As groundwater has been pumped up via the outdoor subdrain pits, this has created a situation where the standing water in the basements could end up higher than the surrounding groundwater.
TEPCO is making an effort to lower the water level in some of the basement rooms of the unit 1 reactor buildings.
This work also showed that much of the water still sitting in certain areas of the basements of unit 1 is highly radioactive as seen in this table below.
TEPCO is installing more water level monitoring sensors to better control the water in the basements.
This could lead to more contaminated water leaking out of the buildings.
Source: Tepco handout