As I posted on Monday, TEPCO announced a 10-fold spike in radioactive materials (all-beta) in the water sample collected from soil just outside the thin (6.4 millimeter-thick) bentonite layer of the in-the-ground water storage pond No.1.
In the successive email alert for the press (which they know post on their homepage press release section), TEPCO sort of tell us what they think was the cause for the spike:
A used bottle that previously collected samples.
TEPCO doesn't say it outright, but instead, from their email alert for the press regarding the pond leak (No. 110) on April 28, 2013:
When we collected the water samples from the drain hole (southwest) of the in-the-ground water storage pond No.1 in the morning and in the afternoon today [April 28, 2013], we used brand-new sampling bottles to avoid contamination of the water collected. Then the amount of [radioactive materials in the water] dropped to the same low level as the one on April 26. Therefore, we now assume the possibility that the water collected on April 27 from the southwest drain hole of the pond No.1 got somehow contaminated.
which I take it to mean they were using used bottles to collect highly radioactive water samples until April 28.
It is little wonder why there is no "end-state" vision for the plant.