On August 22, 2013, TEPCO sent out workers to do the inspection of the 1,000-tonne steel tanks that holds waste water after the RO (reverse osmosis) desalination treatment. The workers did not find any new leaks or signs of past leaks in the H4 area (where the tank No.5 was found with the water level 3.4 meters lower than it should be), but they did find two spots with high beta+gamma radiation levels.
There was no sign of leak on those spots, and the water on the concrete floor turned out to be rainwater (nothing beyond background radiation).
From TEPCO's handout for the press, 8/22/2013:
Location of H3 area:
Half the tanks in H3 area are for the uncontaminated (maybe not for long, but) groundwater drawn for the groundwater bypass plan, and the other half are for RO waste water (tank group H3-A and H3-B). Two locations with high radiation levels are indicated:
The radiation readings of 100 millisieverts/hour and 70 millisieverts/hour are beta and gamma radiation combined, and "dose equivalent". They were measured at about 50 centimeters off the concrete floor of the tank area, but translated into "dose equivalent at 70 micrometers" (70μm線量当量) to express the radiation effect on skin and crystalline lens。 (1 micrometer is 0.001 millimeter, or 1/million meter.)
There are over 300 tanks of the same construction. The concrete platforms on which the tanks are placed look like they were simply poured to 200 millimeter (20 centimeter) thickness. No information whether the soil below the concrete had been amended.
From TEPCO's presentation to NRA Working Group, 8/21/2013, page 10:
The tanks in one group (5 tanks per group) are connected to each other via the pipe like this at the bottom. Thus TEPCO's rationale that only one water gauge was necessary to monitor the water levels of all 5 tanks:
The entire tank areas are on a hill, overlooking the reactor buildings, for those who fear the next tsunami would wipe them out.