hey there, as someone who is tempted by 4-5 pound flushes of reishi off stumps here in brooklyn i would not recommend it. fungi concentrate metals in their
mushrooms. they are so effective that some people (myself included) see them as keystone species but not necessarily a food source. the book below cites dozens of studies that show mycorrhizal and saprophytic fungi being able to break toxic compound bonds and filter heavy metals. one of the studies noted dead mycelium is also greatly effective in filtration.
if you can get to a library and jump on proquest dissertations database you will be able to pick through the most recent studies showing where and to what degree different trees uptake metals and other contaminants, via airborn exposure and from the soil (lead doesn't move and is all over the topsoil from decades back). an interweb search will offer a lot of abstracts too. i love foraging food too but this is one area that city-goers
should be cautious with.
readings and other sources
Harbhajan Singh: Mycoremediation: Fungal Bioremediation
http://link.springer.com/chapter/10.1007/978-3-642-23327-2_18#page-1
http://www.ijpbs.com/ijpbsadmin/upload/ijpbs_50cdc71f58d76.pdf
http://link.springer.com/article/10.1007/BF02817647#page-1
instead of leaving the post with warning, let's talk alternatives briefly. i've collected
coffee grounds from cafes in the neighborhoods around me. most places have been happy help. i bring them a couple clean empty buckets with lids and they give me the full ones. this material, if fresh, has also been exposed to
hot water which will have helped in pushing back competing fungi. i also get sacks of roasted cacao beans and hulls and old coffee beans from fancy chocolate and roasting businesses. and if there are wood workers in your city many artisans get untreated cured lumber so their saw dust and trimmings are fair game too.