Some can even detect chemical composition and contaminates, like lead.

Sometimes the ground is just too contaminated for anything to grow. 

I know there was contaminated land right next to it.

They are required to qualify the scope of lead abatement for contaminated properties and clear abatement projects when completed using the inspection and wipe tests.

Lead base dust could be found in the house due to toys made in China or contaminants from another owner occupant house.

"We're sorry sir, but any outside food or beverage conflicts with our agreement with the state department of health in terms of possible cross contamination, and I need you to please take it out of here before we can serve our other guests at this table."

That said the ordinance was enacted citing empirical data supporting that the preponderance of lead contamination cases stem from poor census zones (why the ordinance starts in specific areas vs others) and that cases occurring in rental properties largely stem from smaller apartment units 1-4 in size.