SAN DIEGO -- Hundreds of birds have turned up dead in a major drinking-water source that serves a large portion of San Diego County, but water officials claim the water is OK to drink.

The Sweetwater Reservoir provides water for 186,000 South Bay residents, but it is possible the water could be contaminated with avian botulism.

Only a handful of Western and Clark's Grebe remain after almost the entire population of the water bird on the Sweetwater Reservoir was wiped out.

"There is always some mortality, just not these levels," said Don Thompson from the Sweetwater Authority.

Thompson does not know the reason for the increase, but in the past week crews have collected about 500 Grebe from the reservoir. He said if it is avian botulism, it is not harmful to humans.

"We are still making that determination; we don't know if it is avian botulism for sure," said Thompson.

Thompson said bacteria that causes avian botulism is found in the deepest parts of lakes where there is no oxygen.

"They can't survive in the presence of oxygen, which our water would have oxygen in it," he told 10News.

Neighbors like Lalane Haenish were clueless about the contamination, and she told 10News, "I'd like to know more about it, but it doesn't scare me right off."

The Sweetwater Authority cut off the water supply from the reservoir last Wednesday. Its customers in Chula Vista, National City and Bonita are now getting water from the San Diego County Water Authority.

The preliminary results for the cause of the contamination are due out Friday.

Officials have ruled out the West Nile virus as the cause of the bird deaths.

If it is avian botulism, officials said it is safe to reopen the reservoir. If it is not avian botulism, it will remain closed until further notice.