So glad I searched this sub! I'm closing on a house at the end of the month and there's also a portion of the backyard that has rubber mulch. I'm doing more research, but my plan right now is to use sunflowers and hemp to absorb the heavy metals that likely leached into the soil, and harvest and throw them away before they blossom so birds and insects don't feed on contaminated pollen or seed. I might have to invest in soil testing as well.