The groundwater sample from the observation hole dug 25 meters from the ground surface in between the turbine buildings for Reactor 3 and Reactor 4 (No. H25J7) was found with:
Cesium-134: 1.6 Bq/Liter
Cesium-137: 2.8 Bq/L
All-beta: 67 Bq/L
after dirt particles were filtered out.
So far, radioactive materials (cesium, all-beta, tritium) have been detected from groundwater samples from the shallower, upper permeable layer. This is the first detection of radioactive materials from the groundwater below the level of the in-the-ground impermeable wall made of waterglass that is still being built closer to the plant harbor.
TEPCO says (handout for the press, 12/20/2013) they don't know whether that means:
The lower permeable layer (25 meters from the ground surface) is contaminated; or
Radioactive materials entered the groundwater when the observation hole was dug; or
The water from the upper permeable layer somehow entered the lower permeable layer; or
The water got contaminated when it was being sampled.
The handout shows the particular location (observation hole No. H25J7) to be close to the turbine buildings of Reactor 3 and Reactor 4:
As far as I'm aware, it is only TV Asahi who covered this news on December 20:
Japanese net citizens on Twitter and message boards who heard about the news (it doesn't look to be many) are all doom and gloom, having already come to the conclusion that the lower permeable layer is contaminated (TEPCO's hypothesis No.1).