Como Lake Alum and Herbicide Treatments

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 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.

The other main concern in Como Lake is an invasive aquatic plant known as curly-leaf pondweed. The plant is dominating Como Lake’s ecosystem, making it difficult for native plants to survive. Decaying curly-leaf pondweed adds more phosphorous to the water when it dies off, so management is key to achieving CRWD’s water quality goals.

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.

Is it necessary to apply herbicides to Como Lake?

Yes, curly-leaf pondweed has overrun the ecosystem, reducing recreation on the lake and making it difficult for native plants and aquatic life to survive.

How is the herbicide applied?

The herbicide Fluridone was applied by a licensed contractor to the lake at a low concentration of 4 parts per billion. That concentration was maintained for 60 days. The Fluridone was applied from a boat over the course of one day. An additional treatment was needed in order to maintain a lake-wide concentration of 4 parts per billion (ppb) for 60 days. The Minnesota Department of Natural Resources reviewed, approved and issued a permit to proceed with the application of Fluridone.

When will the herbicide treatment happen at Como Lake?

The herbicide Fluridone was applied on April 13, 2020 to target curly-leaf pondweed before native plants begin growing.

How will lake visitors and users be impacted by the herbicide treatment?

Visitors saw the lake being treated this spring, but their activity in or around the lake was not impacted. There are no water contact restrictions for the herbicide Fluridone, so visitors, pets and wildlife did not need to avoid the water during the application. The sidewalks, fishing piers, and boat launches remained open. Signs were posted to alert people visiting the lake during the time of application.

Are herbicides safe?

When applied at the correct dosage and time of year, the herbicide Fluridone presents no concern to humans or pets. The EPA chemical rating for Fluridone has no contact restrictions. Fluridone targets plants only and has not been shown to have adverse effects on fish, birds, or mammals in the lake, such as muskrats. Monitoring will occur throughout the treatment and the weeks following to maintain safe concentrations and to determine the effectiveness of the treatment.

Will an herbicide treatment be needed in the future?

It is likely that additional herbicide treatments will be needed in Como Lake to further reduce the curly-leaf pondweed population so native plants can thrive. CRWD has a seven-year variance with the Minnesota Department of Natural Resources to perform herbicide treatments for curly-leaf pondweed, though it will never fully be eradicated from the lake. CRWD will continue monitoring the aquatic plant community in Como Lake to determine what measures are needed to manage curly-leaf pondweed and support the native plant communities.

How did curly-leaf pondweed get into Como Lake?

It was first observed in the early-1990s and has come to dominate 90% of the plant community in the lake. It was likely introduced by a boat, boot treads, or aquarium plant. Curly-leaf pondweed thrives in murky water. The conditions in Como Lake are ideal for the curly-leaf pondweed to be highly successful and outcompete native plants.

How does curly-leaf pondweed contribute to phosphorus levels in Como Lake?

Curly-leaf pondweed plays a significant role in the amount of phosphorus in Como Lake. While it is not the main source of phosphorus, it can accelerate the process of in-lake phosphorus cycling. It has a unique life-cycle that allows it to grow throughout the winter beneath the ice and then dies off late-June. Most native aquatic plants begin growing in early-June. As curly-leaf pondweed grows, it uses phosphorus as food to produce large amounts of plants. When the curly-leaf pondweed dies off in late-June, all of the phosphorus that was taken up by the plants is released back into the water as it decays, becoming available for algae to consume. In addition, the decaying plant matter lowers the oxygen in the water during the decomposition process.

Como Lake Alum and Herbicide Treatments

The Challenge

The Solution

Spring 2020 Lake Improvement Projects

Alum Treatment

Herbicide Treatment

Frequently Asked Questions