Environment, Health and Safety Online (http://www.ehso.com/ehshome/violations.php) has an interactive map that will give you some contact numbers relevant to your area. While the exact answer to this assignment will depend on where you live, the main idea behind your answers is whether state or federal jurisdiction applies. For example, an oil spill in a large body of water is under the jurisdiction of the Environmental Protection Agency, a federal organization. According to their web site (http://www.epa.gov/oilspill/contacts.htm)
"To report an oil spill or hazardous substance release, call the National Response Center (NRC) at (800) 424-8802." You could also try calling the Oil Spill Program Information Line, (800) 424-9346, which is mentioned on the web site. The Coast Guard would likely be mobilized to contain the spill. The Coast Guard's web site also has an extensive listing of state environmental agency phone numbers: http://www.uscg.mil/vrp/faq/statechart.pdf
Many of them are 24 hour, 800 numbers. They could likely direct you to the appropriate answer for all these questions.

All states have a department of environmental quality (although it may have a slightly different name) that reports to the EPA to ensure compliance with Federal law, such as the Clean Water Act. This department should have an emergency response unit, with contact numbers posted on the Web. For example here is one for Wyoming:
http://deq.state.wy.us/out/spills.htm
This would be where I would call if I saw a fish kill, which would indicate a point pollution source, and a violation of the Clean Water Act.

Each year, manufacturers file many environmental reports to various units of government to verify they are operating in compliance with their permits. One report is the "Toxics Release Inventory," which is part of the Community Right-to-Know Act, compiled by the Environmental Protection Agency. Toxic Release Inventory substances violations can be reported directly to the EPA via this web site:
http://www.epa.gov/echo/

Landfill gas is produced from organic waste disposed of in landfill. As conditions become anaerobic the organic waste is broken down and landfill gas is produced. This gas builds up and is slowly released into the atmosphere. Landfill gas contains these VOCs which are regulated by the the Clean Air Act, and Part 40 of the Code of Federal Regulations (CFRs), and so landfill owners must estimate the quantity of VOCs emitted. If the estimated VOC emissions exceeds 50 tonnes, then the landfill owner is required to collect the landfill gas, and treat it to remove the entrained VOCs. Because VOCs are governed by the Clean Air Act you can report violations to your state environmental quality department.