• Excessive pumping of individual wells locally lowers the water table and creates a cone of depression around the well
• Subsidence may occur when water is withdrawn from aquifers in unconsolidated materials causing pore spaces to collapse
• Groundwater pollution may come from single point sources or distributed non-point sources including agricultural fertilizers, landfills, oil wells, mines, septic tanks, road salt and underground storage tanks

Local Overpumping
xcessive withdrawal of groundwater at a regional scale may exceed recharge resulting in groundwater overdraft (see High Plains aquifer section). Overpumping of individual wells will not permanently deplete an aquifer but can impact the use of neighboring wells. Pumping of a well pulls down the local water table adjacent to the well as water is withdrawn faster than it can be replaced. The surface of the water table forms a cone of depression surrounding the well. The change in elevation of the water table (drawdown) decreases with distance from the well. Given sufficient time, the water table will be restored to its original level when pumping stops. Domestic wells rarely yield sufficient water to generate a significant cone of depression but the large volumes of water necessary for irrigation can result in the formation of a sizable cones of depression around irrigation wells during the growing season.

A simple animation of contrasting water levels in four wells as a result of excessive pumping of irrigation well (B). Water levels in lightly used domestic wells decrease due to the influence of heavy pumping of the irrigation well.

The cone of depression for any single well may affect neighboring wells prompting land owners to ask who owns the water below their property? Nearly fifty years ago, Tom Bristor found out the hard way that the state of Arizona considered groundwater to belong to the person who could pump it out of the ground. (For more on Bristor's story see Who owns the groundwater?)

Ground subsidence
Groundwater removal from unconsolidated sediments may result in sediment compaction and the subsidence of the ground surface. The weight of the overlying material (including and any engineered structures) is supported by both the mineral grains and the water in the pore spaces in a confined aquifer. Pressure from water on the surrounding grains keeps the pore space open. When the water is extracted mineral grains may collapse inward on the pore space. This causes a decrease in the volume of the underlying sediment and can result in subsidence of the ground surface. The Leaning Tower of Pisa, Italy, developed its characteristic tilt soon after construction began in 1174 because of subsidence following groundwater withdrawal. Subsidence of up to 9 meters occurred over an area of approximately 13,000 km² in the San Joaquin valley, California, as a result of groundwater withdrawal for irrigation.

Cities built over weak unconsolidated fine-grained sediments associated with geologic environments such as floodplains, deltas or lake beds often show evidence of subsidence. Engineered structures (roads, pipelines, large buildings) may fracture or collapse as a growing city's population extracts increasing volumes of groundwater. Intensive pumping of over 500 million gallons of groundwater per day in the Houston region from 1967 to 1983 lowered the water levels in wells of the Texas coastal lowlands aquifer system. Water levels in the wells declined approximately 60 meters in the past 50 years, and are now more than 110 meters below sea level. 

Subsidence has occurred below some of the world's largest cities. Over 12,000 km2 was affected in Houston, flooding some subdivisions.

Groundwater Pollution
Natural groundwater is far from pure but it typically contains few chemicals in sufficient quantity to cause harm to humans and ecosystems. However, under specific geological conditions, elements such as arsenic or mercury may be concentrated in groundwater. An example of widespread groundwater contamination by arsenic that may become the greatest mass poisoning in history in currently unfolding in the impoverished nation of Bangladesh. Over 20 million people may potentially be exposed to harmful levels of arsenic present in the country's 4 million groundwater wells. (For more on the Bangladesh arsenic crisis see Arsenic & Bangladesh)

Human development has added many potential pollution sources that may contaminate the groundwater supply. Pollution may be associated with specific identifiable point sources such as leaking storage tanks or may not be traceable to a single point of origin but may occur over a wide area (non-point source) such as croplands.

The Clean Water Act (1972) and its amendments banned the most egregious examples of pollution from industrial point sources but many less obvious pollution sources still exist. The Woburn case cited in the Introduction is just one example. Other potential pollution sources in the U.S. include:

• approximately 3,000 landfills and thousands of illegal dumps that may leak a chemical soup of waste liquids;
• 23 million domestic septic systems that serve homes beyond the reach of municipal sewer systems;
• five million active and inactive underground storage tanks used to store products such as gasoline and industrial chemicals;
• over one million abandoned and active oil and gas wells that produced crude oil mixed with brines and water/mud mixtures;
• thousands of active and abandoned coal and metal mines many of which yield acidic run-off that percolates into the groundwater;
• thousands of tons of animal wastes concentrated in areas of livestock (chicken, pig, cattle) farms;
• millions of tons of fertilizers, pesticides, deicing salts and other materials added to the land surface annually.

Potential pollution sources for groundwater.

Non-point sources of pollution in agricultural regions are difficult to detect but can have some of the most far reaching effects as rural well waters are not monitored by municipal water treatment plants. Pollutants that may be present in rural wells include pesticides (herbicides, insecticides, fungicides) and nitrates that are products of fertilizers. Both pesticides and fertilizers are applied to crops and some are washed off into the groundwater and surface water systems. 

The Environmental Protection Agency (EPA) is responsible for enforcing water quality standards for drinking water. Some of the common pollutants that the EPA recognizes in drinking water are listed below.