After the high radioactive water was found in the turbine buildings at Fukushima I Nuke Plant, TEPCO has been testing ground water by testing the water from the Reactors' sub-drain pits.
Radioactive materials measured in the sub-drain pit and which TEPCO is ALLOWED (by Nuclear and Industrial Safety Agency) TO DISCLOSE (iodine-131, cesium-134, -137) have risen materially in the Reactor 1 since April 6, particularly cesium. The result for the Reactor 2's sub-drain pit is also high.
Expect an announcement a month from now that they also found a host of other radioactive materials (like plutonium and strontium).
From TEPCO's press release on April 8 for the tests done on April 6 (top table), and TEPCO's press release on April 14 for the tests done on April 13 (bottom table):
This map shows the locations of the sub-drain pits for each reactors (marked in yellow) and the location of the deep well (blue arrow) (the column in the far right in the tables). The radioactive materials in the Reactor 1 sub-drain pit may have come from the Reactor 2.