Iodine-131 rises in Chiba prefecture.

The most recent measurements of iodine-131 in sewage sludge from Chiba prefecture indicate the highest amount of I-131 since August. The last measurement was actually made on Dec. 3. It is the third highest measurement since May 2011.

Iodine-131 from Gunma prefecture remained relatively low, as of Nov. 26.

Iodine measurements are affected by wind patterns and rainfall, as well as Fukushima emissions. Chiba is south of Fukushima, and a north wind would blow this way more often in the winter. Gunma is west or southwest of the plant. East winds prevail more often in spring and summer.

Iodine-131 is associated with criticalities. Fukushima Diary has a story about groundwater rising at the plant, and appearing at ground level. Water slows neutrons down and makes criticalities more likely, for any corium on or near ground level.