0.22 Bq/liter of cesium-134, and 0.39 Bq/liter of cesium-137 to be exact.
The amount is very small, less than 1/100 of the legal limit of radioactive cesium in the water allowed to be released from a nuclear power plant (90 Bq/liter).
But TEPCO has a credibility problem, as the company has been saying that there are no detectable radioactive materials in the groundwater that they've been pumping up (to prevent some of the water from entering the building basements) and are planning to release, with consent from interested parties, into the Pacific Ocean.
What kind of error that TEPCO admitted, you ask? It turns out that the company measured the sample groundwater with very low radioactivity at Fukushima I Nuclear Power Plant with very high radiation background, instead of at Fukushima II (Daini) with lower background or at Kashiwazaki-Kariwa Nuke Plant in Niigata with no background to worry about.
Who pointed this problem out to TEPCO, who says they didn't have any procedure in place, actually, to measure low-radiation samples at a high-radiation location like Fukushima I?
Nuclear Safety Inspector from the Nuclear Regulatory Agency.
The Nuclear Regulatory Agency is the secretariat of the Nuclear Regulatory Authority, with many former NISA officials and inspectors. Finally doing some good by pointing out the obvious to an oblivious nuclear plant operator.
From TEPCO's handout for the press in Japanese (6/3/2013; summarized by me):
The germanium semiconductor detectors at Fukushima I Nuclear Power Plant have been used to detect gamma nuclides in samples with high radioactivity such as contaminated water in the building basements. For samples that require measurement at lower detection levels, the measurement has been done at Fukushima II Nuclear Power Plant and Kashiwazaki-Kariwa Nuclear Power Plant.
However, we tested the groundwater stored in the temporary storage tank at Fukushima I Nuclear Power Plant, as part of the procedure in the future in carrying out and managing the groundwater bypass scheme [pump the groundwater, store it in the tank, then release it into the ocean].
We explained the scheme to Nuclear Safety Inspector, and on May 30, Inspector pointed out to us that in measuring samples using germanium semiconductor detectors, background self-shielding effect cannot be ignored.
So, we tested the two germanium semiconductor detectors used at Fukushima I by measuring the background without any sample, then with purified water with no contamination in a 2-liter marinelli container.
After examining the test results, we found that the marinelli container's shielding effect against the background affected the results by several Bq/liter.
We then tested the groundwater sample from April 16, 2013 that had been tested at Fukushima I and had been found with no detectable cesium, at Fukushima II Nuclear Power Plant, and the result was below our target of 1 Bq/liter of radioactive cesium.
TEPCO's emphasis is, naturally, "below the company's target of 1 Bq/liter".
"Happy", the worker who tweeted from Fukushima I for much of the past two years, says:
Detection of contamination from the water in the storage tank for groundwater bypass system, where contamination should be below detection level. There is one problem that people don't necessarily notice, and that is the contamination of the groundwater bypass system itself. It would be a huge headache to decontaminate such huge storage tanks and all the pipes.
I think TEPCO may say there is no problem if it is less than 1 Bq/liter, but the problem is the detection of contamination itself when TEPCO initially insisted there was no contamination (or TEPCO's wishful thinking that it would be impossible that there was any contamination?). If they designed the system with the assumption that the groundwater may be contaminated, the tanks and pipes wouldn't have been contaminated.
There should have been a filter installed to remove radioactive materials from the groundwater after it was pumped up. The whole system should have been better considered. The bypass construction was a rush job. I think TEPCO wanted to obtain the consent from the fishery association as soon as possible and make release of groundwater a fait accompli.
His interview with AP's Yuri Kageyama has been pretty much ignored in Japan, by the way, mostly because it is an English article and partly because, I guess, it is about Fukushima I Nuke Plant which many in Japan simply ignore these days. Everything is sort of OK at the plant, isn't it? No major catastrophe has happened since after March 2011, so decommissioning work must be progressing fine, right?
(I have translated the article into Japanese. If your Japanese friends and acquaintances don't know about the interview, send them the link.)