40 CFR Environmental Code of Federal Regulations. 

Wisconsin Administrative Code https://docs.legis.wisconsin.gov/code/admin_code, environmental code is listed under “Natural Resources”

U.S. EPA homepage http://www.epa.gov/

RCRA Online This is the same database the Hotline uses for hazardous waste questions. https://rcrapublic.epa.gov/rcraonline/

EPA Hazardous Waste homepage A good stepping off point for solid and hazardous waste questions. https://www.epa.gov/hw

EPA Technology Transfer If you enjoy the nuts and bolts of air emission management, start here. https://www.epa.gov/technical-air-pollution-resources

WDNR homepage http://dnr.wi.gov/

Library of Congress http://www.loc.gov/

CEO Express A great site that has hundreds (maybe thousands) of links arranged by category (few or none of them are environmental) http://www.ceoexpress.com/

Engineering Toolbox  a reliable source for a variety of information regarding properties of materials, conversions, HVAC and other. http://www.engineeringtoolbox.com