HANNIBAL, MO. -- Some Hannibal residents were surprised to find a notice from the Board of Public Works in the mail Tuesday.

Click here to read the letter.

It's a letter regarding a violation of Missouri DNR drinking water standards that was discovered after testing Hannibal's drinking water back in September. But officials say this is nothing new and not an emergency.

They say it's the second notification since the city discovered some areas had a high amount of disinfection byproducts.

The byproduct is formed when chlorine combines with organic materials in the water. It can lead to health problems later in life if consumed over a long period of time.

The Board of Public Works is already working on removing the byproduct from its system.

"We changed up some chemicals at the water plant, per the direction of Missouri DNR. We talked to them over the phone about it. We changed our aluminum at the water treatment plant, feeding it a different kind that removes the disinfection byproducts. We also changed the carbon at the plant and we've seen promising results of both of those," Heath Hall with the Board of Public Works said.

Because the tested sample showed such high levels of the byproduct, the city of Hannibal automatically receives a violation for the entire year, or all four quarters.

This means residents will receive another notice like this one in the mail come July or August.