The water was from Reactor 6 turbine building basement - underground water seeping into the turbine building. The contamination level is low, according to TEPCO's handout on February 18, 2013. After desalination treatment, this water is being sprinkled throughout the compound. It's also the same water that Goshi Hosono's sidekick drank, back in October 2011, to prove how safe it was.
From TEPCO's photos and videos page, 2/18/2013:
Date and time: At around 7:36 PM on February 16, 2013
Location: Water intake tank which is a part of the desalination
system installed outside of Units 5-6
A cooperative company worker found water overflowing from
the water intake tank which is a part of the desalination system
installed outside of Units 5-6. Though the leaked water has
absorbed into the ground (gravel), it has not flowed out to the
ocean considering that there is no side ditch, etc. in the
surrounding area. The amount of leaked water is evaluated to
be approx. 19.8m3.
Radioactivity analysis results
Cesium 134: 6.8×10-2Bq/cm3
Cesium 137: 1.3×10-1Bq/cm3
All γ: 2.0×10-1Bq/cm3
A small photo that the workers took of the water intake tower shows tubes and pipes reinforced by what looks like duct tape. The location of the leak is circled in red.