Water Quality - August

Water Quality - August
Water Quality - 9/1/2020
It has been a busy summer and I apologize for not providing an update for the past two months. My last update on June 18 was about phosphorus and our efforts to control this nutrient, and today I want to discuss how we are doing so far.

You may have noticed something different about the lake this summer. This is the first time since our hypolimnetic aeration system was installed that we did not activate it during summer season. The result seems to be positive, in that the water quality has been quite good despite the lack of circulation.

As you recall from the June article, most lakes that suffer algae blooms have too much phosphorus. This nutrient can come from internal sources, meaning from below the water line, or external sources which come in from outside the lake boundary. To control algae you need to reduce phosphorus, and aeration is used to reduce phosphorus coming from a significant internal source, the sediment.

The intent of our aeration system when it was initially installed was to circulate water from shallower, more oxygenated depths to the bottom, near the sediment. This worked, and for three years after startup our sampling data confirmed the phosphorus reduction. But the lake continued to grow algae and cyanobacteria, culminating in a 2004 cyanobacteria bloom that required us to restrict lake access due to potential health risks. Although the lake was growing cyanobacteria prior to the bloom we had a significant rainstorm in August that washed a lot of nutrients into the lake from the watershed. This meant internal loading was not the sole source of phosphorus.

In 2005 we started our alum program using two methods to control phosphorus. We inject alum into Oswego Canal and near the headgate to strip phosphorus from water coming in from the Tualatin River, which is our largest external load of phosphorus during summer. We also apply alum from the surface with a goal of reducing phosphorus in the water column and building a layer of aluminum on the sediment to intercept phosphorus releases from that source. Our surface applications are also used to reduce phosphorus from summer rainstorms, so we do not have a repeat of 2004. This has kept phosphorus and algae concentration low for the past 15 years, and despite an occasional bloom we have been able to manage water quality quite well.

So why turn off aeration? We started our system in fall 2001 and ran it without other treatments for three full summers. In 2005 we started applying alum and have been using both alum and aeration ever since. We went three summers with aeration and no alum, but have not tried a summer with alum and no aeration. This summer was an experiment to see how the lake would react.

So far this summer our phosphorus has been very low in the shallow water, but it has increased quite a bit near the bottom. This is to be expected because there is less oxygen down there to keep phosphorus bound to minerals. However, since the lake is thermally stratified this high phosphorus water stays near the bottom where it cannot feed algae. When the lake cools this fall the high phosphorus water will mix with the surface water, but that will not occur until early November and conditions are not likely to be conducive to growing a lot of algae.

However, in an abundance of caution since we are drawing down in mid-October we are applying alum the first week of September to strip phosphorus from the deep water and reduce the potential for a fall bloom. This is because we are removing 10 feet of water from the top of the lake, which will have two consequences: removing this warmer water near the surface will cause the lake to cool earlier, and there will be a smaller volume of "good" water to dilute the high phosphorus "bad" water coming up from near the lake bottom.

The long-range forecast calls for a cool and wet fall starting in September, so the upcoming warm weather may be the last of our summer weather. I am writing this after a night where the air temperature was down into the 40's, putting the lake into the cooling phase. And even though it will get in the 90’s this coming week, most of the day will be below the lake temperature of 75 degrees so the lake will continue to cool. We will start drawing down October 15th so this may be the last opportunity to enjoy a warm lake.