(UPDATE) TEPCO Nuclear's tweet from 30 minutes ago says they finished the release at 12:42PM. 561 tonnes in total.
Again, the groundwater is NOT highly radioactive, as it is drawn before it enters the reactor buildings.
Cesium-134: 0.022 Bq/L
Cesium-137: 0.039 Bq/L
All alpha (including plutonium): ND
All beta (including strontium): ND
Strontium-90: 0.011 Bq/L
Tritium: 230 Bq/L
Having secured the consent from the Fukushima fishermen with the help from the national government (Ministry of Economy, Trade and Industry, which is still in charge of oversight), TEPCO has started releasing the groundwater drawn from the wells before the water reaches the reactor buildings and gets contaminated heavily.
According to TEPCO's alert for the press (5/21/2014),
Release of the groundwater started at 10:25AM on May 21, 2014. The groundwater had been drawn from the groundwater bypass wells and stored in the temporary storage tank.
About 560 tonnes of water will be released today.
The area patrol was done at 10:30AM, and we confirmed there was no leak or other abnormalities [along the pipes].
NHK says the release is from the South Drainage Outlet, which is located south of the plant harbor. That means the water is directly going into the ocean.
As to the contamination levels of the water (which was drawn in April), TEPCO released the nuclide analysis done by both TEPCO and a third-party laboratory (Japan Chemical Analysis Center), as per agreement with the fishermen in Fukushima.
The result of nuclide analysis of the groundwater, from TEPCO (5/14/2014; English labels are by me):
(click to enlarge)
Tritium, measured by Japan Chemical Analysis Center, is 230 Bq/L. The rest of radionuclides, including alpha nuclides, are in negligible amounts or ND.
From my post on 8/21/2013, locations of the wells to draw groundwater (map from TEPCO, annotation by Kontan_Bigcat):
The wells in dark blue circles, pipes to transport the water to the ocean in yellow.
The red circle above marks the H4 tank area where highly contaminated (beta nuclides) wastewater (after reverse osmosis treatment) was found leaked in August 2013.
You can also see the South Drainage Outlet in the upper right corner.