Almost a million people are still waiting for clean tap water in the Vancouver area, and officials have shut off supply from one of three reservoirs to speed up disinfection.

Meanwhile, health officials said it could take until Friday to confirm possible E. coli contamination. A water sample taken at the University of British Columbia tested positive for the harmful bacteria.

Because the water had a high level of chlorine that would have killed E. coli, it's possible the sample was contaminated by the tester. One official believed a nearby compost site affected the water.

"(The test water) was sitting low, and unfortunately some water run-off from a compost pile that was near it came over, and had been sitting around the water test station," the university's Gordon Apperley told CTV Vancouver.

About 1,000 tests have been taken since the boil-water advisory came into effect last Thursday, and not a single person has become sick.

High turbidity levels - the amount of cloudiness in the water, caused by silt - prompted the advisory after torrential rains. The turbidity makes it difficult for officials to effectively chlorinate the water.

That advisory remains in effect for the City of Vancouver, North Vancouver, Burnaby and parts of West Vancouver. In those communities, boiled water or bottled water should be used for drinking, brushing teeth and washing food.

The advisory will end when Greater Vancouver Regional District (GVRD) officials determine the turbidity levels are low enough.

Most people in the Vancouver area get their water from the Capilano, Seymour and Coquitlam reservoirs.

Turbidity levels in Capilano's water supply were dropping so slowly that officials took it off line, although the gradual addition of clean water is helping the situation.

Levels are also changing slowly in Seymour's water, but the supply has more exposure to chlorine.

"We have about 45 minutes in the Capilano system for the chlorine to work on those bugs before you hit the first customer, whereas in the Seymour system we have about three hours," Paul Archibald, water supply operations manager for the Greater Vancouver Regional District, told The Canadian Press.

Vivianna Zanocco of the Vancouver Coastal Health Authority said Seymour is also being helped by the gradual addition of fresh water.

"What we're seeing in Seymour is that the turbidity at the lower levels is quite high, but at the upper levels, where there's new water coming in either from rain or from fresh streams, it's actually very clean," she told CTV Vancouver.

Officials said it could take at least another week to clean the Seymour reservoir, and several weeks to clean the Capilano reservoir.

With a report by CTV Vancouver's Jill Bennett and files from The Canadian Press