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Soil in the city I live in is quite often contaminated with nasty things like arsenic and lead. Levels are sometimes twice or more what's considered acceptable. For this reason, people usually garden in imported soil in raised beds rather than directly in their own backyards.

I'm wondering how risky it would be to use leaves (most commonly maple) from untested soil as mulch or as bedding for vermicomposting worms. Would different varieties of trees be more/less likely to deposit contaminants in their leaves?

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Where are you getting those leaves from? If from your own property I would get a full soil test done (ASAP). Then depending on the results, will greatly dictate the best course of action. If from elsewhere, I personally would not use them until I knew the soil was safe, especially if you plan to use the leaf mulch/compost on/around produce that you (and your family) plan to eat after harvesting. –  Mike Perry Jul 9 '11 at 21:22
    
I've just got back from a BBQ and while there I ended up speaking with someone who worked for the US Forestry for 3 years & now runs a (small, 10 acres) registered organic farm here in Missouri. His advice is, whatever you do, do not bring leaves, compost, soil etc into your environment that even have the slightest possibility of such contamination, the risks (possible consequences) are far too! high. –  Mike Perry Jul 10 '11 at 13:43
    
If you're in New York City there's no need to get your soil tested: it is contaminated. Pretty much a guarantee. I don't know much about other cities, but I've never seen an NYC soil test come back clean, they're always full of lead and cadmium. We've always used leaves from street trees in our compost and it only started to dawn on me recently that we may be introducing heavy metals to our compost. I don't know the answer, but I think the question is: do street trees take up toxins and heavy metals and store them in their leaves. –  Amanda Jul 9 '12 at 19:52
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This study's summary suggests that arsenic in fruit trees ends up mainly in the heartwood. (I don't have access to full text, so I don't know if there are any caveats.)

The book Arsenic, By Assembly of Life Sciences (U.S.). Committee on Medical and Biologic Effects of Environmental Pollutants looks like it may contain the answer to your question, but I can't find it at the moment. A quick reading of a few dozen pages suggests that (a) trees that take up certain forms of arsenic will show signs of toxicity but (b) sometimes you might not be able to tell.

But a general answer may not be what you really want anyway. What I would do:

  • If you're getting leaf compost from an external source (e.g. municipal compost), ask them if they test for heavy metals.
  • If they don't test, or if you're making your own, give your local cooperative extension service a call to see if they will test compost/leaf mulch for heavy metals, and how they recommend taking a sample to send in for testing.
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I don't have a yard myself — I garden in containers on a balcony — so my leaves are coming from bags thrown out by my neighbours. –  intuited Jul 10 '11 at 3:04
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