Phase I Environmental Site Assessment is usually required by banks during commercial property transactions. Basically a building inspection but aimed towards environmental issues (i.e., spills, petroleum, asbestos, etc). There's deed research and interviews performed, as well. This is usually a day or two in the field and the same in the office. Review the ASTM guidelines for requirements.
I've never done an environmental impact statement (EIS), but from my understanding, they are required during the design phase of project with sensitive areas (i.e., wetlands, oceans, lakes, etc.). For example, Giants Stadium that was built most likely required an EIS because it was being built in the swamps. You'd probably have to do several environmental studies such as a noise, fish, bats, etc. I've heard the budgets for EIS are 6 figures. Not cheap and usually the owner tries to change the project to avoid drafting the EIS.