Considering that the White House has been there for 209 years, gone through several renovations including a complete strip and repaint over the decades past, it doesn't surprise me in the least that the ground around it and surrounding neighborhoods is contaminated.
Any moron with half a brain would have at least considered the possibility. Compared to the rest of the country, the East coast is basically a landfill anyway, piled up goop from five centuries of people pooping whereever they could and burying the refuse and waste and all sorts of nastiness. Kind of like Europe except it doesn't smell bad, YET! (Unless your in New Jersey...

)
Nothing a simple test couldn't have provided.