Hydrogeochemistry of Fe and Mn in small boreal catchments

Abstract: It is suggested that the overall understanding of the general hydrogeochemistry of a region is best understood from a landscape perspective. If so, this can provide useful information when planning monitoring programmes and influence of human activity. In this thesis the influence of landscape properties on iron (Fe) and manganese (Mn) is investigated.Iron and manganese are abundant in the Earth’s crust but occur in relatively low concentrations in most aquatic systems, due to the low solubility of their thermodynamically stable oxidation states (Fe(III) and (Mn(IV)). The hydrogeochemistry of iron and manganese is important, since their naturally occurring oxyhydroxides exhibit a strong adsorption affinity for trace elements. Moreover, the redox transformations of iron and manganese occur at pH and Eh (redox potential) boundaries found in natural waters.We investigated the hydrogeochemistry of iron and manganese in a boreal stream network of 15 streams, located in Västerbotten, Sweden. Water samples were collected on a frequent basis during 2004 and 2005, from 15 sub-catchments of the Krycklan catchment (67 km2). Total (unfiltered) and dissolved (<0.4µm) concentrations of iron and manganese were used to investigate the influence of landscape types (i.e. percentage cover of wetlands and forest) on the spatial and temporal variations of these elements.We found that iron correlates significantly with percentage of wetland and also with dissolved organic carbon (DOC). For manganese we did not observe a significant correlation with wetlands, which indicates that this element is less dependent on organic matter in comparison to iron. In particular, the correlation of the Fe/Al ratio to wetlands was highly significant (r2=0.92, p<0.01). However, principal component analyses indicate that iron, during peak discharge at spring flood, is not correlated with wetlands. During this period iron instead correlates with soil variables (i.e. silt) which highlights the importance of particulates during high discharge events.Since the naturally occurring oxyhydroxides have a strong adsorption affinity for other trace elements it is of great importance to further increase the knowledge of how these elements interact with iron and manganese in relation to the landscape composition. Further analyses of relations between trace metals (e.g. Cd, Co, Cr, Cu, Pb), and iron and manganese, but also between trace metals and landscape composition, will highlight the importance of studying the hydrogeochemistry from a landscape perspective.