This sunflower showed up in our garden uninvited, i.e., we didn't plant it. No doubt it was a product of a sunflower seed from the bird feeder that was dropped onto the mulch bed surrounding our intended plants and made itself at home in our garden.
Today I ran across the use of sunflowers in: Phytoremediation: You Can Grow Plants That Help Eliminate Radiation In The Soil
I'm not sure how much scientific proof there is so far, but the theory is that the sunflower's root structure is so dense and strong that it is able extract heavy metals such as arsenic, lead, and radionuclides Cesium 137 and Strontium 90 out of the soil or water.
I'm not sold on it yet, but I thought it was worth passing on.