Many in Pierce County in and around Gig Harbor, Tacoma and Lakewood, Washington allow their chickens to graze in the back yard. It is well documented that the ground in these areas are contaminated with with arsenic and lead. Gardeners already use raised beds for this very reason. Giving them organic feed is not enough. You should also consider the water you give your chickens. Is it from the spigot/faucet? You should consider that water from municipalities, cities, Counties and private companies treat the water with chlorine and add fluoride.
Used raised chicken runs with organic dirt. build a raised bed with 2X4 or 2X6’s. Make them 4X4, 5X10 or 10X12 or bigger depending on how many chickens you have.
Use rain water. Use rain barrels to catch rain water and use to water your garden (raised beds), feed your chickens and any other pets that you have.
Here is a link to the report from the Pierce County Health Department.