The U.S. Geological Survey has developed computer models of how phosphorus cycles through the Boise River that it hopes will help the effort to clean up the river.
The requirement that Boise River communities reduce phosphorus pollution grows out of rules that have been on the books for more than a decade and a federal court order from the 1990s. Both are based on the 40-year-old Clean Water Act’s mandates that all of the nation’s rivers be cleaned up.
The challenge has been that the EPA regulates “point sources” — specific sources like sewage systems — but has no authority over “non-point” sources like farm fields. Because farmers don’t have to do anything, cities end up having to do more. The better the information, the easier it will be to convince regulators that pollution trading and other innovative cleanup measures can help farmers and others reduce pollution cost-effectively.
The Boise River’s cleanup plan has required phosphorus levels from all sources to come down to meet water quality standards in the Snake River below. EPA also is pushing for Idaho to develop a separate, more specific pollution budget on the Boise River called a total maximum daily load, or TMDL.
The TMDL is set as a limit for pollutants to ensure that waterways can support fish, and allow swimming and other beneficial uses. The limit at the mouth of the Boise River is 70 parts per billion, compared with the current level of 320 ppb. Until there’s a Boise River TMDL, Boise and other point sources have to meet that 70-ppb limit.
Using the USGS models, Idaho Department of Environmental Quality managers can simulate the effects of reducing upstream phosphorus loads from point sources such as wastewater treatment plants and non-point sources such as agricultural areas. By setting a specific load allocation for sources such as a wastewater treatment plant, resource managers can model how that allocation would affect downstream phosphorus concentrations.
The models also consider how much water the river and its tributaries gain from and lose to groundwater. As water moves between the ground and streams, it carries phosphorus with it.
To develop the models, USGS scientists collected water-quality samples and measured discharge at 35 sampling sites along the Boise River, its tributaries and in the Snake River upstream and downstream of the Boise River. Additional samples were collected from six wastewater treatment plants and two fish hatcheries.
Samples and measurements were taken in August 2012 during the irrigation season, in October 2012 just after irrigation season, and last March just prior to the irrigation season.