|Measuring water quality|
For some purposes, it is not necessary for water to be potable (drinkable). For example, the water that is pumped down oil wells to help recover the hard-to-reach oil can have chemicals and microorganisms present.
However, drinking water that comes from underground (groundwater) or from sources on the surface (surface water) must meet a higher standard of cleanliness. When harmful chemicals and microorganisms get into the water it is considered contaminated.
Contaminants that cause disease must be taken out of the water that humans drink in order for the water to be potable. If not, drinking the water can cause intestinal upsets such as diarrhea if the water contains harmful bacteria, protozoa or viruses. If the contaminant is a compound such as mercury or lead, then the water can be poisonous. For example, mercury-laden water can cause nervous system difficulties.
Because many different types of chemicals can dissolve in water, water is easily contaminated. Many communities have monitoring programs in place, where the surface and groundwater sources that supply the community with water are checked for contamination on a regular basis. Usually the water is checked for microorganisms more often than for chemicals.
This is because microorganisms can quickly enter the water through human and animal body waste. As chemicals generally must move down through the soil and rock to reach the groundwater, chemical contamination may appear in water over a longer period time. Often, chemicals will be filtered by the layers of rock and soil and will be removed before reaching the groundwater level.
Water scientists consider chemical contamination detected in groundwater a significant problem, as groundwater makes up 22% of Earth’s total freshwater supply. Breaks in the soil-rock barrier sometimes allow chemicals and microorganisms to rapidly move down to the groundwater layer, where they collect and remain.
Groundwater contamination can occur naturally or as a result of man-made processes. For example, if a cavity (or sinkhole) collapses in the rocky limestone layer beneath a cow pasture, then the microorganisms accompanying the animal waste can contaminate the groundwater below.
The cavity creates a more direct route for fluids to move down to the groundwater. In a man-made process, an underground tank such as the gas storage tank below a local gas station, can develop a leak. As the gasoline or oil leaks out of the tank, it moves down to the groundwater.
Because the tank is below the surface, a slow leak may not be detected until it has contaminated the groundwater. As water moves downward to the groundwater, any contaminant that is on the surface, such as pesticides, road salt, or toxic (poisonous) chemicals buried in a landfill can also threaten the nearby groundwater.
Several technologies exist to clean up sites that have been contaminated with gas, oil, or other chemicals. Some of these methods attempt to fix the problem without removing the contaminated soil or groundwater. This is an “in place” or in situ cleanup.
Other cleanup methods require the contaminated soil to be dug up, removed, and the groundwater pumped out of the ground. Treatment is then done at another site. The method that is selected often depends upon the nature of the problem, how urgently the cleanup is needed, the characteristics of the site, and the amount of money available for the clean up project.
Generally contaminated groundwater is pumped to the surface, treated to remove the harmful chemicals and then pumped into a nearby surface water body. The treatment can involve a process where the contaminants are transferred from the water to the air, or passed through filters that remove the harmful chemicals. In some cases, contaminated water can be sent through the normal wastewater treatment process at a treatment plant.
Surface water is more accessible, and so is easier to clean than groundwater. Nevertheless, if the surface water contamination originates from the surrounding land, then clean up can be complicated, and can involve correcting the problem from this surrounding territory.
In situ cleanup of soil.
In situ methods are usually less expensive than the methods that truck the contaminated soil and water away for treatment. On the other hand, in situ treatment is usually a lengthy process.
In one in situ method, known as soil venting, wells are drilled and air is injected in the soil. The air evaporates the liquid that contains the contaminated chemicals, making the chemicals easier to remove.
Another method uses microorganisms that occur naturally in the soil or which have been specially designed. These microorganisms (usually bacteria) can use the contaminating chemical as a food. Even radioactive compounds can be chewed up by certain bacteria. This approach is known as bioremediation, and has also been used in some oil spills.
Off-site cleanup of soil.
When soil is taken from the ground to another site to be treated, several methods are used to clean it. Sometimes the soil is mixed with asphalt to encase it. Other times, the soil is spread out in a thin layer on pavement or plastic to let the toxic chemicals pass off into the air. Both methods are accomplished at specially qualified sites, and the processes are monitored to make sure that surrounding air, water, and soil is not being harmed.
Another off-site clean-up method involves heating the soil to cause the contaminating chemicals to move from the soil into the air. The air is collected in a device where the contaminants are burned. The remaining soil can be re-used or returned to the original contamination site. Contaminated soil may also be disposed of by burying it in a landfill. This is an expensive option and is not frequently used.
Ground- and surfacewater treatment by isolation.
When current methods are not sufficient to clean contaminated groundwater, surface waters, or soil, it is sometimes necessary to seal the site off from the surrounding area. This is a measure of last resort, as the area is lost to other uses for generations.