Final answer:
Groundwater contamination and air pollution are two undesirable by-products created in landfills.
Step-by-step explanation:
Two undesirable by-products that are created in a landfill are groundwater contamination and air pollution. When plastics and styrofoam are dumped in landfills, they do not decompose and can release carcinogens. As landfill sites fill up, there is a risk of increased groundwater contamination. These by-products pose environmental and health hazards.