(UPDATE: additional information on my new post.)
From TEPCO's press conference on December 18. Water with relatively high surface radiation was found where it was not supposed to be on December 18 between the process main building and the solid waste process building.
First it was announced that 125 tonnes of water was found, but it was soon corrected to 230 tonnes.
The surface radiation of the water "puddle" (as TEPCO calls it) is relatively high 3 millisieverts/hour.
It is actually a deep (50 centimeters) trench for electrical wires found filled with water. It is 4.5-meter wide, 50-meter long, with 3-meter deep pool at the end. Water is 50 centimeter deep.
Water is dripping from a duct (5-centimeter in diameter) for electrical wires inside the trench.
Screen capture of the diagram, from the press conference live stream at TEPCO:
TEPCO says it's either groundwater or dew condensation water. Little possibility that the contaminated water from the building is leaking, but not conclusive until the nuclide analysis is finished.
At the location, the background radiation is 1.7 millisievert/hour.
The trench does not lead to the ocean, so TEPCO does not think the water has leaked to the ocean.
The surface radiation of the highly contaminated water inside the main process building is 1,000 millisieverts/hour (1 sievert/hour).
Water is still leaking into the trench, but TEPCO doesn't know the flow rate yet.
What's the surface radiation of sub-drain water? (from Asahi) - not higher than the background radiation.
The last time anyone checked this trench was in spring.
(TEPCO's press conference room is big, but it is near-empty. Well the accident is "over". Matsumoto isn't even in the room.)