Four pounds of trash a day. That's how much solid waste is generated by each man, woman and child in the United States. This waste includes substantial amounts of paper and cardboard (40%), as well as yard waste (18%), metals (9%), plastic (8%) and other products. Where does it all go? The answer: more than 70% of this material is buried directly in the ground-in disposal facilities known as landfills. Today's well-engineered, sanitary landfills are a far cry from the open dumps of previous decades. Those dumps, where waste was burned or simply piled up, often became unhealthy eyesores that attracted insects and rodents. In general, they were unlined and unmonitored-so that contaminated liquids could seep into underground water supplies. By contrast, modern sanitary landfills are designed to protect the environment by containing and isolating waste. Building these landfills requires extensive planning, engineering, monitoring and supervision. This entire process begins before the first shovel-full of dirt is removed and continues long after the facility closes.

C hoosing the best site for a landfill calls for highly skilled engineers and hydrogeologists. These specialists look for areas where natural clay deposits and other structural features will best protect underground water reserves and other important resources. Detailed testing is carried out to provide a complete picture of conditions above and below the ground. This information is then used to develop construction plans and determine what safety features are required. It is carefully reviewed by health and environmental officials who issue the appropriate permits.

Today's landfills include multiple safeguards to contain wastes and isolate them from surrounding water and soil. In many cases, for example, such safeguards involve a protective liner to prevent filtration. These liners may be made of compacted clay or impermeable materials such as plastic. When clay is used, the layer may be as much as 10 feet thick.

Drainage systems are another common feature. Instead of allowing rainwater and other liquids to collect inside the liner, these "leachates" pass through the refuse and are pumped to the surface where they are treated and discharged. Monitoring wells around the perimeter of the landfill allow technicians to check groundwater quality and detect contamination.

Still another feature concerns landfill gas. As wastes slowly deteriorate, they often produce methane, a potential energy source. To harness this potential, some landfills have installed equipment to collect the gas and pump it through pipelines for use as a commercial fuel.

Each day, landfill operators must handle truckloads of trash. To assure proper management, such wastes are generally unloaded at only one designated area. This area is known as the "working face."

Walls of dirt known as berms, built around most landfills, prevent unloaded wastes from being blown by the wind and shield them from view.

Once trash has been deposited, it is spread and crushed by a bulldozer or compactor. As each day ends, the waste is covered with a layer of soil to minimize odors and discourage insects and rodents.

When landfills reach their capacity, they are sealed and covered with a cap of clay and dirt. Depending upon their location, they may be turned into parks, gardens, golf courses, playgrounds or even ski and sledding areas. Environmental monitoring continues long after these facilities have been converted to other uses.