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Developing Total Phosphorus Mass-Balance Models for the Lower Boise River

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Under the authority of the Clean Water Act, to protect beneficial uses for the lower Boise and Snake Rivers, the Idaho Department of Environmental Quality (IDEQ) has established a maximum phosphorus concentration target of 0.07 milligrams per liter (mg/L) at the mouth of the Boise River near Parma, ID. In a previous study, our scientists determined that during certain times of the year, phosphorus concentrations in the Boise River exceed that target.

Resource managers must understand how concentrations of phosphorus in the Boise River vary seasonally in response to changes in phosphorus sources. Groundwater, in particular, is a source of phosphorus that is poorly understood in the lower Boise River watershed.

How the USGS Helps

Discharge water from non-point source to the Boise River
Discharge water from non-point sources
to the Boise River, photo USGS
In August and October 2012, and March 2013, in cooperation with IDEQ, we collected water-quality samples, and evaluated the alluvial groundwater interaction at sites along the lower Boise River between Veterans Memorial Parkway and the mouth of the river near Parma, ID. We analyzed the samples for total phosphorus, total dissolved phosphorus, dissolved orthophosphorus, and suspended sediment concentrations, some of which had been collected from wastewater treatment facilities and fish hatcheries that discharge directly to the Boise River.

When combined with the streamflow discharge measurements that were collected at the same time, these “synoptic” sampling events provided basin-wide “snapshots" of water-quality conditions during key times of the year and under various hydrologic conditions.

Using data from the synoptic sampling events, we developed three simple mass-balance spreadsheet models—one for each of the sampling events. Once the models are calibrated to fit the synoptic data, resource managers can use them to assess how phosphorus concentrations in the Boise River will vary in response to particular management practices designed to reduce phosphorus loading to the river. The models can then be used to establish reduction targets for different phosphorus sources with an overall goal to achieve the phosphorus concentration target of 0.07 mg/L at the mouth of the Boise River.

Products

Etheridge, A.B., 2013, Evaluation of total phosphorus mass balance in the lower Boise River, southwestern Idaho: U.S. Geological Survey Scientific Investigations Report 2013–5220, 70 p.