(UPDATE 2) 10 workers were all TEPCO employees.
(UPDATE) Some researchers and independent journalists are speculating as to what could have contaminated the workers and how. Researchers Jun Makino and @study2007, and journalist Ryuichi Kino seem to have come to the same, albeit tentative, conclusion which can be summarized as:
A mass of air containing radioactive materials was passing the area, and caught by the mist and fell on the workers.
Another unconfirmed story is that the mist generator was contaminated. No information as to how it got contaminated.
This is a follow-up on the last post about workers who were waiting for the bus to take them from the Anti-Seismic Building to the checkpoint near the main gate at Fukushima I Nuclear Power Plant and somehow got their heads and faces contaminated with radioactive materials.
TEPCO at first suspected the water used in the mist generator, but it says the test confirmed the levels of radionuclides were below detection levels.
So what caused the sudden rise in radioactivity in that particular location? TEPCO says it doesn't know, but is certain that it was a one-off, localized event.
The workers had maximum 19 Bq/cm2 (square centimeter) of contamination, not 10 Bq/cm2. Test by WBC showed no internal radiation exposure.
TEPCO did disclose some information about the water source, in the handout for the press on August 12, 2013 (English labels added by me). As I added to my previous post, TEPCO has its own dedicated water source for the plant, which is a river water. As NHK reported, the water is dammed, and then transported, I assume via the duct or pipe, to the plant, just outside the plant perimeter. There is a water purification facility there. The purified water is then fed to the plant for daily use. How it is fed to the plant buildings is not explained: