The work was done from January 7 to 30, 2012. Total 60 workers, 150 bags of contaminated soil and small debris removed by the workers.
Workers used brooms, dustpans, shovels to remove dust and dirt, and washed the surface of the parking lot. Apparently, they didn't bother removing the cars as the photos below show.
The results as announced by TEPCO (2/3/2012):
Air radiation levels at 1 meter off the ground:
Before: Average 82μSv/h,Maximum 355μSv/h
After: Average 54μSv/h, Maximum 115μSv/h
Air radiation levels at 1 centimeter off the ground:
Before: Average 254μSv/h, Maximum ,240μSv/h* (*TEPCO's Japanese version of the document says "1,240".)
After: Average 68μSv/h, Maximum 181μSv/h
For TEPCO's credit, the company didn't call the work "decontamination"; it was "Radiation dose reduction by collecting dust and small rubbles at the parking lot in front of Main anti-Earthquake Building of Fukushima Daiichi Nuclear Power Station".
The same operation anywhere else in Fukushima is called "decontamination", and it is to be done by the residents of Fukushima hired as "decon workers" and by decon volunteers from all over Japan.