Groundwater can also become contaminated when liquid hazardous substances soak down through the soil into groundwater. Contaminants that can dissolve in groundwater will move along with the water, potentially to wells used for drinking water. A combination of moving groundwater and a continuous source of contamination can, therefore, pollute very large volumes and areas of groundwater. Some plumes at Superfund sites are several miles long. More than 88 percent of current Superfund sites have some groundwater contamination.
Some hazardous substances dissolve very slowly in water.
When these substances seep into groundwater faster than they can dissolve, some of the contaminants will stay in liquid form. If the liquid is less dense than water, it will float on top of the water table, like oil on water. These pools continue to contaminate the aquifer as they slowly dissolve and are carried away by moving groundwater. This form of groundwater contamination is called residual contamination . Many processes can affect how contamination spreads and what happens to it in the groundwater, potentially making the contaminant more or less harmful, or toxic.