As you can see from the recent water quality update we realized a temporary benefit from the alum application the end of July that improved water clarity. We don’t have all the data from that time period (particularly nutrients and phytoplankton), but we do know the application stripped algae and cyanobacteria cells from the water column, taking them and the phosphorus contained inside to the lake bed. This led to several weeks of improved conditions, but water clarity has decreased again in some areas as cyanobacteria continues to grow.
We have another alum application scheduled for the week of August 28 that should reduce the phosphorus enough to provide good conditions before our drawdown October 1. Unfortunately this does not help folks who want to enjoy the lake this last week before school starts, but if it is any consolation the entire lake is not experiencing a bloom so going to those areas with less cyanobacteria could be an option. We are spot treating areas that are particularly bad so that will provide temporary relief.
We have been talking about alum a lot this summer, and you no doubt saw our application barge during the late July treatment. We apply Aluminum sulfate (alum) to remove phosphorus, the food source for cyanobacteria. Phosphorus is highly reactive in the dissolved form and will be readily taken up by plants and algae or will bind to minerals in the sediment. Applying alum offers aluminum as a binding site, which means phosphorus is not available to feed algae and cyanobacteria. Our alum injection equipment at the headgate and in Oswego Canal target the soluble form of phosphorus coming in from Bryant Springs and the Tualatin River. This is our primary phosphorus reduction tool during summer because once this phosphorus is incorporated into algae and cyanobacteria it becomes more difficult to remove.
During summer almost all the phosphorus in the lake is taken up by algae and cyanobacteria with very little in the soluble form. In this case we need to physically remove the algae and cyanobacteria cells to remove phosphorus which is why we do surface applications. During an application alum forms a flocculant, or “floc” that physically removes algae and cyanobacteria cells from the water column. This also removes the phosphorus contained in those cells, taking it to the sediment where it keeps it from growing new algae.
You can see the effect of an alum application in the two photos below. The top one is right after the barge passes showing a cloud of alum in the water. The second photo is a few minutes later as the alum floc starts to form. Over the next several hours the floc settles through the water column, capturing algae on its way down. This is why water clarity increases after a treatment, and if we successfully remove enough phosphorus it will keep the lake clear for several weeks.
Alum just after the application barge passes. The white liquid is just starting to form a floc.
A few minutes after application the floc has formed and starts to settle through the water column.
This has been a long summer and we are using all the tools available to reduce phosphorus and maintain water quality. We are successfully removing most of the phosphorus that is coming in from the headgate and are using the aerators to keep phosphorus in the sediment. Our extensive data collection this summer will show if these methods have remained effective or have become less so as the season progresses. As always, we use this data to inform decision the next year and take steps necessary to incrementally improve water quality.
