In the past, household wastes were often left to rot in the open, whether in a centralized dump or on the landowner's property. Today, garbage is disposed of in regional or municipal sanitary landfills. These landfills are lined with clay or a strong polyethylene liner to prevent any materials from leaching into the groundwater. Leachates create the potential for soil and groundwater contamination. At the end of each day new compacted garbage is covered with a layer of soil. When the landfill is full, it is capped with clay to make it air and water-tight. Materials do not decompose very quickly in the absence of air and water. This process minimizes the amount of material that could decompose and perhaps contaminate groundwater or the surrounding land. The clay is covered with fill (poor grade soil) and topsoil to provide a medium for the growth of vegetation or other uses. Contouring the site creates proper drainage and prepares it for seeding. Today, sites for sanitary landfills are chosen after soil and water studies are carried out and impacts on the environment, including people, have been considered.
