Potential Techniques to Prevent Eutrophication Through Aluminum Sulfate Addition or Oxygen Injection : A case study in Lake Svartbyträsket, Boden, Sweden

Eutrophic lakes characterized by algal blooms and oxygen depleted bottoms symbolize a worldwide environmental challenge and a clear indicator of human impact on aquatic ecosystems. Eutrophication occurs when nutrient input in the forms of nitrogen (N) and phosphorus (P) is exceptionally high, contributing to excessive algal growth in the lake. The decomposition of dead algae consumes oxygen, resulting in degraded water quality and loss of habitat for aquatic organisms. Numerous efforts have been implemented to mitigate algal blooms and enhance lake water quality. However, there are few methods proven to be effective in achieving long-term recovery from eutrophication. The purpose of this study was to evaluate two different methods that aim to reduce phosphorus and permanently bind it in the sediments. The first method involved oxygen injection into the water, thereby forming iron hydroxides (Fe(OH)₃) under oxic conditions, which could adsorb phosphorus. The second method was performed by adding aluminum sulfate, Al2(SO₄)3 into the water that could form aluminum hydroxides (Al(OH)3 or AlOOH). These aluminum complexes also had the potential to absorb phosphorus in a way that immobilizes it, making phosphorus unavailable for nutrient uptake. This study focused on Svartbyträsket, a lake in Boden, Sweden which exhibits evident signs of nutrient enrichment and eutrophication. The main purpose of the research was to evaluate two eutrophication mitigation strategies and determine which one that could be best suited for application in Svartbyträsket. The experimental procedure involved column experiments conducted at laboratory scale using sediment and water from the deepest point of the lake (11 meters, SV26). Three column experiments were performed: (1) oxygen injection, (2) reference, and (3) addition of aluminum sulfate. In addition to the column experiments, probe measurements were carried out in April 2025 at the sampling location, (SV26) where pH, oxygen, electrical conductivity (EC), and temperature were evaluated at one-meter intervals. Ultrafiltrated water samples (<1 kD) and total concentrations of elements in the water were also collected during this occasion. To determine the composition of the sediments, SEM-EDS analyses were conducted both before and after the column experiments to observe how the sediments were affected by the experiment. The results show that the deepest part of the lake becomes oxygen-depleted at a depth of 4 meters during the winter, and that the water column becomes stratified. No major geochemical changes happened in column 1 and column 2 during the experiment. A significant pH decrease occurred in column 3, and the aluminum concentration in the water increased markedly after the addition of Al₂(SO₄)₃. Framboidal pyrite (FeS₂) had also formed in column 3 after the column experiments. The main conclusions of this study were that the method involving aluminum sulfate addition is not suitable for improving water quality in Svartbyträsket. That is primarily due to the high levels of aluminum, which contributed to acidic water and thus dissolution of aluminum hydroxide complexes. The oxygen injection method, on the other hand, could potentially be applicable in the lake. However, further investigations are required to obtain a deeper understanding of the lake.