of water table.
Another problem associated with excessive irrigation on poorly drained soils is waterlogging. This occurs (as is common for salinisation) in poorly drained soils where water can’t penetrate deeply. For example, there may be an impermeable clay layer below the soil. It also occurs on areas that are topographically poorly drained.
What happens is that the irrigation water (and/or seepage from canals) eventually raises the water table in the ground the upper level of the groundwater from beneath. Growers don’t generally realise that water logging is happening until it is too late tests for water in soil are apparently very expensive.
The raised water table results in the soils becoming waterlogged. When soils are water logged, air spaces in the soil are filled with water, and plant roots essentially suffocate lack oxygen. Water logging also damages soil structure. Worldwide, about 10% of all irrigated land suffers from water logging.
Saltwater incursion involves contamination of freshwater aquifers with saltwater. Fresh water floats as a lens on denser salt water. If fresh water is removed in large amount, a cone of depression is created in the fresh water lens.
Lowering the water table by 1 foot results in raising the level of salt water by 40 feet. This situation occurs primarily in island or coastal communities, but is also threatening the Salinas Valley in California. It can be counteracted by:
1. Reducing groundwater withdrawal.
2. Re-injecting treated wastewater into recharge wells.
3. Construction of recharge ponds.
Sewage is the most common source of groundwater pollution. Landfills, underground storage tanks, and hazardous waste disposal sites are other sources of contamination. Surface sources of pollution can affect groundwater where the ground is very permeable or where conduits to the water table are present.
Pollution spreads with the flow of groundwater. Cleanup of contaminated groundwater is extremely difficult and expensive. High-level nuclear waste disposal may threaten groundwater quality in the future.