A report has been sent to prosecutors after 6,000 homes and businesses in North Lanarkshire were told not to use their water.
Customers were warned not to cook, drink or wash with tap water following concerns over its taste and smell.
The Drinking Water Quality Regulator has been investigating the problem which closed nine schools in June 2015.
The affected properties were in parts of Carfin, Newarthill, Chapelhall, New Stevenston and Dalziel Park.
Restrictions were lifted two days after customers first complained of an “oily based substance” with a strong odour in their water.
According to the BBC, in her annual report, Sue Petch, drinking water quality regulator for Scotland, said: “One major incident affecting drinking water quality occurred during June 2015 when over 6,000 properties in North Lanarkshire suffered restrictions on the use of their water for two days after contamination of the supply.
“We have investigated this event thoroughly and submitted a report to the procurator fiscal.”
The BBC reported that a spokesman for Scottish Water said: “Providing safe, wholesome drinking water to our five million customers is Scottish Water’s top priority.”
The report showed drinking water standards reached a record high in 2015.
Out of 308,356 samples of water taken from consumers’ taps, 99.92% met the required standards.
Of the 35 incidents that were investigated, only the case in North Lanarkshire was classified as major.
Ms Petch said: “Consumers in Scotland are generally more satisfied with the taste and appearance of their tap water.
“Last year, only 0.2% of consumers reported concerns with the quality of their supply, almost half that of numbers reported six years ago.
“When concerns are reported, they are thoroughly investigated and, where necessary, action is taken to resolve the situation.
“It is vital that the progress that has been made is maintained and further built upon.
“Scottish Water must ensure their assets and procedures are resilient and able to cope with a wide range of challenges.”