it wasnt done after cleanup, it was done 6 days after the event:
sept 17,2001
"Also yesterday, the Pennsylvania Department of Environmental Protection said it would begin taking soil and well water samples today to determine whether contamination occurred from Flight 93's aviation fuel, hydraulic fluids and other hazardous materials following the crash.
then from oct 2nd 2001
"Betsy Mallison, a spokeswoman for the Pennsylvania Department of Environmental Protection, said that investigators still don't know how much jet fuel was spilled at the crash site. But whether it burned away or evaporated, much of it seems to have dissipated, Mallison said."
http://html.thepittsburghchannel.com/pit/news/stories/news-100064120011002-151006.html
thats about 2 weeks after the event. notice it doesnt say that the jet fuel was cleaned up?
and then ocit 3rd-
"By today, Environmental Resources Management Inc. of Pine, a contractor hired by United, expects to return 5,000 to 6,000 cubic yards of soil to the 50-foot hole dug around the crater left by the crash.
The soil is being tested for jet fuel, and at least three test wells have been sunk to monitor groundwater, since three nearby homes are served by wells, Betsy Mallison, a state Department of Environmental Protection spokeswoman, said.
So far, no contamination has been discovered, she said."
http://www.postgazette.com/headlines/20011003crash1003p3.asp
thats probably where your article picks up since they didnt find any then the site is "clean".