From Jiji Tsushin (10/28/2011):
On October 27, the Institute for Radiological Protection and Nuclear Safety (IRSN, Institut de Radioprotection et de Surete Nucleaire) of France announced its research report in which the researchers estimated the total amount of radioactive cesium-137 leaked from Fukushima I Nuclear Power Plant into the ocean was 27,100 terrabecquerels from March 21 to mid July. The IRSN estimate is 20 times as much as the estimate announced in June by TEPCO.
According to IRSN, it is the largest amount of radiation leak in a single accident. 82% of the leak took place by April 8, because of the spraying of water to cool the reactors.
The research paper (in French) is here.
(I don't have energy to look for TEPCO's estimate. But here's the paper (English) by Japan Atomic Energy Agency that put its estimate as 4 times as much as TEPCO's for cesium-137.)
Early April was when someone finally noticed the highly contaminated water from Reactor 2 had been leaking through the "crack" near the water intake canal. Remember the "tracer" (bath salt) to identify the location of the leak? Ah the good old days...