The amount of radioactive iodine in the ocean water is now more than 3,000 times above normal.
Officials said there's still not enough contamination to be an immediate health threat, but fishing in the area has been suspended.
Leaking radiation has seeped into the soil and seawater nearby and made its way into produce, raw milk and even tap water as far as Tokyo.
Nearly three weeks after the disasters, Tokyo Electric Power Co. is still struggling to bring the facility in northeastern Japan under control.
In another effort to reduce the spread of radioactive particles, TEPCO plans to spray resin on the ground around the plant. The company will test the method Thursday in one section of the plant before using it elsewhere, officials said.
The government also is considering covering some reactors with cloth tenting, TEPCO said. If successful, that could allow workers to spend longer periods of time in other areas of the plant.
Japan's emperor and empress visited earthquake and tsunami survivors at one evacuation center on Wednesday.
Hundreds of thousands of people were left homeless by the disasters. The official death toll is 11,257, but that number is expected to rise above 18,000.
The government said damage is expected to cost $310 billion, making it the most costly natural disaster on record.
The Associated Press contributed to this report.