High fluoride concentrations in surface water - example from a catchment in SE Sweden

摘要

We studied the fluoride occurrence and its relationship to local geological properties in a small catchment (2700ha) located at the Baltic Sea coast in southeast Sweden. The aim was to investigate the possible impact of a granite intrusion (1. 4 ga old G?temar granite) on the fluoride (F) concentrations in a stream, both on a spatial and temporal scale. This type of anorogenic granite is younger than the surrounding bedrock types and typically recognized by its richness in fluorine. The catchment is dominated by exposed bedrock (51.2%) and a thin till cover (30.3%) and 86 % is covered by coniferous and mixed forest. The intrusion is situated in the lower reaches just north of the main stem. Since the stream, which is perennial, is located in the boreal zone (N 57°) it is recognized by strong discharge fluctuations due mainly to snow melt during spring. Time series of surface water chemistry and discharge have been analyzed and combined with targeted sampling within the catchment. On a continental scale European stream waters (n=808, 25 countries) have concentrations (95 percentile) below 0.36 mg l~(-1 1). In the stream highlighted in this study (K?rrsvik) this is the case for the upper reaches of the catchment with maximum and median concentrations of 0.79 mg l~(-1) and 0.37 mg l~(-1) respectively. However, towards the stream outlet the F~- concentrations increase 1.6 to 4.7 times (during equal discharge conditions). The highest concentration measured in the lower reaches was remarkably high for a surface water with 4.16 mg l~(-1) (median 1, 13 mg l~(-1)). In comparison, the WHO guideline value is 1.5mg l~(-1) for drinking water~2. The results describe a spatial and temporal behavior of F~- that confirms the hypothesis of the G?temar granite as a source for elevated fluoride concentrations in the surface water of the catchment. The mechanism is weathering of glacial deposits, partially consisting of G?temar granite, and greisen fractures (which are strongly connected to the intrusion and, as well, rich in fluorite). This knowledge can be of significant importance in areas where overburden waters frequently exceed the maximum limit of fluoride, as occurs in parts of Sweden.