[Mistletoe]

Not saying it is perfect, but modern landfills are sealed on the bottom.

> Modern landfills are completely sealed to reduce contamination of the nearby groundwater. First, the ground is lined with clay. A thin layer of flexible plastic is placed on top of the clay layer. That allows the collection of leachate, the liquid that passes through the landfill and may draw out toxins from the trash. The leachate is collected though a drainage system that passes this contaminated water through pipes to a pool where it can be treated to remove the toxins before being released back into the environment.

https://education.nationalgeographic.org/resource/landfills/...