Monitoring Shows Progress Towards Reducing Boise River Phosphorus Levels

By Lauren Perreault, USGS

For decades, the lower Boise River downstream of Lucky Peak Reservoir has been highly enriched with phosphorus. Too much of a good thing, the high concentrations of phosphorus create a cycle of excessive plant growth, decreased oxygen for fish, and even algal blooms. But things may be turning around. Water-quality monitoring by the U.S. Geological Survey (USGS) shows that phosphorus concentrations in the lower Boise River are down one third since 2015.

From below Lucky Peak Dam to the confluence near Parma, the Boise River winds through cities, towns and farmlands, and that takes a toll on its water. While the river is relatively cold and clear upstream at places like Barber Park, it becomes warmer, carries more sediment, and contains more aquatic plant life as it moves downstream to its mouth near Parma. These changes are caused in part by increased amounts of phosphorus in the river, which cause problems by increasing plant growth. Besides being a nuisance to recreationists, this excess plant matter consumes oxygen that is dissolved in the water when it decays, and too little dissolved oxygen is harmful to fish and other aquatic life.

The Boise River near Barber Park above the City of Boise is relatively cold and clear, versus the Boise River near Parma is warmer, carries more sediment, and has more excess aquatic plant growth.

So where does the excess phosphorus come from? The short answer is: all of us. A little bit of it comes from water and soil runoff from forested lands upstream. Municipal wastewater contains lots of phosphorus, and while much of it is removed during the treatment process, some is still in the water when it’s released back into the river. Fertilizers often contain phosphorus, and soil and water runoff from lawns and fields can carry some of this phosphorus into the river. Some phosphorus also comes from shallow groundwater. This phosphorus may come from leaky septic systems and from the application of both fertilizer and irrigation water that already has a lot of phosphorus in it from upstream sources. After it’s applied to fields, this water and phosphorus can infiltrate into shallow groundwater and eventually makes its way into the river.

Recognizing these impacts, the Idaho Department of Environmental Quality (DEQ) established total phosphorus limits for the lower Boise River in 2015 to address water quality issues. The U.S. Geological Survey has worked with DEQ and the Lower Boise Watershed Council to monitor water quality in the Boise River since the 1970s. This partnership brings local stakeholders on the Council, including cities, irrigation districts, and industries, together with local regulators and federal resources to help understand and solve problems here in Idaho.

Most recently, the USGS has been collecting water quality samples six times a year from the Boise River between Star and Middleton, and from near Parma. These samples are analyzed for a variety of constituents, including phosphorus and other nutrients. Partnering with the City of Boise, the USGS also uses an automatic sampler to collect samples every 49 hours from the Boise River near Parma; these samples are analyzed for total phosphorus (phosphorus in all forms). By comparing the phosphorus in these recent samples to previous data, the USGS has recently shown that phosphorus in the Boise River near Parma decreased by about one third (36 percent) over the past two years. From 1987 to 2012, the average annual phosphorus concentration (the mass of phosphorus per a volume of water) was 0.32 milligrams per liter (mg/L). The average total phosphorus concentration from 2015 through 2017 was 0.21 mg/L. So in the past two years, total phosphorus has declined by almost half of the amount needed to achieve the water quality target, which is 0.07 mg/L.  Although the high flows in spring 2017 helped dilute phosphorus in the river, the overall decline was apparent well before these record spring flows.

Monthly average measured phosphorus in the Boise River near Parma is lower than previous phosphorus levels. The difference is biggest during the winter. The difference is smallest during irrigation season (May through October) and storms.

This reduction is good news for the Boise River, and shows that work done throughout the basin to reduce phosphorus inputs is having an impact. Best management practices installed to reduce soil erosion and water runoff are likely making a difference in agricultural inputs. In the meantime, cities throughout the Treasure Valley have been upgrading their wastewater treatment facilities and removing more phosphorus than before. One way to understand the impact of these upgrades is to look at loads. Similar to concentration, a load is the mass of phosphorus but per a set unit of time. In this case, the load refers to the mass of phosphorus that moves through the Boise River near Parma in a day.

From 2012 to 2013, the average annual wastewater phosphorus load from Boise, Meridian, Nampa and Caldwell was about 1387 pounds per day (lb/day). During 2015 to 2017, this load decreased by about 50 percent to 644 pounds per day. During the same time, the total Boise River load near Parma decreased about 30 percent from its previous level, 2398 lb/day, to 1653 lb/day.

The municipal wastewater phosphorus load has decreased by about 50 percent over the past two years. This has helped decrease the total phosphorus load in the Boise River near Parma, but further reductions are needed, particularly during storms and the irrigation season.

However, the biggest improvement comes during the non-irrigation season, when the municipal load represents the greatest proportion of the total phosphorus load in the Boise River near Parma. This shows that although good progress has been made towards meeting the phosphorus target – and improving water quality – there is a lot more progress to be made by all of us.

Fortunately, many irrigators are working to install new water and erosion control structures, municipalities continue to upgrade their facilities, and innovative projects are intercepting and cleaning up phosphorus-laden waters throughout the valley. Continued water quality monitoring in the lower Boise River will track the progress made by these improvements, as fish, recreationists and the river reap the benefits.