Grey (or gray) literature consists of literature or documents that are not available through the usual bibliographic sources such as databases or indexes.  

Ask yourself: What do I need to know? Who else cares about that, and might be keeping track of that or talking about it? What region am I interested in? What county is involved, what towns or cities?  In other words, who would be producing or keeping this information? 

Governmental agencies, non-profits, advocacy groups and others don't necessarily publish their works in conventional library databases. So to search for it, you'll need to be thorough and perhaps unconventional with your research. Start with Google and look for sites whose URLs end in .gov  or .org to help identify non-profit and governmental agencies, as well as .edu sites that may point you toward the unpublished (grey!) work of academic researchers. 

However, know that Google can only go so far. It won't bring you back everything, and you may find that you need to search broadly to find sites that you then can examine more closely.  You will need to not just search, but browse, which means identifying the organizations and agencies that are relevant to your issue, and then examining their information sources carefully. 

• Brainstorm who your stakeholders are! Think about government agencies, tribes, political organizations, educational organizations, religious organizations...
  •   Example: Washington Watershed Restoration Initiative (found by searching "washington state watershed advocacy")

• Not all media is indexed by library databases: check for independent websites for radio and TV stations, and use their search functions.
  • Example: KIRO 7 (found by searching "Seattle Tacoma TV stations")

• Check for social media accounts (Does the group you're interested in have a Facebook page? Twitter? Instagram?) and then see who they are following!
  • Example: Save Our Wild Salmon (found by searching "salmon seattle facebook") or Pacific Salmon Foundation (found by searching "salmon restoration twitter")

When trying to identify useful resources, consider your search terms in these cateagories:

Geographical: state? city? county? watershed? bodies of water? 

Relevant departments: federal (EPA, BLM, Dept of Interior, DOE, USGS...?), state (environmental protection/quality? which departments regulate what?) 

Key words: what are you interested in? case studies, remediation, clean up, contamination, mining, 

The Internet Archive (www.archive.org) is a nonprofit organization whose mission is to "archive the Internet."  This is a good way to do historical sleuthing!