ELKHART, Ind. (AP) — Dentists' offices in Elkhart will be asked to use amalgam separators to capture mercury used in some fillings as the city works to reduce levels of the metal in wastewater discharged from its treatment plant.

The Indiana Department of Environmental Management notified the city in a Dec. 31 letter that average mercury levels for June, August and October exceeded the allowable limit of 1.6 nanograms per liter. A nanogram is one billionth of a gram.

The 1.6-nanogram limit took effect in June and is a sharp drop from the previous limit of 120 nanograms per liter, The Elkhart Truth reported.

Laura Kolo, utility services manager for the Elkhart public Works and Utilities Department, said the city must act on the violation of the more stringent limit and will focus on dentists' offices because Elkhart doesn't have any industrial operations that could be behind the mercury.

She noted in a response letter to IDEM that a 2002 report had found dental clinics are the primary source of mercury emissions at public wastewater treatment plants.

The amalgam separator program will be voluntary but could become mandatory if there is low compliance, Kolo said.

She said she hopes to have a formal treatment program drafted by late June.

Kolo said the highest mercury reading was 4.4 nanograms in a June 2013 measurement.

Mercury can impair neurological development in infants and children.