I am still in the middle of trying to understand the whole picture, but there is a compelling argument (here (link in Japanese, post and comment section) and here) that construction of the impermeable wall along the ocean side, east of the turbine buildings at Fukushima I Nuclear Power Plant, may have caused and/or accelerated the leak of groundwater contaminated with high levels of tritium, strontium, and in one observation hole high cesium.
Construction of the impermeable wall started in early April this year, with steel pipe sheet piles driven into the pre-loosened soil using a vibratory hammer and a hydraulic hammer. Loosening of soil had started in June last year to prep the site for driving the steel pipe sheet piles using high-power hammer, probably to mitigate damage from the impact in the area probably already fragile from the earthquake and tsunami in March 2011.
From TEPCO's Photos and Videos, 4/2/2013:
At the end of April, they started to drive down the sheet piles in the sea right off the Reactor 1 turbine building. In early May, tritium levels inside the port started to increase.
The first section of the impermeable wall was finished by mid June, and that's when the samples taken from the observation holes dug to monitor the groundwater started to show elevated amount of tritium and all-beta including strontium, particularly in the observation holes No.1 (1-1, 1-2, and now 1-3).
From TEPCO's Photos and Videos, 6/26/2013, a section of the impermeable wall near Reactor 1-4 water intake:
From the same document, the location of the photograph in diagram:
The locations of groundwater observation holes, from NRA's document, with the finished impermeable wall marked in green:
I am still trying to understand the argument. I'll update.
But if they are right in their argument, will it be "soh-teh-gai" (beyond expectation) or "soh-teh-nai" (within expectation)? At this point, those who have followed the accident and have been attending TEPCO's press conferences are saying it's the latter.
(H/T TSOKDBA blog with meticulous, detailed study of the Fukushima nuclear accident, and the commenter "inja")