Dynamics of Humic Substance in Bolmen Lake, Sweden

Abstract: Humic substance (HS) in natural water is a complex compound, and causes problems in the production and distribution of drinking water. In addition to the adverse effect of HS on the aesthetic of water quality, it also causes a health problem. The objective of this thesis was to analyse the dynamics of HS in Bolmen lake. Bolmen lake is the tenth largest lake in Sweden. The lake is situated in the south-western Sweden and used as a drinking water source for the majority of South Sweden (Skåne). In this thesis the long term and short term variation of HS, measured as the water colour, of the lake was analysed. HS in the soil is formed from the microbial degradation of plants and animal matter in the soil, which is affected by the air temperature. HS from the catchment is transported by the runoff. The impact of air temperature on the formation and discharge on the export of HS from the catchment to the lake can be shown by their relation with the water colour correlation. Excel 2007 was used to show the correlation of water colour with air temperature, precipitation and discharge. Storån is the largest tributary river, drains about 40% of the Bolmen catchment. Therefore, the water colour dynamic of Storån river was analysed to show the dynamic of Bolme’s water colour dynamics. The 2007 mean annual colour value shows that Storån river contributes the largest portion of water colour to the lake, about 87% with Lillån river. The long term variation shows that there are two ten years; one with smaller annual mean colour value (1988-1997) and the other with higher annual mean colour value (1998-2007). Yearly peak value of water colour mainly occurs in summer and autumn, and the yearly minimum value occurs mainly in winter and spring. The correlation between flow rate and colour value also depends in the season of a year. The correlation was stronger and positive in summer and winter; and it was very weak in the rest of a year. The correlation of water colour with air temperature as well as with precipitation was very weak. The climate change in Sweden is predicted to have increased runoff. Since the discharge has a positive correlation with water colour value, the climate change could have the increasing impact on the water colour of Bolmen lake.