But it should be noted that not all contamination events pose a threat to our health. Pollutant concentrations become diluted when they enter water sources and are further reduced by biological degradation, filtration, and adsorption to soil. Some chemicals, such as the man-made chlorinated hydrocarbons, are very stable in the environment. Some of these compounds accumulate in living organisms and are not readily metabolized and excreted. The impacts of contamination events to lakes and reservoirs are more severe and persistent than streams and rivers because there is not a natural flushing process as is characterized by the flow in streams and rivers.
Contamination is even more persistent in groundwater due to lack of biological degradation.
The most biologically active bacteria live within the soil above groundwater supplies. In contrast to surface water there is not a lot of mixing when a contaminant enters groundwater. When a contaminant first enters the soil it will travel down vertically with gravity until contact with groundwater. The contaminant will then spread out three-dimensionally like smoke from a chimney and is called a plume. Groundwater does not exhibit turbulent flows as found in surface water.