It is the first time that radioactive strontium was detected from the groundwater at Fukushima I Nuclear Power Plant.
The sub-drain pits are located outside the turbine buildings to collect groundwater so that the basements don't get flooded.
From TEPCO's press release on June 12, English labels added by me (measurement unit: becquerel/cubic centimeter):
In addition, strontium-89 and -90 were detected from the seawater samples taken on May 16 at 3 locations: at the water intake canals for the Reactors 1 through 4, inside the silt screen for the Reactor 2, and inside the silt screen for the Reactor 3.
From TEPCO's English press release on June 12 (measurement unit: becquerels/liter, or 1000 cubic centimeters):
The amount of strontium-90 in the seawater inside the silt fence for the Reactor 3 was 240 times as high as the legal limit allowed for nuclear plant operations.
Note that the sub-drain water analysis is in becquerels/cubic centimeter, and the seawater analysis is in becquerels/liter (1000 cubic centimeter). Obfuscate, obfuscate. I'll translate the numbers for the sub-drains into liters to compare. (Or if someone can do it for me, I'd much appreciate.)