Como Lake Alum Treatment

Capitol Region Watershed District (CRWD) is committed to improving water quality in Como Lake, a beloved water resource in Saint Paul for over a century. Despite efforts to address the water quality in the lake over the years, decades of stormwater runoff and pollution have made a new Como Lake plan necessary.

The 70 acre Como Lake drains more than 1,711 acres of land in Saint Paul, Roseville, and Falcon Heights. With a maximum depth of 15.5 feet, it is a shallow body of water. The shallow depth, coupled with significant nutrient and pollution in stormwater runoff, has had a significant impact on Como Lake water quality. In fact, CRWD began when a group of residents raised concerns about the water quality in Como Lake. Accordingly, the District has placed a high priority on improving Como Lake, as it is one of the most recognizable and well-loved water resources.

The Challenge

The biggest challenge facing Como Lake today is too much phosphorus, a pollutant carried to the lake through runoff in storm drains. Phosphorus comes from decaying organic matter like leaves, grass clippings, pet waste, and soil. Over time, high levels of phosphorus — three times higher than Minnesota standards — have led to algae blooms that choke the oxygen from the lake, leading to fish kills and strong odors mid-summer. CRWD and its partners have worked to reduce phosphorus flowing to the lake in stormwater runoff by 20% over the past two decades. Como Lake’s water quality remains poor due to the high levels of phosphorus that have built up in the lake over time. Excess phosphorous in the lake must be addressed to reach water quality goals.

"Capitol Region Watershed District is committed to improving Como Lake. Reducing phosphorus and controlling curly-leaf pondweed are critical for restoring balance." - Britta Belden, CRWD Water Resource Project Manager

The Solution

CRWD, with guidance and support from the City of Saint Paul, Ramsey County and the Minnesota Department of Natural Resources (DNR) have outlined the new Como Lake Water Management Plan implementing the industry’s best practices, science and technology. The Como Lake plan identifies strategies to address the water quality issues, which includes treatment of the water over the course of several years, beginning in the spring of 2020 with herbicide and alum treatments.

View Como Lake Management Plan

Spring 2020 Alum Treatment

Alum Treatment

Alum (aluminum sulfate) was applied to Como Lake this spring to reduce phosphorus being recycled inside of the lake.
The treatment is expected to dramatically reduce algae growth and improve water clarity and quality.
Alum is commonly used in drinking water treatment and has been a safe lake management tool for decades.

View Alum Treatment Factsheet

View Alum Treatment Updates blog post

Frequently Asked Questions

What is an alum treatment?
An alum treatment consists of applying liquid alum below the surface of the water from a boat. When the alum comes into contact with the water, it turns into a non-toxic substance, or floc, called aluminum hydroxide. The floc settles to the bottom of the lake, it binds to phosphorus in the water and forms a barrier that prevents it from being consumed by algae.
What is alum?
Alum (also called aluminum sulfate) is a chemical compound that has historically been used in drinking water treatment and as a scientifically proven lake management tool.
Alum is safe for humans, pets and other aquatic wildlife. It has no known adverse effects.
When will the alum treatment happen at Como Lake?
The alum treatment was completed by a licensed contractor during the week of May 18-22, 2020.
How will lake visitors and users be impacted by the alum treatment?
Visitors can expect an immediate change in Como Lake with clearer water and fewer algae blooms.

Visitors saw the lake being treated this spring, and access to paths around the lake and the east parking lot were restricted at times. Signage was posted around the lake to keep visitors informed.

Water quality monitoring occurred throughout the treatment to ensure it was being applied safely and correctly for the best results.
How does phosphorus get into Como Lake?
Phosphorus is carried to the lake by stormwater runoff. There are 22 storm drains that flow to Como Lake, each of which transport runoff from the surrounding neighborhoods. Runoff carries organic material like leaves, grass clippings, pet waste, and soil—all of which contain phosphorus. CRWD and its partners have constructed stormwater best management practices (BMPs) such as rain gardens, stormwater ponds, and underground infiltration systems in the Como Lake watershed over the last 20 years that have reduced phosphorus being transported to the lake by 20%. The long-term goal is to reduce external phosphorus by 60%. CRWD will continue looking for opportunities for BMPs in the watershed to meet this goal.
What happens to phosphorus once it's in Como Lake?
Internal sources refer to phosphorus that is already inside the lake—often called “in-lake” phosphorus. This is phosphorus that was transported to the lake in runoff and has built up over time. Typically, this in-lake phosphorus is stored at the bottom of the lake because it chemically binds to sediment. However, phosphorus can break its chemical bond with sediment when the lake water is experiencing low-oxygen conditions, causing it to release back into the water and be available for algae to consume. Low-oxygen conditions persist in Como Lake for much of the summer months, so in-lake phosphorus is constantly available to algae during this time. The result is extended algae blooms, odor and continued low-oxygen conditions.