How do you know pcbs are in your backyard?  Those associated with power
lines were contained in transformers, if some years ago one of these
leaked or somehow broken then it might be possible some is there.

Forget about the power lines. Nothing to worry about there.

PCBs are everywhere in the world, in all environments, but at extremely low
concentrations. The only real risk comes from biomagnification, the effect of eating
high off the food chain. As your garden produce puts you at the very lowest level of
the food chain, the risk is minimal to nonexistent. If it still bothers you, there
is one thing you can do....increase the organics in the soil. Compost, manure, that
sort of thing. PCBs and dioxins and the like will tend to partition into any organic
substrate, so having high soil organics gives the PCBs a sink, an alternate to being
partitioned into the plants you want to eat.

Lar