HARBIN, China – Experts warned Tuesday that dangers from a huge chemical spill in this northeastern Chinese city could last for years because of toxins – including cancer-causing benzene – imbedded in ice and mud at the bottom of the Songhua River.

Their concern came as city officials in Harbin and down river in Russia’s Far East, where the 50-mile-long chemical slick was headed, sought to reassure residents their tap water was clean.

“Harbin’s water is now safe to use and drink,” Xiu Tinggong, vice director of the city’s health inspection bureau, said on local state television. “Everybody can rest assured.”

In Khabarovsk, Russia, a top environmental official drank a glass of tap water on television to show his confidence in its purity. Officials estimate the benzene spill flowing from the Songhua into the larger Heilong River, called the Amur in Russia, should reach the border city around Dec. 10 to 12 – or sooner.

Water was shut off for five days in Harbin, the capital of the northeastern province of Heilongjiang famed for its annual winter ice festival, after the Nov. 13 explosion at a nearby chemical plant. The blast, which authorities said killed five people, spewed 100 tons of benzene and related toxins into the Songhua, which passes through Harbin and provides most of the city’s drinking water.

Running water resumed Sunday for Harbin’s 3.8 million people, but many residents said they were sticking with bottled water. In parts of the city, water taps ran dirty.

“We still can’t be sure that it’s safe,” said bank worker Sun Ning as she loaded a shopping cart with bottled water at a supermarket.