Nearly 1,000 South Lake Tahoe customers of the Lukins Brothers Water Co. are advised to boil their tap water or buy bottled water this weekend.

The system was put under a boil order Thursday after routine water samples tested positive for coliform bacteria. The order is in effect until further notice and applies to all of the company’s 963 customers.

People affected by the order should either boil their tap water for at least one minute before using it for drinking or cooking or buy bottled water to use. Failure to follow those precautions could result in stomach or intestinal illness, the company warned.

Coliform bacteria were detected in a “grab sample” from the company’s water system. The samples came from the Tahoe Island Drive area, according to company spokeswoman Jennifer Lukins.

Such samples are pulled from various parts of the system each month for water quality testing.

“It came from an isolated area in the system and we are aggressively researching what caused it to prevent it from happening again,” Lukins said.

The company issued the boil order as a safety precaution for customers. Lukins said it’s the company’s first positive test for total coliform.

Coliform bacteria are common in soil and surface water and generally harmless. They are used as an indicator for the potential that other harmful organisms may have entered the water system, according to the U.S. Environmental Protection Agency.

Within an hour of learning about the positive coliform test from California Department of Public Health the company started quarantining and aggressively chlorinating the water system, Lukins said.

Boil order notices were posted on the company’s website and orange flyers were put on customers’ front doors.

The focus now is on finding and fixing the problem and getting a negative total coliform test from the state health department. Lukins said the company hopes to have the water system back to normal within one week.