I am putting in a raised garden that will sit on the ground. I'm doing so because I have contaminated soil below and wish to put in fresh black dirt above ground. What do you recommend I put on bottom of garden (12 inches down) to serve as barrier between soil and the raised portion of the garden? I'd like to separate the two so there i no mixer of soil but still allow for drainage. Thoughts? Weed barrier would allow for water to pass through but will it deteriorate and contaminate the soil? Maybe a tarp with holes?