Answer: That is an ongoing controversy with no easy answers! The arsenic from the treated timbers does leach out into the soil to some degree over time. However, the issue of how much leaches, and more importantly, how much is taken up into various plant parts (leaves, fruit, roots) is still largely undetermined.
Gardeners have been using treated timbers for years with no apparent ill effects. However, if the issue is a concern for you, I would suggest you use other materials instead such as rock, cinder blocks, or even no border materials at all.
Thanks for the question!
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