Lake Carmi Water Quality Expected to Improve After Alum Treatment

Lake Carmi, a 1,375-acre lake in the northwest corner of Vermont, has been plagued by cyanobacteria blooms for decades. These blooms reduce water clarity, often have a foul odor, and can turn toxic, forcing the closure of swimming beaches and preventing recreation. Cyanobacteria blooms have had a significant impact on the local Lake Carmi community and economy.  This fall, the lake is undergoing an aluminum sulfate (alum) treatment to address persistent internal phosphorus loading, which continues to cause cyanobacteria blooms.

“We expect this treatment to be effective in limiting internal phosphorus loading and cyanobacteria blooms for at least a decade on this lake,” said Mark Mitchell, Lay Monitoring Coordinator at LCSG and the Vermont DEC. “The results are almost immediate, so by next summer we should be seeing a much healthier lake for swimming and recreation.”

History of Water Quality Work on Lake Carmi

The Vermont Department of Environmental Conservation (DEC) has listed Lake Carmi as impaired for phosphorus since 2008. Large amounts of phosphorus have accumulated in the lake bottom sediments, due to decades of runoff and erosion from agriculture, shoreland development, roads, and other land uses in the watershed. During warmer months, temperature and density differences between water layers in the lake become more distinct, forming layers of water that don’t mix together. The lower layers can become deprived of oxygen because they do not mix with the atmosphere. When the lakebed becomes deprived of oxygen (anoxic), phosphorus is released from the sediments, allowing phosphate to disperse into the water column. This phosphate feeds cyanobacteria blooms, particularly in late summer and fall.

Significant effort has been devoted to reducing phosphorus inputs into Lake Carmi from multiple sources within the watershed. Typically, land-based watershed management techniques are considered the best practice to improve water quality in  a lake or pond. However, because of the persistent algae blooms that occur in Lake Carmi and their associated health risks, in-lake management techniques were explored and used to address the built up phosphorus in the lakebed sediments.

The Vermont DEC funded the installation of an aeration system in 2019 in an attempt to eliminate those anoxic conditions and reduce cyanobacteria blooms. Ultimately, the aeration system was unsuccessful at consistently oxygenating the lake bottom sediments to limit phosphorus release and cyanobacteria blooms. However, residents and the Vermont DEC remained resilient. With federal funding available through Vermont’s Clean Water State Revolving Fund Program, they began considering an alum treatment. In 2024, the Vermont DEC contracted Barr Engineering to conduct a comprehensive feasibility study to evaluate the expected water quality benefits and costs of an in-lake alum treatment to control internal phosphorus loading from anoxic sediments in Lake Carmi. Alum has been used successfully in lakes for decades, including twice for Lake Morey in Vermont, where it has significantly improved water quality after treatments in 1986 and 2024.

What Is an Alum Treatment?

When applied to a lake, alum binds with existing phosphate, which then precipitates into a solid in what’s called a “floc” that settles on the lake bottom. This process immobilizes phosphorus release from the sediment regardless of oxygen levels, making it unavailable to cyanobacteria. The treatment is paired with an ambitious Watershed Action Plan in the Lake Carmi watershed to minimize incoming phosphorus runoff in addition to addressing legacy phosphorus.

The alum treatment will take about three weeks to complete. During this time, five tanker trucks carrying liquid alum and sodium aluminate arrive daily from Maine, and the team treats the lake from a barge with tanks and about 20 delivery hoses positioned to inject the chemicals about one meter below the water’s surface. The lake is divided into seven zones, each with its own alum concentration requirements. Treatment for each zone takes approximately 90 minutes and must be completed three times in total.

One potential challenge associated with alum treatments is a drop in pH levels in lake water, which could be harmful to aquatic life. To prevent this, a buffering agent (sodium aluminate) is applied simultaneously with the alum, and rigorous testing of any water quality changes is conducted consistently throughout the process. To determine the appropriate levels of alum and sodium aluminate to use during the treatment process, the project relied on water monitoring data from the Vermont DEC (including the Lakes Lay Monitoring Program) and the Barr Engineering feasibility study.

“There has been really great community involvement in this project,” said Kate Lucas, Water Resources Scientist at Barr Engineering Co. “Everyone seems to be supportive of this effort and is excited for the benefits we expect to see next summer.” In fact, community interest has been so strong that most residents voluntarily pulled their docks and boats out of the water, even though treatment takes place only in deeper waters, meaning that boating infrastructure would not have impeded the alum treatment barge or the pontoon boat that conducts water monitoring during treatment.

This project has been decades in the making. It represents a huge step forward for the Lake Carmi community and economy to mitigate chronic cyanobacteria blooms caused by excess phosphorus. Monitoring will continue throughout the process to track how this treatment impacts the lake, with results available on the Vermont DEC webpages for Restoring Lake Carmi and the Lakes Lay Monitoring.

While the alum treatment is anticipated to mitigate cyanobacteria blooms for an extended time period, it is essential that phosphorus runoff to the lake is minimized over time. It is imperative that landowners, businesses, and visitors to the Lake Carmi watershed (or any watershed for that matter) understand that while the alum treatment helps to immobilize phosphorus within the lake, phosphorus entering the lake from across the landscape can reduce the effectiveness of the alum treatment. Everyone must take action to limit phosphorus runoff to the lake. Projects that prevent erosion from agricultural lands and from streambanks, and that minimize stormwater runoff to the lake’s tributaries and the lake itself can help reduce how much phosphorus enters the lake moving forward. We all have a role to play to help that to happen.