There is lead in the dirt that is taken up by the plants. This lead then gets into the digestive system of the animal from which the protein will be derived. This causes lead to be present in animal based protein supplements.

However, a significantly smaller amount will pass from the plant to the animal than from the dirt to the plant. As such, plant based protein powders will contain a higher concentration of lead unless it is removed at some point during the process, as the plants just have more lead in them than the animals.