Tokyo Electric Power Co. on Wednesday started pumping groundwater into tanks before it passes through the crippled Fukushima No. 1 nuclear plant and gets highly contaminated.

Pumped from wells, the water will first be stored in tanks, where its level of contamination will be checked. Once its safety is confirmed by analysis, which is expected to take about a month, Tepco will release the water into the Pacific Ocean.

The so-called groundwater bypass system is a key measure to slow the pace of increase of highly radioactive water accumulating at the plant. Local fishermen have OK'd the dumping of groundwater on condition that it passes stringent safety criteria.

About 400 tons of groundwater a day is seeping into the basements of reactor buildings and mixing with water used to cool three reactors that suffered meltdowns in the 2011 disaster, adding to the total volume of highly radioactive water at the plant.

Although Tepco is installing more tanks to avoid running out of storage space, it also wants to stop the total volume of radioactive water from further increasing. With the groundwater bypass system, Tepco says it can reduce the amount of water seeping into the reactor buildings by up to 100 tons per day.