Japan began dumping thousands of tons of radioactive water into the Pacific Ocean on Monday, an emergency move officials said was needed to curtail a worse leak from the crippled Fukushima Daiichi nuclear power plant.

In all, about 11,500 tons of radioactive water that has collected at the nuclear facility will be dumped into the sea, officials said Monday, as workers also try to deal with a crack that has been a conduit for contamination.

The radiation levels were highest in the water from reactor No. 6, the officials said.

These are the latest, but hardly the only challenges facing workers at the embattled power plant and its six reactors, which have been in constant crisis since last month’s ruinous earthquake and tsunami.

Officials with Tokyo Electric Power Company, which runs the plant, proposed the release of excess water that has pooled in and around the Nos. 5 and 6 reactors into the sea. But most of the dumped water — 10,000 tons — will come from the plant’s central waste treatment facility, which will then be used to store highly radioactive water from the No. 2 unit, an official with the power company said.

The water in reactors Nos. 5 and 6 is coming from a subdrain and wasn’t inside the building itself, officials said. Tests suggest that groundwater is the source of the contamination in these two units, but they are not certain.

Japanese Chief Cabinet Secretary Yukio Edano called the dumping “unavoidable.” The liquid was most likely contaminated in the process of trying to cool nuclear fuel rods.

Though Japanese officials say the water being discharged is less radioactive than the water now leaking into the sea, its top concentration of radioactive iodine-131 is 20 becquerels per cubic centimeter, or 200,000 becquerels per kilogram. That’s 10 times the level of radioactivity permitted in food. But since it’s being dumped into the Pacific, it will be quickly diluted, according to Dr. James Cox, a radiation oncologist at Houston’s MD Anderson Cancer Center and a CNN consultant.

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