Balticsea 2020

Phosphorus binding in shallow eutrophic bays – which substances work?

The effects of eutrophication in shallow bays around the Baltic Sea are persisting, despite a reduction in nutrient inputs from land. One reason for this is a deterioration in the capacity of bottom sediments to bind phosphorus. This project will investigate potential phosphorus-binding methods as a way of tackling eutrophication.

A lack of oxygen in sediment is unusual in shallow coastal bays. The water is renewed on a regular basis, which contributes to good oxygen conditions in bottom water. However, despite excellent water renewal, phosphorus released from sediments is a huge problem and one that contributes to eutrophication. It is clear that bottom sediments lack sufficient phosphorus-binding substances.

As part of BalticSea2020’s large-scale demonstration project “Living coast”, an aluminium treatment was used to great effect on the anoxic sediment in the Björnö bay during the summers of 2012 and 2013, enabling the sediment to bind phosphorus again. Following the treatment, the bay was found to have the same good water quality as it had back in the 1950s. However, studies show that aluminium treatment in shallow eutrophic bays works less well, particularly if the water has a high pH (over 9), because the aluminium-phosphorus binding process is destabilised. In order to identify alternatives to aluminium, this project will examine other substances that can bind phosphorus in shallower water effectively.

The Gran bay
The Gran bay, near Östhammar in Norduppland, has been chosen as a suitable bay for development of measures to tackle internal phosphorus loading in shallow bays. This bay is shallow and eutrophic, which is predominantly due to phosphorus being released from the bay’s bottom sediment. Östhammar Municipality has, in recent years, surveyed and dealt with the majority of external loading from the bay’s catchment area, but because the sediment is still releasing phosphorus, eutrophication has persisted.

Iron – a suitable phosphorus binder
In the summer of 2017 a small-scale pilot study was conducted in a laboratory environment, on behalf of Östhammar Municipality and with local water conservation (LOVA) funding, which revealed that the addition of iron can reduce phosphorus release from sediments and that iron should bind phosphorus as effectively as aluminium. Iron is also naturally the dominant phosphorus binder in sediments. This project will examine the function and efficacy of adding iron as a way of stopping phosphorus from being released from the sediment in the Gran bay. It will also evaluate the efficacy in relation to other potential binders, such as aluminium chloride (same as in the Björnö bay) and marl.

Method
The project will use so-called enclosures installed in the Gran bay (10 in total). An enclosure is a delimiting area in the bay where the efficacy of chosen methods can be evaluated in a controlled manner. Here four additives will be examined in two enclosures each: iron in a dissolved state, iron in granular form, aluminium chloride and marl. Two of the enclosures will be left untouched as a control.

Measurements will be taken every two weeks to evaluate the effect of the chosen methods. Finally, the project will be described in a final report that will include an evaluation and recommendations (please visit the Swedish website for more information).

The project is being funded by BalticSea2020 with grants from Östhammar Municipality (incl. LOVA grants).