Answer: There are varying studies which conflict on whether treated lumber leaches harmful materials into the soil. Many fine studies state it does not, many fine studies state that it does. Primarily though, this affects folks who want to build a raised bed with treated lumber and plant vegetables in the soil contained within. Even if the studies that say chemical treatments do leach into the soil are correct, I do not believe your situation is at all risky. If treated lumber does leach I believe it is under conditions where it is constantly exposed to the soil and degradation. I don't believe that water running off of your deck and on to the vegetables poses a problem. If you are concerned, and have not planted vegetables yet, consider having a soil test done in the area that is subjected to run off. This will let you know if your soil contains any contaminants. Contact your local county extension office for details.
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