Potential toxicity and chemical processes of aluminium addition for sediment phosphorus control in Östhammarsfjärden

The main goal of this project was to determine the potential effects on biota from aluminum addition to Östhammarsfjärden (Östhammar county) for reduction of internal phosphorus loading. Potential risks to biota in the water and sediment were summarized from reports and scientific publications as well modeling based on data from an enclosure experiment where aluminum was added to the sedi- ment in Löparöfjärden (Norrtälje county) during the summer 2011. Aluminium salts have been used to reduce phosphorus in water and limit internal phosphorus loading in lakes for over forty years. Aluminium toxicity in lakes is considered negligible under neutral pH conditions because reactivity and bioavailability are generally low when pH is between 6 and 9. The toxicology literature, however, largely supports the conclusion that there is some potential for alumi- nium toxicity from treatment using aluminium salts when pH is maintained in the neutral range in fresh waters. Continuous exposures to aluminium are more likely to have adverse effects on aquatic life than single dose applications of Al-salts. Hence, the potential for adverse effects with one-time aluminium treatments lie primarily with significant aluminium floc accumulation and subsequent dis- ruption of benthic invertebrate habitat. Other short-term effects are also possible, including reduced benthic invertebrate populations and stress to certain species of fish. Elevated concentrations of natural dissolved organic matter (DOM) capable of binding aluminium in the water of Östhammarsfjärden are likely to limit toxicity during and after application. Previous studies have shown that the effects are short (invertebrates) to medium term (fish), and aquatic communities affected by treatment with Al- salts generally rebound and improve from pre-treatment conditions due to increased water and habitat quality. A direct injection of aluminium to the sediment will likely limit much of the potential treat- ment impacts to plankton and fish in the water column. Given the lack of total and dissolved alumi- nium data in marine and brackish systems, it is difficult to compare the results from freshwater sys- tems directly. Although some authors recommend less restrictive limits for aluminium in marine wa- ters due to differing water chemistry, others propose more restrictive limits due to the potential for a greater diversity of aquatic species and potential toxicity to these species. Benthic species diversity appears to be somewhat poor in Östhammarsfjärden, potentially limiting any impact from treatment with Al. In addition, the experiment conducted in Löparöfjärden (which has similar water chemistry to Östhammarsfjärden) showed no significant effects on plankton, epifauna or benthic invertebrates after addition of aluminium to the sediment. Enclosure experiments were conducted at Löparöfjärden in 2011 where aluminium was added directly to the sediment and chemical data were collected. Modelling shows that elevated concentrations can be expected during the early phase after aluminium addition and that aluminium levels (mostly in the form of Al(OH)3 or soluble Al(OH)4- ) may be higher than what is expected from theoretical solubility calculations and in studied reference systems. Without additional data, it is difficult to explain why this occurred but aluminium solubility may be slightly enhanced under the pH and ionic strength (salt content) conditions found in the brackish water of Östhammarsfjärden. Organisms that are present in the water column may be exposed to elevated aluminium concentrations during a prolonged time pe- riod lasting from several days to weeks depending on the physical mixing of the water column after the addition. Based on the information in this report, use of aluminium salts is likely to be an effective method for reducing internal phosphorus loading in Östhammarsfjärden. Short term effects on aquatic biota should be expected, but long term improvements in the abundance and diversity of species should 27increase due to the improvement in water quality. Given the results from the Löparöfjärden experi- ment, the risk for negative effects on plankton, epifauna and benthic fauna present in Östham- marsfjärden should be low. Because there are very few data on full scale brackish water aluminium treatments, however, we recommend a complete chemical analysis including pH, alkalinity, total or- ganic carbon and major cations and anions during a full scale treatment for the sake of better under- standing of the on-going chemical processes during aluminium addition to brackish water. The data may also be useful for describing the process or process behind the slightly elevated aluminium con- centrations seen during the Löparöfjärden experiment. We also advise examination of sediment alumi- nium and phosphorus content as well monitoring of aquatic biota following treatment to assess the potential for long term impacts of treatment on sediment chemistry and the aquatic community.