Groundwater, defined as water moving through soil and rock, is a common route for chemical movement. The source of groundwater contamination can include surface impoundments in which hazardous materials are disposed or stored, such as ponds and lagoons, leaking underground storage tanks, or any spill where contaminants can seep downward. The type of soil configuration is crucial in groundwater contamination. Some soil layers, such as clay, are harder for contaminants to move through (less permeable) and can protect the underlying groundwater.

While contaminants in rivers or streams are generally churned and diluted by movement as they are in the air, contaminants can move great distances in groundwater without dilution. Also, chemicals in the groundwater last longer; chemicals cannot evaporate, and they resist breakdown in the absence of air and light. It is difficult and sometimes impossible to purify contaminated groundwater.