SANDPOINT — Officials plan to use dye tracing in the Pend Oreille this summer to help dial in the herbicide treatments of Eurasian milfoil.

The project, which is being conducted by the U.S. Army Corps of Engineers, the Idaho State Department of Agriculture and Bonner County, is set to begin Tuesday at the mouth of the Pack River Flats if weather permits.

There are also tentative plans to do dye tracing in the Pend Oreille River at the Long Bridge, Gypsy Bay, Thama and east of Laclede. County noxious weed superintendent Brad Bluemer said a decision will be made next week when the river sites will be traced.

The tracing will be done using rhodamine, a fluorone-based dye which has the ability to stain plants, silt and dirt. Rhodamine WT, product selected for the tracing, ultimately dissolves in the water, according to its manufacturer.

To the naked eye, Rhodamine WT appears fluorescent red.

However, county officials doubt the dye will be visible to people on the water.

“It might be possible in some rare cases. If a person was out there looking real hard you might be able to find it if you knew exactly where it was put,” Bluemer said.

Corps officials will track the movement of the dye using a fluorometer, a device used to measure the parameters of fluorescence.  

Clean Lakes, the contractor hired by the county to apply herbicides on the milfoil infestations, will inject Rhodamine WT into the water using drip lines similar to the ones used to apply herbicides.

The only label restriction requires that applications not exceed 0.1 parts per billion in drinking water.

“We’re not going to be near any water intakes,” said Leslie Marshall, the county’s noxious weed director.

By analyzing the water flow’s speed and other movement characteristics, county officials hope to make herbicide treatments more precise so less product has to be used.

“The whole idea is to be more effective,” said Bluemer.