There are literally hundreds of databases on environmental issues, both at the state and federal level, and many have had little scrutiny from the media. Federal data are typically 2 years old by the time it is released, while you can usually get more current information from the state level.
Budget extra time to get the information. It could take months from your first request until you actually get the data. Environmental agencies at the state level seem to be even less experienced than other state agencies in handling FOIA requests.
Beware that environmental laws can vary from state-to-state, which will change not only the record keeping but also the interpretation of the data. This will also make it difficult to draw comparisons between states.
Study the topic thoroughly before diving into the data - particularly the regulations for the topic and state/region you're looking at. Covering the environment has enough stumbling blocks because of the technical jargon, but computerized environmental data can be even more cryptic.
The usual warnings about dirty data are even truer when it comes to environmental data. Be prepared to look through as many paper records as possible.