The operator of Japan’s damaged Fukushima nuclear plant said Monday that it had detected a rise in levels of radioactive tritium in seawater near the plant, a sign that contaminated groundwater may be seeping into the Pacific Ocean. The operator, the Tokyo Electric Power Company, said a sample taken on Friday near the docks that serve the Fukushima Daiichi plant contained 1,100 becquerels of tritium per liter, or about a quart, of water. While that is well within Japan’s safety level, it was more than double the amount detected at the same location two weeks ago and the highest since the accident in 2011, the company said.