Lake Mälaren, located in eastern Sweden, supplies water for drinking use to more than two million people in Stockholm and the surroundings. Water quality is generally rather good; although concern has been raised in the last years due to observed increasing trends in both color and dissolved organic carbon (DOC) in other surface waters in Scandinavia. This may eventually occur in Lake Mälaren. DOC can carry along contaminants and toxic compounds affecting the functioning of water treatment plants. A better understanding of the DOC dynamics within the catchment is essential. This requires an investigation of the spatial and temporal patterns in DOC quality and quantity. The main aim of this project is to produce the basic knowledge that will be useful to predict DOC quality and quantity in Lake Mälaren. The project is structured in
two parts: (1) areal mass transport fluxes of color and total organic carbon (TOC) within the catchment, and (2) simulation of DOC concentrations in Fyrisån subcatchment using the HBV and INCA-C models. TOC concentrations and color show similar dynamics with some differences. Catchments with lower open water proportion present browner waters due to lower retention times. TOC exports are well correlated to discharge in suggesting that TOC concentration is controlled by flow. However, there is a risk of increasing carbon concentrations after peaks in runoff, which are related to wet years. The INCA-C reproduces well the intra- and interannual variation in DOC concentration in Fyrisån, however, the model fails to capture some of the high peaks. Further studies, are needed in order to both understand the new trends in DOC concentration and
develop the INCA-C model to predict these trends. Once the new studies based on this report are carried out successfully the model could be used to predict DOC
concentrations in the future
