Letters will be mailed this week to customers of Shelby County&8217;s water system notifying residents of a violation of drinking water standards.

Utilities Manager Charles Lay received testing results in December showing the county water system had exceeded maximum contaminant levels for trihalomethanes, a byproduct of the disinfection process.

&8220;This is not an immediate risk to our customers,&8221; the letter reads. &8220;If it had been, you would have been notified immediately.&8221;

However, people who drink water containing excessive amounts of trihalomethanes for many years may experience problems with their liver, kidneys or central nervous system and may have an increased risk of getting cancer, according to the letter.

Lay said the county was &8220;frustratingly close&8221; to meeting requirements and the amount of contaminants causing the violation was &8220;extremely small.&8221;

Tests showed the county water system with an average level of trihalomethanes at 0.083 milligrams per liter in 2006. The standard level for trihalomethanes in drinking water is 0.080 milligrams per liter.

&8220;The violation was 0.003 parts per million,&8221; Lay said. &8220;To give you an idea, this is the equivalent of one second in 10.6 years.&8221;

Still, Lay said the county is not taking the violation lightly and has already begun steps to correct the problem, including changes in the water treatment process. &8220;I do expect this will take care of it,&8221; Lay said