See Government Agencies section. In many states, the state health department may have a registry of diseases such as cancer. A call to your state health department should let you know if the registry can give you information specific to your community. Your local health agency may also be aware of research that may have been performed in your community by universities or other researchers. Many large companies do.
Some companies keep extremely detailed medical records on employees.
Larger companies may have an epidemiologist who studies the incidence of disease in workers. Smaller companies may not have as detailed medical records on employees and may not keep them for long. The water pollution control agency or public works department in your county or state has an office dealing with the safety of public water supplies. Ask them for the office to contact with questions and concerns.