The U.S. Geological Survey (USGS) is a federal scientific agency and a good source of information about the Earth, natural hazards and the environment.
The USGS Publications Warehouse provides access to publications written by USGS authors. If you are trying to find data on Illinois soils, try, for example, soil AND Illinois as search strategy.
USGS publishes the National Geochemical Survey Database, which contains geochemical data obtained by analysis of stream sediments and soils in the US. See the maps of elements concentrations on a county-by-county basis, or download data of chemical elements analyzed in one county.
The Environmental Protection Agency (EPA) is a federal agency that develops and enforces regulations to protect human health and the environment. The EPA website contains regulatory information and standards about many different topics, like the air, land and cleanup, pesticides, toxic substances, waste, and water. Cross-cutting topics like lead and mercury are covered in detail. Additionally, there is background information on environmental issues, like chemicals and toxics, land and cleanup, and water.
Envirofacts is a portal to several types of EPA data, including environmental activities that can affect air, water, and land. You can search by geographic location, and answer questions like what hazardous waste is being generated in a particular area (e.g. city or zip code) or what facilities have toxic releases. See here for the different opic searches that you can do.
EPA's site also contains information about land and waste management, including management of contaminated sites. Superfund is a program established by EPA to clean abandoned and hazardous waste sites. Superfund sites can be located by state.
You can search by metal (e.g. lead) and refine to the results of the Hazardous Substances Data Bank (HSDB) to find a wealth of information about human health effects, environmental fate and exposure, toxicology, or regulations.