Tokyo (CNN) -- A top Japanese official apologized Tuesday for the dumping of 11,500 tons of radioactive water into the sea, acknowledging the toll the ordeal has taken on fishermen but deeming it a necessary evil to avoid even more problems, and contamination, outside the embattled Fukushima Daiichi nuclear facility.

The process, which began Monday, of expelling contaminated water into the Pacific Ocean will take five days, according to an official with the Tokyo Electric Power Company, which runs the plant. Authorities explained they were quickly disposing of water, that had been in the plant's water treatment facility and around four of its six reactors, to alleviate worse issues such as the excessive leaking of especially dangerous liquid out of the No. 2 unit's turbine building.

"The water contains a high level of radiation," Chief Cabinet Secretary Yukio Edano said of the liquid being dumped. "This is a regret. We are sorry for this decision we have to make."

The most contaminated batches of this water comes from outside the No. 6 reactor, likely having gotten in via groundwater (and not a breach in the unit itself), officials said. It has a concentration of iodine-131 that would be 100 times more than the maximum amount of tapwater that infants could drink, and 10 times more than what would be OK in food. Overall, the dump equates to about 3 million gallons, notes Gary Was, a nuclear engineering professor from the University of Michigan.

Yet Hidehiko Nishiyama, an official with Japan's nuclear and industrial safety agency, said, "We've decided that discharging the contaminated water into the sea poses no major health hazard."

Experts say that this is a fair assessment, given the likelihood the contamination should quickly dilute and especially if the tainted material is largely iodine-131, which loses half its radiation every eight days. Last week, well before the dump, this isotope was measured at 4,385 times the regulatory limit at monitoring posts 330 meters (361 yards) away from the plant.

"To put this in perspective, the Pacific Ocean holds about 300 trillion swimming pools full of water, and they are going to release about five swimming pools full," said Timothy Jorgensen, chair of the radiation safety committee at Georgetown University Medical Center. "So hopefully the churning of the ocean and the currents will quickly disperse this so that it gets to very dilute concentrations relatively quickly."

John Till, president of Risk Assessment Corp., said he does not expect to see any permanent effects on marine life, even close in to the plant. However, he added that officials should continue to monitor the radiation levels closely -- both in the ocean as well as in seafood that reach restaurants and markets.

"What we have to watch is how these materials accumulate in food products and then could be consumed by people," Till said.

During Tuesday morning's press conference, Edano acknowledged reports that young sea eels may be adversely impacted by the radiation spike. He also apologized to those in the fishing industry, who have been told not to fish within 20 kilometers (12 miles) of the stricken nuclear plant -- not to mention been forced to deal with blowback from consumers, fearful anything from seawaters off Japan isn't safe.

To address such concerns, Edano said Tuesday that authorities will step up their monitoring of radiation in ocean water and seafood. Japanese authorities have not imposed any bans on seafood due to radiation fears, unlike prohibitions in place on certain leafy vegetables, milk and other items farmed in prefectures near the nuclear plant.

"We will continue to intensify monitoring," said Edano, the government's point-man on the nuclear crisis. "We urge the public to act calmly, and not based on unfounded rumors."

One piece of good news, according to Japanese government reports, is that airborne radiation appear to be steadily falling around northeast Japan. Also, Tokyo Electric officials have described conditions recently in the Fukushima Daiichi plant's six reactors and spent nuclear fuel pools as generally stable, with seemingly slim prospects of any more explosions or a sudden and dangerous overheating of nuclear fuel.

That said, the situation with water gushing directly into the Pacific Ocean from a cracked concrete shaft outside of the No. 2 unit has been described as urgent. Water there measured at least 330 times more radiation than a person in a developed nation naturally ingests in a year, although it likely was worse because the dosimeters being used maxxed out with the 1,000 millisieverts per hour reading.

In fact, Edano said Monday that the decision to dump tainted water from other reactors and wastewater treatment facility was "unavoidable" in order to ensure "the safety" of the No. 2 reactor core.

Getting rid of the less contaminated water elsewhere will open up space for liquid from the No. 2 unit in the treatment facility.

The idea is to expeditiously pump the tainted water from the No. 2 reactor's turbine building, lowering levels inside so water no longer gushes out and reaches the sea, a Japanese nuclear safety official said. This is after two tries to plug the crack through which the water is escaping -- on Saturday by pouring in concrete, then Sunday using a chemical compound mixed with sawdust and newspaper -- both failed.

Officials were still awaiting test results to confirm the water pouring into the ocean is leaking from the highly radioactive No. 2 reactor.

"We don't know clearly, but we feel it is somehow leaking from Unit 2," Nishiyama said. Even if the water is confirmed to have come from the reactor, neither Tokyo Electric nor government officials know how it is making its way from the reactor to the leaking pit, he said.

Once the water is pumped out of the waste treatment reservoir, the agency believes it can safely transfer the water from the basement of the No. 2 turbine plant to the reservoir without further leaks, he said.

Reactors No. 1 and No. 3, which have lower levels of water, need to be drained as well. Tokyo Electric's plan is to pump that water to other storage tanks, including some that still need to be set up. Water in and around the Nos. 5 and 6 reactors will be jettison directly in the sea, officials have said.

In the sea itself, workers plan to install a silt fence along a damaged sea wall surrounding the plant, Nishiyama said. The aim of this screening, usually used to halt erosion at construction sites, is to prohibit the spread of radioactive particles into the sea.

Workers also have put a dye tracer into the water to allow them to track the dispersal of such particles, the nuclear safety added.

CNN's Matt Smith, Tsukushi Ikeda, Yoko Wakatsuki, Junko Ogura, Midori Nakata, Susan Olson and Martin Savidge contributed to this report.