Interesting proposal. Here burials require metal lined waterproof coffins, to prevent heavy metals concentrated in the human body form leaching into the soil and potentially contaminating the ground water. (Whether this legislation is based on actual science or lobbying by the funeral industry to increase the cost of burial is open to conjecture.) However, there is no question that there is nowhere near enough water to support the number of trees required for a one person one tree burial pattern. Natural gas for crematoriums — yes, water for that many trees — no way.
