Formation of Plumes

The Transport of Chemicals into Groundwater

Chemicals that are spilled or released into soil may dissolve in any water that is present. In some cases, the chemicals may be transported into groundwater by water slowly seeping downward. How fast this occurs and the details are complex, but water generally moves downward. Lateral movement is generally not significant until water reaches the water table. The contaminant concentration reaching groundwater is less than the concentration at the source because of dilution and breakdown that occur along the way.

As contaminants are slowly transported downward, they can chemically interact with other things in the water or soil. This can temporarily or permanently remove the contamination. Natural defense mechanisms such as biological degradation also act to naturally mitigate contamination to varying extents by removing or altering the contaminants. For instance, some bacteria that live naturally in soil and groundwater can metabolize many of the contaminants in gasoline.

Contaminants become “pollution” if they reach concentrations high enough to be judged harmful to humans or the environment. Groundwater standards are set by the United States Environmental Protection Agency (EPA) and state agencies. Contamination can originate from a “point source” such as a leaking underground tank, or from a “nonpoint” source such as infiltration from agriculture (i.e. pesticides and fertilizers) or urban run-off (i.e. road salt or trace metals left on roads from tires and car emissions).

When contaminants enter the groundwater, they are slowly dispersed to form a diluted cloud or “plume.” Chemical reactions and biological breakdown can continue. Pumping, complex flow patterns, chemical and biological processes all affect the travel, size and shape of plumes. Some contaminants may be removed by a process called “adsorption.” Adsorption is the process by which a chemical adheres to grains in the soil. Although this does not remove the contaminant, it may immobilize it and help prevent it from spreading.

The size and extent of plumes are determined using “monitoring wells.” Such wells also help track clean-up efforts. Information from monitoring wells and other sources are used to determine appropriate mitigation recommendations. Clean-up methods can vary widely depending on local site conditions.