CALGARY, Alberta (Reuters) - A spill of contaminated water in the Athabasca River from Suncor Energy Inc's oil sands operations last month was not a threat to human health, Alberta's Environment Department said on Friday, though the water did contain higher-than-allowed amounts of some metals and other compounds.

The department said the water, which spilled into the river after a pipe froze and broke on March 25, was fatal to rainbow trout in a 96-hour test because of high concentrations of naphthenic acid.

While it is unlikely to pose a health risk to downstream communities, Alberta is still studying the effects of the spill after it was diluted with river water.

Suncor, Canada's No. 1 oil producer, and other oil sands companies store contaminated water, a byproduct of stripping tar-like bitumen from the sands, in holding ponds.

Those ponds became the focus of environmental protests in 2008, when 1,600 ducks died after landing on a tailings pond operated by Syncrude Canada Ltd.

While new regulations introduced after the mass duck deaths aim to eliminate the toxic ponds, they remain controversial because of the risk of spills into the Athabasca River, which runs through the heart of Alberta's oil sands mining region.

The tests showed the undiluted water contained higher than acceptable levels of chloride, ammonia, arsenic, boron, cadmium, selenium and zinc. However the department added that these were present only at trace levels and are not expected to harm fish and other aquatic life.

(Reporting by Scott Haggett; Editing by Bob Burgdorfer)