Even though the ground is an excellent mechanism for filtering out particulate matter, such as leaves, soil, and bugs, dissolved chemicals and gases can still occur in large enough concentrations in groundwater to cause problems. Underground water can get contaminated by industrial, domestic, and agricultural chemicals from the surface. This includes chemicals such as pesticides and herbicides that many homeowners apply to their lawns. Contamination of groundwater by road salt is of major concern in northern areas of the United States. This web page provides an overview of USGS's research about groundwater quality. It includes related research, publications, and a collection of data and tools (Introduction).