UPDATE: These are the 20-liter plastic containers that transported the contaminated water from Fuku-I to Fuku-II. They are wrapped in plastic. That's spill-proof, isn't it? TEPCO claims the lid in one of the containers was loose. From METI's press release on 3/27/2012:
TEPCO transported 140 liters of the water after being treated by the cesium absorption towers (SARRY, Kurion) from Fukushima I Nuclear Power Plant to Fukushima II (Daini) Nuclear Power Plant for nuclide analysis, but somehow the water spilled and contaminated the buildings in Fukushima II.
Duh. Why TEPCO needed to transport a large quantity of contaminated water just for analysis, no one knows. The water contained maximum 700 becquerels/cubic centimeter of radioactive materials, so the 140 liters of this water could contain 700 x 1000 x 140 = 98 million becquerels of radioactive materials.
First, the overview of the incident from Jiji Tsushin (3/27/2012):
"140 liters" information comes from the ad hoc NISA press conference on March 27 at 9:15PM (that's unusual these days).
Contaminated water from Fukushima I for analysis spilled at Fukushima II Nuclear Power Plant, says TEPCO
TEPCO announced on March 27 that the contaminated water spilled from the container and contaminated desks and corridors at Fukushima II Nuclear Power Plant. The water contained radioactive strontium and other nuclides, and was brought from Fukushima I Nuclear Power Plant to Fukushima II Nuclear Power Plant for analysis. The Nuclear and Industrial Safety Agency under the Ministry of Economy, Trade and Industry has instructed TEPCO to report the details, as the company is likely to have been in violation of the regulation that specifies how the contaminated water should be handled.
According to TEPCO and the NISA, contamination was found at 7 locations in the service building for Reactors 3 and 4 at Fukushima II Nuke Plant, on the desks and corridors. Maximum 700 becquerels per cubic centimeter of radioactive materials have been found from the locations of the spill. There is no worker exposed to the contamination.
More detailed information from the press conference, by Ryuichi Kino:
東電の発表では、汚染は２０６Bq/cm2 [sic] という話だったが、保安院の発表では、最高７００Ｂｑ／ｃｍ３と聞いているとのこと。ただ、運んでいた量が全部で１４０Ｌ。なんでこんなに多量の汚染水を運んでいたのかは、今のところ不明。保安院によれば今回の漏洩量は少ないが場合によっては汚染された水のすべてが漏洩した可能性があり、その場合に想定される放射性物質の量などからすると、国際原子力事象評価尺度（ＩＮＥＳ）の対象になり、暫定でＩＮＥＳレベル１と判断しているとのこと。
TEPCO announced the contamination as 206 Bq/cubic centimeter, but NISA says max 700 Bq/cubic centimeter. Total amount of the water was 140 liters. It's not known why such a large quantity of contaminated water was being transported. According to NISA, the amount of leak this time was small, but depending on the situation the entire amount could have leaked. If that was the case, INES (International Nuclear Event Scale) should be applied to the incident because of the amount of radioactive materials that would have been released; consequently, NISA considers the incident as INES Level 1, on a provisional basis.
Another question. TEPCO touched on this spill very lightly at the 6PM press conference, and said there was only one contamination. But NISA says there were 7 locations that were contaminated. The spill happened at 12:42PM, and TEPCO didn't have the details at the press conference, 5 hours after. Too slow.
Kino also reports this was the second time TEPCO transported a large quantity of contaminated water from Fukushima I to Fukushima II.
Kino also says that the contaminated water is routinely sampled by the affiliate companies (probably Toshiba, Hitachi, and other top-tier contractors) for testing at their facilities.