Environmental reporters can find oceans of data for stories if they can navigate their way to their targets. The federal Environmental Protection Agency is rich with publicly available data — despite industry and White House efforts to block access.
The WatchDog has devoted much space to the best-known databases, like TRI (releases of toxics) and ECHO (enforcement and compliance). There's much more. And finding your way to it or through it is easier if you start from EPA's online master index.  It's worth exploring. If journalists don't use the data, they run the risk of losing it.
Example: right now local drinking water utilities are coming out with their annual "Consumer Confidence Reports"  or contact your local utility. But you could put that information in better context if you knew about EPA's National Contaminant Occurrence Database  (NCOD). It lists sampling data on contaminants, both regulated and unregulated, that have been found in drinking water across the U.S. It's one of many listed in EPA's master index.
Or when was the last time you took a look at CERCLIS,  the list of all hazardous waste sites either proposed or slated for Superfund cleanups? It's listed in the master index, too, and EPA makes it really easy to find the ones near you.
The index goes way beyond just listing databases, though. It includes all kinds of publicly available software, as well as many specific computer models for assessing pollutant dispersion or toxic risk, for example.
EPA more and more is developing and using data tools built on geographical information systems (GIS), allowing reporters not only to find pollution problems in their own neighborhoods — but to display them on maps that a newspaper's graphic/web designers can turn into flashy features. Just for starters, check out "Window to My Environment." 
Remember, too, that state environmental and resource agencies often maintain similar databases (where do you think EPA data comes from?) Try hunting for local stories there as well.
The data universe for environmental reporters extends well beyond the insular concerns of EPA. Many of these are public as well. Some favorite examples (a few among many):
- The National Conference of State Legislatures keeps a state-by-state database  of state legislation on a wide variety of environmental topics. It's searchable and online. Your state legislature likely has its own legislative database, often searchable by keyword.
- The USDA has an Agricultural Resources and Environmental Indicators (AREI) Database and Mapping Tool,  which displays a variety of statistics in map form.
- The Environmental Working Group (possibly the geekiest environmental group) keeps a database  (called "Skin Deep") of the ingredients in cosmetics — and their possible health effects. Or you may be more interested in EWG's Farm Subsidy Database. 
- The World Resources Institute (a think tank respected by both sides of the political spectrum) keeps a site called "EarthTrends."  While they call it a "database," it could better be described as a user-friendly emporium of well-digested information on a broad array of environmental topics.