Why are modern landfills lined with plastic or clay?

Modern landfills are lined with clay or plastic to prevent leachate from seeping into the ground. Leachate is runoff from garbage dumps that contains chemicals and other wastes. Without a liner, this waste may be able to seep into the ground and enter the groundwater. If the waste reached the groundwater, the waste could contaminate the drinking water and make people and/or other organisms ill. Likewise, the soil could become polluted in nearby areas. This would be disadvantageous to the organisms that inhabited to such areas.

Neither clay nor plastic garbage dump liners work perfectly. Additionally, clay can be degraded by some chemicals. Similarly, some chemicals can degrade plastics and/or make the plastics brittle. Degradation of the clay or plastic liners is concerning because the weakened liners become cracked and/or may allow the toxic leachate to seep through.