The geochemistry of tungsten (W) in the environment is poorly studied. Tungsten usually occurs in low concentrations in natural waters and is not very mobile. For this study, we analyzed W together with molybdenum (Mo) in the dissolved and particulate fractions of two boreal estuaries during different seasons. Additionally, we sampled first-order streams that drain different landscape types and the receiving northern Baltic Sea. Furthermore, surface sediment from the estuaries was analyzed to obtain a comprehensive overview of the distribution of W and Mo in a boreal environment.

Both elements showed different distribution patterns during different seasons. While they decreased in the dissolved fraction during spring discharge, in winter, their concentrations were elevated. Molybdenum exhibited non-conservative behavior along the salinity gradient in winter, which was probably caused by its release from underlying sediments. In the particulate fraction, we found opposite behaviors for Mo and W, with higher particulate W and lower particulate Mo during spring discharge.

Molybdenum and W underwent fractionation from land to sea, indicating the different mobilities of these oxyanions. The Mo/W ratio in the dissolved fraction was mainly determined by the Mo concentration, as the W concentration varied only in a narrow range from first-order streams to the Bothnian Bay. In the particulate fraction, the Mo/W ratio appeared to be affected by scavenging processes and showed only small variations.