These two Federal Government agencies probably are the most valuable for environmentally-themed data:
Other Federal Government agencies also are major repositories of data useful in environmental studies.
If you have little or no idea about what agencies or organizations collect data in a particular subject area, these two databases are great starting places.
Contains references to tens of thousands of data sources from the U.S. Federal Government, U.S. state governments, international intergovernmental organizations, and private sector groups, such as trade and professional associations.
These are tools for finding information from the Congress and from Executive agencies of the U.S. Federal Government.
This is almost certainly the most comprehensive database of Congressional publications, some dating back as far as the late 1700's. Includes the complete texts of hearings and most other documents. An interesting sub-set of the database provides legislative histories. (Law Library subscription.)