Chemical characteristics of surface waters in the Forsmark area : Evaluation of data from lakes, streams, and coastal sites

Abstract

This report is an evaluation of the chemical composition of surface water in lakes, streams, and at coastal sampling sites in the Forsmark area. The aim with this study is to characterise the surface water systems in the area, and the further aim with this characterisation is to be used as input material to the safety analyses and environmental impact assessments for the potential deep repository of used nuclear fuels. The data used consist of water chemical composition of lakes, streams and coastal sites from the period March 2002 – April 2004. The sampling have been performed predominantly on a monthly basis. The emphasis of the assessment have been on surface waters (0.5 m), as the water depth at all sampling locations have been limited, and thereby the water systems are rarely stratified for prolonged periods. The characterisations have been restricted to the most commonly measured chemical parameters. The assessment has been divided into three parts: • Comparisons within and between the lakes, streams, and coastal sites, respectively. • Temporal and spatial variation, predominantly within lakes and stream sites. • Relationships between the various chemical parameters. Beside comparisons between the sampling sites within the Forsmark area, comparisons have also been made with regional and national data from the latest Swedish National Survey (2000). The analyses of temporal and spatial variation have been concentrated on the freshwater systems in the Norra Bassängen catchment area. This catchment area is the most comprehensively investigated, and it also includes the Bolundsfjärden sub-catchment, which is the area where the continued site investigations will be concentrated. The relationships among the sampling sites, the catchment areas, as well as the chemical parameters investigated, were examined by applying PCA analyses on the lake and stream data. In general, the freshwater systems in the Forsmark area are characterised by small and shallow oligotrophic hardwater lakes, i.e. they have low nutrient levels, whereas the amount of dissolved ions is very high. The primary production is dominated by benthic Stoneworths (Charales) and by cyanobacteria in the surficial lake sediments. This kind of lakes is typical for the coastal areas of the Uppsala county, which comparatively recently has emerged from the Baltic Sea. Additionally, the streams in the Forsmark area, which also are small, are often partially dry during summer. The water chemical composition in the freshwater systems is to a large extent affected by marine deposits from the latest glaciation period. In addition, intrusions of brackish water are more or less frequent in the lower stretches of some catchment areas. This area is comparatively young and the altitude is low, which implicates that some lakes are still in their latest stages of separation from the Baltic Sea. Contrastingly, some of the coastal sampling sites, mainly in the bay Kallrigafjärden, are heavily affected by inflowing freshwater, which e.g. dilute the sodium and chloride content typical for marine environments. The four different catchment areas investigated are to a large degree similar in their water chemical composition, but there are also numerous differences both between the catchment areas and within them: 1) The alkalinity is very high in the whole Forsmark area, but the level in the Gunnarsbo-Lillfjärden/ catchment is among the highest of all investigated freshwaters. Beside the elevated calcium level, most other dissolved ions are found at lower levels comparable to the other freshwaters in the area. However, the levels are generally markedly higher than other Swedish lakes. 2) The chemical composition in the Norra Bassängen catchment varies considerably, both at single sampling sites and between the sites. The large variation could partly be an effect of the comparatively large catchment area. The catchment area can be divided into three different sub-catchments, which partly possess different chemical composition. The Eckarfjärden sub-catchment is in the upper part of the Norra Bassängen catchment. The water in the outlet of Eckarfjärden has lower levels of many dissolved ions compared to the other parts of the catchment area. The main exceptions are Ca, K, I, Li, Fe, and Mn. Occasionally, the outlet possess high ammonium concentrations due to nitrogen released from the lake sediments during episodes with low oxygen levels. There are also a considerable variation within the Eckarfjärden sub-catchment. In general, the composition in the outlet and in the lake is roughly the same, whereas the inflow deviates markedly. Many dissolved ions and nutrients are found at comparatively higher concentrations in the inlet, whereas the sodium and chloride concentrations are higher in the lake and in the outlet. Two possible explanations to the differences have been proposed. First, the inlet does not cover the whole drainage area of Eckarfjärden and the differences may be a result of inflowing water with different origin. Secondly, the lake may act as a sink for certain substances due to biogenic precipitation of CaCO3. Substances like phosphate and iron may co-precipitate together with the CaCO3. However, this is not an irreversible process, and the nutrients and the calcium may be released when dissimilative processes are dominating. Besides the calcium concentration, the water from the sub-catchment Bolundsskogen generally contains more dissolved ions than the water from the Eckarfjärden area. However, in comparison to the lower parts of the Norra Bassängen catchment, i.e. the sub-area around Norra Bassängen and Bolundsfjärden, the ion levels are markedly lower, at least during episodes with brackish water intrusions that heavily affect the chemical composition in these lower stretches of the water system. Actually, there seems to have been a severe brackish water intrusion in this area, all the way up to Bolundsfjärden, before the investigations started in March 2002, as the level of many dissolved ions decrease considerably during the whole first year of investigation. 3) The water in the Fiskarfjärden catchment area is generally, together with the inlet to Bredviken, the most nutrient rich of the freshwaters in the Forsmark area. Like in Eckarfjärden, do the water in both Fiskarfjärden and in its outlet, Flottbron, possess high levels of ammonium during episodes with low amounts of dissolved oxygen. 4) The Bredviken catchment is only monitored at the inlet to the lake, and its chemical composition is the most deviating of all the freshwaters in the area. It possess among the highest levels of nutrients and many dissolved ions, whereas the levels of organic carbon, and the marine ions Na, Cl, Br, and I are low. The alkalinity is considerably higher than in the other areas. The reason why this chemical composition deviates is unclear, but it may be caused by the agricultural activities in the drainage area and, probably most important, the soil composition in the area, which is dominated by clayey moraines. The main conclusion from the investigations of the coastal sites is that the water in the bay Kallrigafjärden is largely affected by inflowing fresh??water. This inflow dilute e.g. “marine” ions like sodium, chloride, and bromide, whereas the concentration of other substances like TOC is elevated, as the level in freshwater is considerably higher. The other two coastal sites possess a fairly stable chemical composition, except from occasional inflow of water from melting snow or ice

Published in

SKB rapport
2005, number: R-05-41
Publisher: Svensk Kärnbränslehantering AB, SKB

SLU Authors

• Sonesten, Lars

  • Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Economics and Business
Fish and Aquacultural Science
Environmental Sciences related to Agriculture and Land-use


Permanent link to this page (URI)

https://res.slu.se/id/publ/6234