City explores options for treatment of phosphorus in lake
The aerial photo of Lake Stevens shows phosphorus levels along the shoreline.
The City is asking for public input regarding the lake’s aerator at an upcoming city council meeting on January 14.
Lake Stevens continues to have an influx of internal and external phosphorus loading. While phosphorus is important to the health of the lake, high levels of phosphorus can result in water quality deterioration and unwanted algae blooms. In 2012, several occurrences of algae blooms with three that were considered to be toxic.
Two of the events required posting of public warning around the lake. The photo above shows a blue-green algae bloom that occurred in the spring of 2012 in Lake Stevens’ southwest shore area. This is an indicator of high phosphorus nutrients in the water.
Phosphorus has been an identified problem since the 1950’s. In 1994 the first action was taken to address the phosphorus problem through the use of an aeration system. The aerator’s function is to provide oxygen to the sediment to maintain a phosphorus-iron bond.
During the summer months oxygen levels are depleted, especially in the deeper water, and the aerator is activated to replenish the oxygen in the water column. The aerator typically operates from late June through October.
The activation is determined based on oxygen level readings of the lake (performed by Snohomish County). The cost to operate and maintain the aerator system is shared between the City and the County with the City covering the majority of the costs.
The share paid by each agency is based on the amount of watershed area contributing to the lake. The annual cost to operate the aerator is approximately $35,000 which includes power consumption and staffing. However, for the past six years the estimated average annual cost including maintenance (repairs) has been estimated at over $110,000 per year.
The aerator operates five months out of each year and only treats a small portion of the phosphorus in the deepest sediment portion of the lake. The aerator has provided an acceptable level of phosphorus control. However, the long-term viability of aeration as the single treatment method for excessive phosphorus is unsustainable because of the limitation of the iron bonding capacity within the lake being exceeded by the total volume of internal and external phosphorus loading.
In addition, the aerator is very costly to operate and maintain and it is approaching the end of its life-span. With or without the use of the aerator, lake conditions will deteriorate unless a suitable in-lake treatment plan is implemented to help reduce phosphorus levels.
So the City is faced with a question on how it will continue to address the phosphorus problem in the lake.
There are three options currently under consideration at the City that includes: 1) continue the operation of the aerator; 2) perform alum treatments only; 3) a combination of alum and aerator treatment.
A Phosphorus Management Plan has been prepared by the City and provided background information and analysis of these three options. This Plan is currently under review by the city council and the council is seeking public input on this document. The council will be discussing this topic at the council meeting held at the Educational Service Center at 12309 22nd Street NE on January 14, 2013 beginning at 7 p.m. The public is encouraged to attend.
A copy of the Draft Phosphorus Plan is posted on the City’s Web site at http://www.ci.lake-stevens.wa.us.