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Groundwater Contamination

Groundwater Contamination

The following four graphics illustrate how pollution can impact groundwater quality

Step 1: Pollutant is Released

A hazardous substance is released at or near ground surface

A spill occurs at ground surface and a hazardous substance enters the unsaturated area just below the surface. This unsaturated area is known as the vadose zone. Once released, the contaminant mixes with soils and sediments. Precipitation percolating down through the soils and sediments will carry the contaminant towards the water table, which marks the beginning of saturated sediments. Many organic compounds can linger in the soil for years before either evaporating into the atmosphere, or breaking down through microbial action.

Step 2: Spill Percolates Through Soil

The contaminant seeps downward and begins to disperse in the aquifer. The contaminant now has formed a contaminant plume.

In Step 2, the effect of a complex pollutant, such as gasoline, can be seen. A complex pollutant is one containing more than one compound. When these pollutants reach the water table, the different compounds often separate and either sink or float within the aquifer.

All contaminants are affected to some extent by the flow of groundwater, which slowly causes the contaminant to spread out into a plume. The plume is the path in which the contaminants are moving through the aquifer. The size of the plume will vary with the physical and chemical nature of the contaminant(s) and the aquifer (see Step 3).

Step 3: Spill Moves Through Aquifer Towards Wells

The contaminant continues to spread. As the plume expands, parts of the plume are drawn towards the wells by the suction force created from pumping the well

The rate at which a contaminant will move through the aquifer will vary not only with the characteristics of the aquifer but also with the characteristics of the contaminant. Some contaminants are heavier than water and will continue to sink until reaching an impervious surface. These types of substances are often called “sinkers” or DNAPL’s (dense non-aqueous phase liquids). Many chlorinated solvents such as perchloroethylene (“per-kloro-ethyl-een”), a cleaning solvent, are sinkers. Other contaminants are lighter than water and “float” near the water table. These types of substances are called “floaters” or LNAPL’s (light non-aqueous phase liquids). Petroleum is an example of a floater.

It is important to remember that although most pollutants tend to float or sink, all can dissolve in water to a certain level, further increasing the severity of contamination and difficulties involved in remediating the contamination.

Step 4: Chemicals Drawn Into Public Water Supply

Contaminant is drawn into the wells and enters public water supply

Once contaminated, groundwater may remain so for a very long time and cleanup can be very difficult and expensive. In some cases, it may not be possible to remove the contaminant completely. The costs to a community can be significant.

A water supplier must now decide how to respond to the contamination. The first step is shutting down the contaminated well until the impact of the contamination can be fully assessed. The water supplier may have to purchase additional water from another supplier if the remaining non-contaminated wells cannot produce enough water to meet community demands. The water supplier must then determine:

  1. What the source of the contaminant is;
  2. If the contaminant can be removed through the normal water treatment process
  3. If the contaminant can be removed in the well field, possibly by pumping the contaminated groundwater to prevent continued spreading of the plume; or
  4. If the well or well field must be totally abandoned and a new source of water found for the community.


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