Ojika Peninsula is located in the northeast part of Miyagi Prefecture, where towns like Ishinomaki City and Onagawa-machi are located (see the map at the bottom of the post). Tokyo Municipal government has been importing the disaster debris from Onagawa and burning it in the regular garbage incinerators (many of which are located in the middle of residential areas) all over the central Tokyo since March 2012.
The Tokyo University researcher who was the first to measure Neptunium-239 in Iitate-mura in early April 2011 says he thinks he has been able to identify the radiation contamination in Ojika Peninsula to have happened on March 12, 2011, from Reactor 1.
Radioactive materials such as iodine-131, cesium-137 and tellurium-132 started to leak hours before the Reactor 1 venting operation in the early afternoon and the hydrogen explosion in late afternoon on March 12, 2011.
From Dr. Shouzugawa's tweets to other researchers who were discussing the Fukushima nuclear accident and the resultant contamination in various locations in Kanto and Tohoku:
I have obtained the result that proves the contamination in Ojika Peninsula is from Reactor 1 on [March] 12. The paper is currently in review process. I have also been trying to attribute contamination in various parts in Fukushima to Reactor 2 and Reactor 3, but this is not yet complete.
The activity ratio of nuclides in the contaminated water taken from the Reactor 1 building matched that in the soil sample taken in Ojika Peninsula, and this activity ratio is distinctively different from the activity ratios for the other reactors.
22 months after the start of the accident, painstakingly slowly, the picture may emerge of the radiation contamination mechanism. About time, but it is exciting nonetheless.
I'm searching for the available data at TEPCO's site, but if you want to beat me to it, feel free and post the links in the comment section. (It is possible though that the Reactor 1 data is only available in detail to researchers.)
(Let's see, who was that university professor who claimed there was no radiation contamination in the disaster debris from Ojika Peninsula...?)
From Professor Yukio Hayakawa's Radiation Contour Map Version 7, location of Ojika Peninsula:
Professor Hayakawa puts contamination of Ojika Peninsula just past the midnight, at 1:50AM on March 13, 2011 (according to the backside of the Version 7 map).
That's when the radiation measured at Onagawa Nuclear Power Plant jumped to 21 microsieverts/hour.