They're still trying to figure out the source
From NHK, translated:
If it is from the mist generator, radioactive water may now have infiltrated (what is assumed to be) clean water sources within the plant. More speculation from NHK:When they left the plant premises at 00:40 PM on 12 August, 10 employees were found to be contaminated mainly on their face and head. The highest contamination was 19 Bq/cm², which is 5 times Tepco's internal standard. The cause is being investigated. The mist generators installed as a countermeasure against heat strokes in the surroundings of the earthquake-isolated building are suspected. The contaminated employees were working indoors, but they took the bus in front of the earthquake-isolated building which is used as a base. Also, an air radiation alarm rang in this area in front of the earthquake-isolated building.
Another possibility is that radioactive dust is getting kicked in the air, making contact with the mist and settling on the workers, but that would also mean the airborne isotopes are subject to inhalation as well.The contaminations are between 4 and 19 Bq/cm², which is 5 times Tepco's internal standard but less than the legal limit requiring decontamination. Tepco said [such contamination] "does not bear consequences on health". In order to investigate the cause, Tepco will check the mist generator's sprinklers, check each contaminated employee's movements, and examine the possibility that the bus could have projected contaminants upwards.