The news yesterday (12/16/2013) was that 63,000 Bq/Liter of all-beta that includes strontium-90 was detected in the groundwater sample from an observation hole right on the embankment near the open channel in the plant harbor. The fear was that the groundwater high in all-beta, well exceeding the TEPCO's in-house limit of 10 Bq/L for strontium, was leaking unchecked into the harbor, because the in-the-ground impermeable waterglass wall is not in place at this particular observation hole.
This sudden spike made no sense, as this observation hole, No. 0-3-2, had never had such a high number for all-beta - for that matter, not even for gamma nuclides (cesium).
Then the next day it all made sense. At some point between the collection of the samples and the measurement, TEPCO managed to mix up the sample from the hole No. 0-3-2 with the sample from the hole No. 1-16, which was found with 1,700,000 Bq/L of all-beta on the same December 16, 2013.
Another example of the deteriorating quality of the workers at the plant, AND of the managers who are supposed to train and oversee the workers.
From TEPCO's handout for the press, 12/17/2013 (English labels are by me):
Now, an interesting question is: Where is the groundwater that is sampled at the hole No. 1-16 coming from?
Note that the amount of radioactive cesium is below detection levels (2.8 Bq/L for cesium-134, 1.5 Bq/L for cesium-137), but all-beta (at least half of it is strontium-90) is 1.7 million Bq/L.
At first, I thought this water may be coming either from the leaky tanks that store waste water after cesium absorption and desalination (reverse osmosis), or the tanks that store treated waters, as that would explain the lack of cesium and abundance of all-beta.
Then, on close examination of the numbers for all-beta (see the table; click to enlarge),
Water sample from the hole No. 1-16: 1,700,000 Bq/L
Treated water (after cesium absorption and desalination): 19,000 Bq/L (8/9/2013)
Waste water (after cesium absorption and desalination): 75,000,000 Bq/L (8/9/2013)
The water from the hole No.1-16 contains too much all-beta if it comes from treated water tanks, and contains too little all-beta if it comes from waste water tanks that are leaky. Besides, both treated water and waste water do contain radioactive cesium above the detection limit.
So my brilliant idea is not so brilliant after all, but there is one another, more likely possibility (h/t Kontan_Bigcat).
The water that is finding its way to the hole No. 1-16 may have come from the trenches, either from Reactor 2 or Reactor 3, and filled with the highly contaminated water from April/May of 2011 when the water leaked from the reactor buildings into the turbine buildings after it cooled the broken reactors then into the ocean via the trenches that connect the turbine buildings and water intakes at the embankment.
The reason for the non-detectable levels of radioactive cesium may be that radioactive cesium has been absorbed by the dirt/soil after two years and 9 months.
Whether or not this trench water has been leaking has been discussed at the meetings of Nuclear Regulation Authority, which usually end with one or more commissioners and/or experts scratching their heads at TEPCO's non-explanation. But as this blog wrote in July this year, detection of significant amounts of radioactive materials along the embankment started right after TEPCO started driving the sheet piles into the embankment and into the sea floor of the open culvert in the plant harbor in April in its effort to "stop" the groundwater from flowing into the harbor.
Looking at the cesium content of the trench water (7/31/2013 post) as of 7/31/2013, it is not likely that this water is currently leaking from the trench into the surrounding soil (cesium content is too low in the sample water from the observation holes).