MATTOON, Ill. (WCIA) — The tap water in Mattoon continues to be deemed unsafe for drinking due to algae blooms, which can induce harmful effects on both humans and animals.

Initially, authorities detected the algal bloom toxin at a concentration of 0.35 parts per billion. The latest tests conducted on Saturday revealed the concentration was still present at 0.314 parts per billion. It’s important to note that a “do not drink order” is advised when toxin levels reach 0.3 parts per billion.

Robert Hirschfeld, the Director of Water Policy at Champaign’s Prairie Rivers Network, explained that the toxin in question is called microcystin, and its impacts depend on the amount consumed.

“It can cause skin irritation, nausea, vomiting, in extreme cases even liver failure and death,” Hirschfeld said. “Typically, pets are going to be more susceptible. A person might not drink water directly out of a stream, but a pet might.”

Hirschfeld said he doesn’t know exactly what caused the bloom in Mattoon. He did, however, explain what tends to form algal blooms.

Much of Central Illinois is used for agriculture, and when heavy rainfall occurs — like recent weather — fertilizer runoff can pollute rivers, streams and lakes. 

Agricultural runoff is full of nutrients such as phosphorus and nitrogen; nutrients that blue green algae feed on. 

“You get nutrient loading in the water and nutrients cause that algal bloom,” Hirschfeld said. 

He added that blooms are fairly common, but the situation in Mattoon is rare.

“It’s more uncommon when levels are so high the municipal filtration system can’t filter it out,” Hirschfeld said.

He also noted intense heat and bright sun — a common occurrence this summer — aids in algae’s growth as well. Additionally, he said Lake Mattoon’s design is ideal for fertilizer-induced algae.

“It’s dammed up,” he said. “With the reservoir, that nitrogen and phosphorus can just collect there.”

The Moultrie County Rural Public Water District alerted residents Thursday that any Mattoon water users in the following areas should refrain from using the water:

• Village of Gays
• Southshores
• Lakeside Villa
• South of Bruce Findlay Road
• Coles Station
• Customers along the 1000 N Road

Further updates will be available Sunday, when the newest algae level results will return in the afternoon.