River Discharge of Humic Substances and Humic-bound Metals to the Gulf of Bothnia

Water from 24 Swedish and Finnish rivers running to the Gulf of Bothnia (comprising the Bothnian Sea and Bothnian Bay) was sampled on five occasions in 1991–93, and analysed for concentrations of total organic carbon (TOC), humic substances and metals (Fe, Al, Cu, Zn). In general, TOC is higher in the Finnish rivers, which drain forest and peatlands, than in the Swedish rivers, which drain vast mountainous and forested areas. The pH is slightly lower in the Finnish rivers. Humic substances comprise approximately 80% of the total amount of organic matter. Calculations of the specific transport of organic matter (i.e. the total transport per catchment area) showed that the average release from the Finnish catchments is more than twice the release from the Swedish catchments. Fe and Al exhibit the same concentration pattern as TOC. To estimate the humic fraction of some metals (Fe, Cu and Zn), a speciation study, utilizing a weak anion-exchange resin, was performed on water from rivers with different hydrochemical properties. The humic fractions of Fe, Cu and Zn were 20–40, 40–60 and <10%, respectively, of the total metal content. The total load to the Gulf of Bothnia of humic substances and Fe, Cu and Zn associated with the humic matter was calculated. It was found that the discharge of humic substances to Bothnian Bay originates primarily from the Finnish rivers, and the largest amounts contributed to the Bothnian Sea come from the Swedish rivers.