This report is a deliverable within sub-action C11. It is published by the Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences. It can be accessed in Sweden from the university’s website (Report 2024:17).
Summary
Lake Norrviken is lake located in the cities of Sollentuna and Upplands-Väsby outside of Stockholm, Sweden. The lake has a surface area of 2.6 km2 and maximum and average depths of 12.7 m and 5.2 m, respectively. The lake has had problems with eutrophication since at least the middle of the 1900s, and had historically received untreated or inadequately treated in-flows from a yeast factory and other waste water for decades. Because it is located in a highly urbanized setting, the lake still receives stormwater runoff from its 29,913 km2 watershed. A large portion of the historical and currrent nutrient inputs are stored in the sediment, continuing to contribute to eutrophic conditions in the lake via internal loading processes, even though the historical point sources have been controlled to some extent.
As part of the EU Life IP Rich Waters project, the lake was treated with aluminium (in this case polyaluminium chloride, or PAC) in order to permanently bind the excess, legacy phosphorus (P) in the sediment, reduce sediment P release, and improve water quality in the lake. During an aluminium treatment, aluminium salts like PAC are applied and the amorphous mineral aluminium hydroxide (Al(OH)3) forms and either falls to the sediment (water treatment) or is injected into the sediment over a specified sediment depth (sediment treatment). The aluminium mineral that forms is naturally found in soils and sediment where it binds with P. It is added to sediment in eutrophic lakes to restore the balance between the amount of sediment P, and the binding capacity of the sediment.
As part of this study, sediment cores were collected across Lake Norrviken, both before and after treatment, in order to determine the effectiveness of treatment. Two sets of sediment cores were collected during each sampling period in order to determine: 1- the sediment release rate of P and 2- the amounts of different releasable and inert forms of P in the sediment.
Data from sediment incubation experiments showed that the average sediment release rate in Lake Norrviken decreased from 9.4 mg/m2 /d before treatment (2017) to 0.8 mg/m2 /d after treatment (2021). The release rate, however, had increased (although still low) by 2023 to 1.9 mg/m2 /d. P fractionation data had similar temporal changes, with releasable (mobile) fractions of P decreasing initially 2021 due to treatment, but then increasing by 2023. This indicates that external P loading is still elevated, and that the treatment effects will decrease over time. Interestingly, the amount of P bound to the Al mineral has increased since treatment and the initial sampling in 2021, indicating continued, permanent burial of P in the sediment even as new P-rich sediment is being deposited due to continued, elevated external loading from the watershed. The lake, as of 2024, continues to achieve Good status for phosphorus in the lake after treatment in 2020 according the EU Water Framework Directive
