Green Valley Lake to lose the green: aluminum sulphate to restore water quality

Green Valley Lake will be receiving a chemical makeover starting in April. The Iowa Department of Natural Resources and Barr Engineering Co. held a public meeting on Wednesday, March 26 to discuss the undertaking.

Union County visitors to the lake would remember a green tone in the water, contributed by algae blooms, causing issues with water quality. In 2015, the DNR noted a hepatotoxin called microcystin toxin, recommending visitors to avoid open contact with the water.

The majority of the presentation was lead by Barr Senior Aquatic Ecologist Joe Bischoff, who outlined how the company, with assistance from the DNR, will inject Green Valley Lake with aluminum sulphate to activate phosphorus, a limiting nutrient, in the sediment layer of the lake.

Phosphorus

Phosphorus is collected at the sediment level of a body of water through both natural and man-made sources. In lake ecosystems, phosphorus is valuable for plant life to grow, but in excess causes algae blooms and creates anoxic areas, or areas which are devoid of oxygen, choking the local ecosystem.

Water quality restoration is the simple goal. Bischoff outlined how increasing water clarity and reducing algae in Green Valley will provide a healthier ecosystem in a lake which is a popular location for hobby fishermen.

Bischoff showed data collected by Barr on the anoxic phosphorus flux (measured in mg/m²d). According to Bischoff, a measurement of two is healthy water quality. Three distinct areas of Green Valley Lake measured about 14, 23 and 26. This means the cycling of phosphorus from the sediment layer into the water is notably higher than what’s sustainable.

Green Valley Lake was noted to have an excess of iron-bound phosphorus, or phosphorus bound to iron compounds which is found in the soil and sediment of the lake.

Alum

Barr and the DNR have proposed dispersing liquid aluminum sulfate into the water of Green Valley Lake. The chemical permanently binds phosphorus in the sediment, reducing phosphorus cycling into the water. As the substance is spread over the sediment layer, it strips the phosphorus attached to the iron compounds and feeds off it, diminishing the ability for it to cycle into the water.

“We call this geoengineering a lake,” Bischoff said. “Basically, what we’re doing is we’re manipulating the biogeochemical cycle to our advantage.”

Aluminum sulfate, or alum, doesn’t release the phosphorus, even under extreme conditions. As alum is injected into the water, it forms a substance called aluminum hydroxide which will strip and rebound phosphorus attached to iron in the sediment. Bischoff called phosphorus bound to alum as a “solid” and “permanent” bond.

A portion of Bischoff’s presentation was dedicated to health concerns with alum, which was endorsed by the North American Lake Management Society as a safe and effective management tool for lakes. Aluminum is used as a food additive as a pickling agent, with the FDA concluding that aluminum is generally safe to be used in this way.

The CDC noted the average adult in the U.S. eats 7 to 9 milligrams of aluminum per day in their food. Exposure to the alum after it’s applied to the sediment layer is unlikely, unless swimmers were to dig into the sediment.

Temporary effects on the macroinvertebrate ecosystem were also noted as side effects of the process. Some species, such as plankton or phytoplankton, would see a noticeable decline during the application process, but would quickly regain their population in a couple years. Long-term improvements were noted to follow, based on multiple studies in lakes across the country.

Alum’s toxicity was noted during the presentation. The solute is safe if the pH remains at 6 to 9. In an application, the substance can be buffered with sodium aluminate at a 2:1 alum to sodium aluminate. In order to apply alum safely, the lake water’s pH would need to be monitored.

Monitoring application

Barr’s application of alum and active monitoring of the lake’s pH levels before, during and after application was explained by Bischoff. pH would be monitored by both Barr and the DNR.

Vessels called application barges will carry tanks of alum as it’s applied across Green Valley Lake. The barges are equipped with GPS equipment similar to those used in precision ag vehicles to map the lake’s application using satellite monitoring. Bischoff called the process “painting” the lake. The alum liquid injected 18 inches deep from an applicator spreads into a layer of alum, called a floc, as it settles further into the water and forms aluminum hydroxide.

Flocs will be visible as they are applied to the lake, forming a noticeably lighter color as it spreads. Bischoff jokingly said the process looks like “someone snotted into the lake.”

The lake would be open during the application process for boat launches, fishing and swimming. Bischoff, however, recommended avoiding swimming into the floc of alum following application barges on the lake.

A daily treatment begins with collecting sediment cores in the area of application. After determining if alum treatment is necessary, alum will be applied, repeating the cycle of collecting sediment cores and applying doses until alum is no longer necessary. Water quality will be monitored to track effectiveness after the application process is over.

Schedule

Two minimum applications are planned for the process. Numbers for the spring application were given, set to commence on April 22 as the first in a 13-day application schedule. An estimated 328,683 gallons of alum will be applied to the main lake, with 18,873 gallons to sedimentary basins with a buffer of sodium aluminate.

Multiple passes per area will be done. The main lake will begin application following mobilization on April 22 from April 23 to April 28 as alum is applied to the main lake. On April 29, the process is mobilized to the northeast basin for application on April 30. One final mobilization on May 1 to the northwest basin will precede three more days of application from May 2 to May 4.

Following additional monitoring of the sediment and another application in the fall (schedule not yet decided), more application could be needed, alongside planned application to sediment basins.

Bigger picture

While the project will assist Green Valley Lake in the short-term, considerable attention was given by one visitor to the long-term problems associated with pollution and watershed. John Norris thought more attention should be given to limiting locally-controlled sources of pollution, such as wastewater and fertilizer runoff, instead of spending millions of taxpayer money on temporarily cleaning Iowa’s water bodies.

“When are [Iowa legislators] going to fix the problem, not the symptom?” Norris said. “We can’t keep doing this in these lakes and every lake across the state when we know what the real problem is.”

Norris’s questioning dominated the discussion part of the meeting, which at times was accusatory to parties not involved with the public meeting. Bischoff noted the cleaning of the lake is an immediate problem which would need to be solved no matter what legislature could be coming down the pipe.

Michelle Balmer, one of the researchers for the lake restoration program for the DNR, outlined the importance of research which would be conducted for the project. The cost of the application in total would be $1 million. Balmer noted the cost is due to the higher dose of alum needed to counter the extremely high phosphorus content.

“We have... really high phosphorus release rates,” Balmer said. “It’s warranted you would have a much higher dose here... We have to keep in mind the size of the lake you’re trying to treat.”