Japan's Environment Ministry says tritium in seawater off the crippled Fukushima Daiichi nuclear plant remains below the detectable level.

The ministry on Wednesday released the results of its eighth weekly survey of water samples. It has been carrying out the surveys since Tokyo Electric Power Company started releasing treated and diluted water from the plant into the ocean on August 24.

In the latest survey, the ministry took samples from 11 points off Fukushima, Miyagi and Ibaraki prefectures on October 17 and 19. The locations include one near the outlet of the water discharge, and another about 70 kilometers away.

The ministry said analyses at a research lab found that the concentration of tritium was below the minimum detectable level of 10 becquerels per liter at all collection points.

In the previous seven surveys, the concentration of tritium was also below that level. The results of the first survey were released on August 27.

The ministry plans to keep monitoring tritium levels once a week for the time being and publish the results on its website and social media.

The Fukushima Daiichi plant suffered a triple meltdown in the 2011 earthquake and tsunami.

Water used to cool molten fuel there has been mixing with rain and groundwater. The accumulated water is being treated to remove most radioactive substances, but still contains tritium.

Before releasing the treated water into the sea, the plant's operator dilutes it to reduce tritium levels to about one-seventh of the World Health Organization's guidance level for drinking water.