The city of Columbus has issued a nitrate alert for residents who get their drinking water from the Dublin Road Water Plant.
Officials say infants and pregnant women should avoid drinking tap water or drinks made with tap water.
Columbus Public Utilities spokesman George Zonders said last week's heavy rains and local flooding are to blame.
"The exact source of the nitrates is difficult to pinpoint, but there are numerous potential sources including lawn chemicals, we do have some home septic systems which potentially overflow, agricultural operations. A number of different sources," Zonders said.
The area affected includes downtown, Upper Arlington, Marble Cliff, Grandview, Grove City, Hilliard, Urbancrest and Lincoln Village.
Zonders said it's difficult to say how long the advisory will last - maybe a few days; maybe a week. He said rain could affect the advisory.
"Additional rain can have the affect of either washing additional nitrates into the system, or it can dilute what's in the system and push what's in the reservoir further downstream more quickly and thus result in a quicker advisory," Zonders said.
Officials caution residents not to boil tap water since that only increases the nitrate levels.
City officials say high nitrate levels occur naturally. They can last a few days or a few weeks.