River, it flows through the urbanized area of Ada County and then through Canyon County, which is primarily made up of heavily irrigated farmland. Water quality in the river degrades as it reaches the confluence of the Snake River and becomes more concentrated with both sediment and phosphorus.
Due to the high concentrations of phosphorus in the Boise and Snake Rivers, wastewater treatment facility (WWTF) National Pollutant Discharge Elimination System permit limits required the point sources in the watershed to invest in major upgrades to treat and remove 98% of the phosphorous leaving their facilities. Although the phosphorus load from the point sources is reduced through requirements in the NPDES permits, much of the improvement in water quality throughout the watershed is not realized in the lower stretch of the river due to the nonpoint sources of phosphorus, which return to the river. Water used for irrigation picks up sediment and phosphorus and then returns back into the Boise River, unregulated, through drains and canals, which constitute a major source of pollutant load in the lower watershed near the confluence of the Snake River.
The Dixie Drain is an agriculturally influenced tributary in the lower stretch of the Boise River watershed that contributes up to 40% of the total phosphorus load from the Boise River into the Snake River. Due to the large quantities of sediment and phosphorus the Dixie Drain delivers back to the Boise River, at locations far below the City of Boise’s WWTF, an opportunity for ingenuity was sought. The City of Boise was able to treat 93% of their phosphorus removal requirements through their treatment plants; however, the remaining 5% of phosphorus required in its NPDES permit would require very expensive modifications to existing facilities for only a slight reduction in phosphorus load in the lower stretches of the river.
Through a trusted partnership, Idaho Department of Environmental Quality (DEQ), U.S. EPA, and the City of Boise collaborated on an alternative plan to address the pollution in the Dixie Drain that would offset the remaining 5% of phosphorus removal required in Boise’s NPDES permit. For the same cost as upgrading their existing treatment plant to remove the remaining phosphorus required in its NPDES permit, the Dixie Drain facility treats 1.5 times the amount of phosphorus that would be treated at its upgraded WWTF, resulting in a greater environmental return on investment.
Results to Date:
The Dixie Drain project results in less sediment and phosphorus in the Boise and Snake Rivers. The sediment- and phosphorus-laden drain water is diverted and cycled through constructed treatment ponds, where the sediment and phosphorus is removed and then returned back to the drain clear and clean of these contaminants. The City of Boise, in partnership with DEQ and EPA, developed an innovative solution to meet their permit limit while addressing nonpoint source phosphorus and sediment downstream. This project is an excellent example of city, state, and federal collaboration to look beyond merely spending money for the sake of regulation and instead addressing a problem with a watershed-based solution.
The facility currently is removing 25 pounds of phosphorus per day and will increase removal to 80 pounds per day in 2020. Running at full capacity, the Dixie Drain facility is capable of treating 130 million gallons per day and removing up to 140 pounds of phosphorus per day, or roughly 10 tons a year.
Senior Water Quality Analyst
Idaho Department of Environmental Quality