Weeping willows have invasive root systems that proliferate when given the right amount of water, nutrients, and oxygen. A leach field sets up the ideal conditions for root growth of these trees. It's likely that your tree will eventually send roots in the direction of your leach field and clog it. Before problems develop, it would be wise to remove the tree. Remember that tree roots can extend 3 to 4 times the expanse of the branches, so even if your willow is planted a good distance from the leach field, the roots may eventually grow into it.