Potentiell fosforfrigörelse från sedimentet i Bornsjön, sydvästra Stockholm
The potential leakage of phosphorus from sediment in Lake Bornsjön, south western Stockholm
Lake Bornsjön is situated 30 km southwest of Stockholm in a rift valley lake and consist three basins outstretched in northwest-southeast direction. The surface area is 6,7 km2 with a maximum depth of 18,3 meters, a mean depth of 9,8 meters and a retention time of the water of 6,5 years. The drainage area is 42 km2 and consists mainly of arable land (ca 1000 ha) and productive forestry land (2500 ha), but in the drainage area there are also restored and fairly newly constructed wetlands.Today the Stockholm area receives its drinking water from Lake Mälaren, but if there would be any pollution of this source, Lake Bornsjön would act as the water source. The area surrounding the lake was bought already in 1899 by Stockholm Stad for protection of the lake and was taken over 1993 by Stockholm Vatten AB. It has been a water protection area from 1988 and it was also accepted as a nature reserve in 1995. This involve some restrictions like prohibition against fishing, bathing and to tread the ice on the lake in the winter and only back-up water activities are allowed on and in the lake. According to information from the 1970s the phosphorus content increased in the surface and bottom waters and later in the mid 1980s the phosphorus decreased (Lännergren 1996). 1986 Lake Bornsjön showed some indications of a deteriorated lake status which lead to an investigation the following year that showed an accumulation of phosphorus in the sediment of the deepest parts of the lake (Pettersson 1987).The purpose of the study was to investigate the present leakage of phosphorus from the sediment to the water column in Lake Bornsjön today and to compare the results with the investigation carried out in 1987. The study aims is also to investigate in which basin the leakage of phosphorus is most severe and to try to explain the variation between the basins. Three different methods were used to analyse the content of the phosphorus in the sediment; phosphorus fractionation, ICP-AES and XRF. In a fractionation the phosphorus is determined in six different forms such as NH4Cl-P (loosly sorbed P, porewater-P and in hardwater lakes also CaCO3- associated P), BD-P (P absorbed to iron and manganese and these forms are sensitive for low redox potential), NaOH-rP (P associated mainly with aluminium), NaOH-nrP (organic-P including bacteria-incorporated P), HCl-P (assumed endured apatite) and finally residual phosphorus witch you calculate by taking total phosphorus (TP) minus the extracted forms of phosphorus. The residual phosphorus consist refractory organic P and inert inorganic P. The ICP-AES - method (Inductively Coupled Plasma Atomic Emission Spectroscopy), is used to investigate the amount of elements, in this study phosphorus, iron and aluminium. ICP-AES is a multi element technique and the method can be used to determine around 40 different elements. In the XRF - method (X-ray-fluorescence) the sample is irritated by X-ray beams which results in electron excitation and when the electron falls back in to their original shells it establish fluorescence radiation (Hulterström 2006). The radiation energy is registered by a half detector and dependence on from which shell that the electron falls from it create different kinds of energy and this makes it measurable. With a pulse counter the different energies and elements are separated. The XRF- technique is more sensitive in detecting heavier element.Field work was carried out from a boat using a gravity corer to collect lake sediment cores. In total 18 sediment cores was sampled which were taken at three different occasions in the fall 2006. The sediment sampling was carried out at the same time as the ordinary water samples were collected. The study also included a phosphorus leakage experiment in vitro where sediment cores with the water column above were incubated in 22 days.The results of the investigation show that there is highest share accumulated total phosphorus, deeper than 10 m, with an assumed sediment density of 1 000 000 g/m3 (personal communication Rydin Emil) in the Skårby Basin (BS) 14,90 g/m2 but the highest amount of mobile phosphorus was found in the Bornsjö Basin (BB) 7,08 g/m2 based on data from sediment layer 0-17 cm. The average of total phosphorus in the sediment for the whole lake deeper than 10 m was at the current sampling occasion (2006-10-17) 12,08 g/m2 and mobile phosphorus, 6,54 g/m2. These results are presumably an affect of the artificially induced oxygen in the bottom water by the hypolimnion unit. The hypolimnion unit is placed in the Bornsjö Basin (BB) on 14 m water deep and through aeration of the bottom water the phosphorus accumulates in the sediment. The oxygen that is brought down to the bottom of the lake forms an aerobic environment near the sediment surface which prevents phosphorus leakage (internal loading). Because of a high amount phosphorus in the sediment of Lake Bornsjön is bound to ferric iron (Fe3+ or Fe(III)) in aerobic condition, there is a big threat that this phosphorus will be released when Fe(III) will be reduced to Fe(II) and there is a large risk that it will react with SO42 under anaerobic conditions. The water depth and the location in the lake plays an important role of the amount accumulated phosphorus which is statistically shown in the study. The hypolimnion unit was initiated in 1987 and has since then been active every year between June - October except 2004. Under 2004 a pumping project took place in the Edeby Basin. During this project the bottom water was pumped out of the lake and the result in this rapport show some indications that this project was successful to reduce phosphorus content in Lake Bornsjön. The phosphorus amount in the water during 2006 May-October was 17,27 ?g P/L and by this value the lake is a mesotrophic lake.The results from the comparison between different methods to analyse phosphorus show that the methods give quite similar results but the fractionation method is the most informative method about how phosphorus is connected to other substances and consequently the most adequate method to use to calculate the mobile amount of phosphorus.It can be concluded that the phosphorus in Lake Bornsjön mainly derive from the internal loading of the lake, but also from the diffuse inflow from the arable land surrounding the lake and from the densely populated parts in the drainage area. The hypolimnion unit that artificially induces oxygen in the bottom water and makes the phosphorus accumulate in the sediment is positive because phosphorus stays in the lake and is not transported to other lakes or the Baltic Sea.