The US Environmental Protection Agency (EPA) maintains an excellent web site explaining its various programs.  Check out the topics page at http://www.epa.gov/epahome/topics.html for information on particular programs.

Federal environmental policy is based on an extensive body of legislation.  Here's a partial list of US environmental laws up through 1990 (mostly taken from "Creating a healthier environment: how EPA works for you," EPA Journal, winter 1995), with their original dates.  Many have had subsequent amendments and reauithorizations.

• 1938 Federal Food, Drug, and Cosmetic Act -- created the Food & Drug Administration
• 1947 Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) -- controls pesticide registration, distribution, sale, and use, now under EPA
• 1948 Federal Water Pollution Control Act (FWPCA, since 1977 known as the Clean Water Act) -- gives EPA authority to set water quality standards for all contaminants in surface waters, to set technnology-based effluent standards, and outlaws pollutant discharges from a point sources into navigable waters without a permit
• 1955 Clean Air Act -- EPA regulates air pollution from area, stationary, and mobile sources, establishes National Ambient Air Quality Standards (NAAQS), directs states to develop Statewide Implementation Plans (SIP's) for compliance.
• 1965 Shoreline Erosion Protection Act
• 1965 Solid Waste Disposal Act
• 1966 Freedom of Information Act (FOIA) -- insures citizens access to all non-confidential, non-classified government information
• 1970 National Environmental Policy Act -- created the EPA
• 1970 Occupational Safety and Health Act (OSHA) -- created the Occupational Safety and Health Administration to regulate worker and workplace safety.
• 1970 Pollution Prevention Packaging Act
• 1970 Resource Recovery Act
• 1971 Lead-Based Paint Poisoning Prevention Act
• 1972 Coastal Zone Management Act -- makes oil companies liable for tanker spills
• 1972 Marine Protection, Research, and Sanctuaries Act
• 1972 Ocean Dumping Act

1973 Endangered Species Act -- authorizes Fish & Wildlife Service listing of  threatened (~190) and  endangered (~632) species, protects them and their habitats. 
• 1974 Safe Drinking Water Act -- EPA sets maximum safe standards for various contaminants; 1996 amendments require benefit-cost analysis of drinking water regulations
• 1974 Shoreline Erosion Control Demonstration Act
• 1975 Hazardous Materials Transportation Act

1976 Resource Conservation and Recovery Act (RCRA) -- EPA oversees "cradle-to-grave" control of hazardous waste
• 1976 Toxic Substances Control Act (TSCA) -- EPA tracks industrial chemicals
• 1977 Surface Mining Control and Reclamation Act
• 1977 Clean Water Act -- updates FWPCA, protects wetlands (Sec. 404)
• 1978 Uranium Mill-Tailings Radiation Control Act
• 1980 Asbestos School Hazard Detection and Control Act
• 1980 Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) -- creates “Superfund” to clean up uncontrolled or abandoned hazardous-waste sites, provides for cost recovery from responsible parties.
• 1982 Nuclear Waste Policy Act
• 1984 Asbestos School Hazard Abatement Act
• 1986 Asbestos Hazard Emergency Response Act
• 1986 Superfund Amendments and Reauthorization Act -- updates CERCLA (1980); authorizes Emergency Planning and Community Right to Know Act
• 1988 Indoor Radon Abatement Act
• 1988 Lead Contamination Control Act
• 1988 Medical Waste Tracking Act
• 1988 Ocean Dumping Ban Act
• 1988 Shore Protection Act
• 1990 National Environmental Education Act
• 1990 Oil Pollution Act -- EPA regulations storage facilities, cleans up catastrophic oil spills with a trust fund from a tax on oil
• 1990 Pollution Prevention Act -- "reduce, reuse, recycle"

You know how a bill becomes law, working its way through committees in both houses, through floor votes, House/Senate reconciliation and presidential signature.  This is a long process, but it's only half the story.  Once the bill becomes an act (law), the appropriate Executive agency drafts regulations governing how the act is to be implemented, publishes the draft regulations in the Federal Register, conducts lots of hearings, accepts and responds to public comment from all sorts of interest groups, and revises and codifies the final regulations in the Code of Federal Regulations (CFR). The CFR is organized by topic into 50 "titles;"  Title 40 contains most environmental regulations.  Even after codification, interest groups that don't like the  regulations can sue to have regulatory provisions struck down, arguing that they do not correctly reflect Congress's intent.

The Environmental Protection Agency was established in 1970 by the National Environmental Policy Act, and assumed various environmental regulation functions from the Depts. of Interior (water quality), Agriculture (pesticides), Health, Ecucation & Welfare (solid waste mgt. and air quality); the Atomic Energy Commission (radiation), etc.  Adminstratively, the EPA is divided into 10 regions (see map), with Delaware located in Region III (EPA regional office is in Philadelphia).

As noted above, the EPA develops the regulations to implement federal environmental laws, manages some of its own programs and delagates enforcement of many other environmental policies to the states.  Remember that the foundation of Federal authority over the environment is the Constitution's interstate commerce clause.  The EPA can't actually force the states to enforce its policies, but Congerss has given it the power to withhold various Federal funds from states that fail to submit adequate implementation or compliance plans.