What is the Influence of Phoslock® on legacy phosphorus?
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Influence of Phoslock® on Legacy Phosphorus
Introduction to Phoslock® and Legacy Phosphorus
Phoslock® is a lanthanum-modified bentonite clay increasingly used to control the release of legacy phosphorus (P) from lake bed sediments into overlying waters. This geo-engineering tool aims to mitigate eutrophication, a process that leads to excessive growth of nuisance algae, particularly cyanobacteria, in freshwater bodies.
Phoslock® Application and Phosphorus Binding
Phosphorus Reduction in Water Columns
Phoslock® has been shown to significantly reduce total phosphorus (TP) levels in water columns. Studies on various lakes and ponds have demonstrated that Phoslock® effectively binds with phosphorus, reducing its availability in the water. For instance, in two golf course irrigation ponds, Phoslock® application resulted in a significant decrease in water column TP levels and shifted mobile sediment P fractions to more stable forms . This reduction in available phosphorus helps in controlling algal blooms and improving water quality.
Impact on Sediment Phosphorus Fractions
Phoslock® not only reduces phosphorus in the water column but also alters the composition of phosphorus in sediments. Post-application studies have shown an increase in the sediment's lanthanum content, which enhances its phosphorus-binding capacity. For example, in Clatto Reservoir, the application of Phoslock® increased the sediment's P-binding capacity by 250 kg, theoretically binding a significant portion of release-sensitive phosphorus . Similar results were observed in Loch Flemington, where Phoslock® increased the mass of phosphorus in more refractory forms, thereby reducing the potential for phosphorus release from sediments .
Environmental Conditions Affecting Phoslock® Performance
pH and Anoxia
The effectiveness of Phoslock® is influenced by environmental conditions such as pH and anoxia. Phoslock® performs optimally at pH values between 5 and 7, with its adsorption capacity decreasing at higher pH levels . Additionally, Phoslock® retains its bound phosphorus even under anoxic conditions, making it a reliable option for lakes experiencing low oxygen levels Ross2008Funes2020.
Alkalinity and Lanthanum Concentrations
The concentration of lanthanum ions (La³⁺) in treated lakes varies with alkalinity. In lakes with moderately low to high alkalinity, La³⁺ concentrations remain very low, minimizing potential ecological impacts. However, in lakes with very low alkalinity, La³⁺ concentrations can be higher, necessitating further evaluation of potential ecological effects .
Long-term Effects and Recovery
Recovery Trajectories
Post-application studies indicate that the recovery of lanthanum concentrations in both surface and bottom waters follows a second-order decay relationship, with recovery periods ranging from 3 to 12 months. Surface waters typically recover faster (3-8 months) compared to bottom waters (11-12 months), likely due to differences in physicochemical conditions and sediment disturbance processes .
Algal Assemblage Shifts
The application of Phoslock® also influences algal assemblage composition. By altering nutrient ratios, particularly increasing the total nitrogen to phosphorus (N/P) ratio, Phoslock® can reduce the dominance of cyanobacteria and promote a shift towards less harmful algal forms. This shift was observed in treated ponds, where cyanobacterial dominance decreased, and planktonic forms became more prevalent .
Conclusion
Phoslock® is an effective tool for controlling legacy phosphorus release from lake sediments, thereby mitigating eutrophication and improving water quality. Its performance is influenced by environmental conditions such as pH, anoxia, and alkalinity. Long-term studies show that Phoslock® can significantly reduce phosphorus levels in both water columns and sediments, alter nutrient ratios, and shift algal assemblage composition. However, further research is needed to fully understand its ecological impacts, particularly in lakes with very low alkalinity.
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