ELKHART, Ind. (AP) — Elkhart officials ordered to reduce mercury levels in wastewater discharged from the city's treatment plant are focusing their efforts on dentists' offices.

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 Elkhart Truth reports the 1.6-nanogram limit took effect in June. It's a sharp drop from the previous limit of 120 nanograms per liter.

Utility services manager Laura Kolo says the city will ask dentists to start using amalgam separators to capture mercury that's used in some fillings. She says the program will be voluntary but could become mandatory if there is low compliance.