At the Fukushima Daiichi nuclear plant, nothing is more problematic than the contaminated water that covers the basement floors, leaks into the environment and endangers any worker who goes near it.

After dousing its reactors for 21 / 2 months in jury-rigged cooling efforts after the March 11 earthquake and tsunami, the Tokyo Electric Power Co. (Tepco) must deal with the side effects of that strategy by removing at least 26 million gallons of water — enough to fill the first five floors of the Empire State Building.

But engineers planning that unprecedented cleanup job face questions about where they will put the water and how effectively they can filter its radioactive particles.

Tepco’s problem “resembles a board game with 16 squares and one empty spot,” said David Lochbaum, a nuclear engineer with the Union of Concerned Scientists.

Workers must inject the reactor cores with water to keep them cool. But that step guarantees that water will leak through the quake-damaged plant and into the basement-level turbine rooms. The resulting radioactive water makes repair work all the harder. That means workers, still struggling to fix the usual recirculation system, must continue to “feed and bleed” the reactors from above.

And that means water levels continue to rise down below.

“They’re just perpetuating the problem and making a bigger and bigger mess,” said Lake Barrett, a nuclear engineer who directed the cleanup of the hobbled Three Mile Island plant in Pennsylvania.

A potential turning point may come in about two weeks, when Tepco plans to begin a treatment process in which water will be sucked from the basement rooms and fed into a tank, then treated with chemicals that eliminate radioactivity. The process will create a byproduct of radioactive sludge, which is generally mixed with bitumen, poured into drums, sealed and buried. The water can be cycled back into reactors or discarded into the ocean.

The treatment system is being set up by Areva, a French company that uses the technology at its La Hague nuclear reprocessing plant off the Normandy coast. Since 1997, Greenpeace — after taking water samples from La Hague’s discharge pipe — has repeatedly asserted that the supposedly decontaminated water contains radioactivity levels above the regulatory limit.

The process “is not 100 percent, but it’s better than nothing,” Lochbaum said. “The alternative: You let the water simply evaporate, and radioactivity carries to all parts far and wide.”

Japan has experienced substantial environmental problems from the failure at Fukushima, with authorities at the plant discharging contaminated water into the Pacific Ocean on at least three occasions. During a visit to Japan last week, Greenpeace officials presented data showing higher-than-legal radiation levels in seaweed and shellfish that were collected more than 12 miles from the plant. The samples’ high concentrations of iodine-131 — which has a half-life of eight days — indicated that leaks from Fukushima Daiichi were ongoing and “much larger than has been declared by Tepco so far,” said Jan Vande Putte, a Greenpeace radiation expert.

In recent days, Japanese government authorities have voiced similar concerns and complained about what they say is Tepco’s lack of transparency. On Monday, Tepco released its first detailed map of water leakage at the plant, with diagrams of nine separate areas, including the turbine buildings for each of the plant’s six reactors.

In some places, according to the maps, contaminated water is ankle-high. In other areas, it’s nearly seven feet deep. The unit 4 turbine building is almost entirely underwater. Some of Tepco’s data are more than a month old, and accurate readings are hard to obtain from the most dangerous areas. Still, Tepco’s readings indicated that in some areas airborne radiation levels were once at 1,000 millisieverts per hour. A worker could stay in such an area for 15 minutes before reaching his or her annual dosage limit.

Tepco is having difficulty securing areas to store the radioactive water — more than 100,000 metric tons of it, according to the latest estimates. Last month, workers installed tanks capable of holding 6,400 and 6,200 metric tons. A seaborne storage tank hauled away about 10,000 metric tons in the middle of the month. Engineers are also trying to use zeolite, a mineral, to absorb radiation from the water.

The onset of Japan’s rainy season this month has prompted concerns that contaminated water levels could rise, with overflow spilling into the environment. Experts have also said there is a risk that water seeps through potential cracks in the floors of the Fukushima facility, contaminating the soil.

Under normal circumstances, Areva’s system can decontaminate 50 tons of water per hour. But experts acknowledge that it is hard to predict how efficiently the system will handle water that contains not only radioactivity but also debris, oil and salt. Water might need to be treated numerous times, not just once, before it can be dumped into the ocean.

“Normally, the processing is done at small volumes and you have carefully controlled chemistry,” Barrett said. “Here you have massive volumes and a very heterogeneous chemistry.”

“Honestly, it’s hard to say how it will work,” said Patricia Marie, an Areva spokeswoman. “We hope everything will be fine.”

Special correspondent Akiko Yamamoto contributed to this report.