Cause unknown, according to TEPCO.
No surprise there.
From Jiji Tsushin (4/28/2013):
Fukushima I Nuke Plant: amount of radioactive materials in the soil [outside] the in-the-ground water storage pond No.1 10 times larger, compared to two days ago
Regarding the leak of radioactive waste water from the in-the-ground water storage ponds at Fukushima I Nuclear Power Plant, TEPCO announced on April 28 that the amount of radioactive materials in the water sample taken from the soil outside the Pond No.1 on April 27 was more than 10 times the amount from two days prior. There are two sampling locations outside the pond, and the location that saw the increase this time had had low amount of radioactive materials. TEPCO says "We don't know the cause. We will continue to watch carefully."
According to TEPCO, it was [the water from] the soil on the southwest side of the Pond No.1 that saw the increase in radioactive materials. 1.1 Bq/milliliter (cubic centimeter) of beta nuclides including radioactive strontium was detected from the water taken from there on April 27. From the same location, it was below detection level on April 26, and 0.099 Bq/milliliter on April 25.
TEPCO's handout for the press on 4/27/2013, showing 1.1 Bq/cm3 of beta nuclides (red rectangle):
Measurement on 4/28/2013 of the same location shows the amount dropped down to 0.074 Bq/cm3 and 0.048 Bq/cm3 (they measured twice, the second time only beta nuclides):
In the meantime, Fukushima Prefecture sent in a group of officials and experts to the plant on April 24 to observe the in-the-ground storage ponds firsthand. They said they were appalled at what they saw. One of the experts, Professor Susumu Nakamura of Nippon University (seismic engineering) said, according to Mainichi Shinbun, "Amateurs. If it were at least 50 to 100 centimeter-thick, the leak of highly radioactive waste water may have been prevented."
Oh but that would cost too much, in a nation whose central bank prints 7 trillion yen (about US$70 billion, at least the conversion calculation is getting easier by the day) each month out of thin air to buy up JGBs from financial institutions to create inflation.
And this thin blue hose is what is transporting extremely contaminated waste water from the Pond No.1 to the metal storage tank(s) (TEPCO will use two, one tank meant for filtered water, and another in the H2 area):
(Photo from Mainichi)