Alberta Health Services says one of four dialysis patients treated with contaminated water in an Edmonton hospital last week has died.

AHS says it is not known if the patient's death on Sunday was connected to the water problem two days earlier at Royal Alexandra Hospital.

The health agency says another patient remains in hospital, one has been moved off the acute-care unit and another has been discharged.

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AHS says a wrong valve was opened during regular system cleaning and a chemical was released into the water used for dialysis.

The error was discovered during monitoring and the dialysis water supply was immediately turned off.

AHS says portable dialysis was used while the water was flushed and cleaned.

"Our heartfelt condolences go out to the family of the patient who died," Vickie Kaminski, president and CEO of Alberta Health Services, said in a release Tuesday.

"We apologize that this incident occurred. It should not have happened and we will do all we can to support the patients' families and loved ones through this difficult time."

Kaminski said AHS is installing extra lockouts on valves, improving valve indicators and using better signage to ensure the water supply is protected from contamination.

It is also conducting a review to identify ways to prevent something similar from happening again.