Land pollution involves many kinds of wastes. For many years, all wastes were dumped (1) sanitary (卫生的) landfills. Sanitary landfills are large pits where garbage is (2) under layers of dirt. When hazardous wastes are put in these (3) , toxic (有毒的) substances can seep into the groundwater and enter the food chain. Hazardous wastes are those kinds of wastes that are (4) or dangerous to the (5) . They can be poisonous, corrosive (腐蚀性的), flammable, explosive, or radioactive (放 射性的). They can pollute the air or (6) fires or explosions. These wastes can also cause (7) problems for humans and animals. For these reasons, it is (8) to dispose (9) hazardous wastes in se- cured landfills (10) they cannot leak. A secured landfill is (11) on clay ground, and the pits are lined with plastic and nylon sheets to (12) the hazardous wastes in the pit.
One kind of hazardous wastes, radioactive waste, (13) a special disposal problem Radioactive waste is created by industries and nuclear power plants (14) use radioactive (15) . Radioactive materials (16) off energy as their atoms change. This energy is invisible, but very powerful. It can harm (17) tissues in plants and animals. Radioactive waste can (18) hazardous for over 100 years. They must be stored in containers that can hold them without (19) for at least (20) amount of time.