Hydrogeochemistry of Fe and Mn in small boreal streams: The role of seasonality, landscape type and scale

Streamwater from a stream network of 15 small boreal catchments (0.03–67 km) in northern Swedenwas analyzed for unfiltered (total) and filtered (<0.4 lm) concentrations of iron (Fetot andFe<0.4) andmanganese (Mntot andMn<0.4). The purposewas to investigate the temporal and spatial dynamics of Fe,Mn and dissolved organic carbon (DOC) as influenced by snowmelt driven spring floods and landscape properties, in particular the proportion of wetland area. During spring flood, concentrations of Fetot, Fe<0.4,Mntot,Mn<0.4 andDOC increased in streamswith forested catchments (<2%wetland area). In catchmentswith high coverage of wetlands (>30%wetland area) the opposite behavior was observed. The hydrogeochemistry of Fe was highly dependent on wetlands as shown by the strong positive correlation of the Fetot/Altot ratio with wetland coverage (r 2 = 0.89, p < 0.001). Furthermore, PCAanalysis showed that at base flowFetot andFe<0.4 were positively associatedwithwetlands andDOC,whereas theywere not associated during peak flow at spring flood. The temporal variation of Fewas likely related to varying hydrological pathways.At peakdischargeFetot was associatedwith variables like silt coverage,which highlights the importance of particulates during high discharge events. ForMn there was no significant correlation with wetlands, instead, PCA analysis showed that during spring flood Mn was apparently more dependent on the supply of minerogenic particulates from silt deposits on the stream banks of some of the streams. The influence ofminerogenic particulates on the concentration of, in particular,Mnwas greatest in the larger, lower gradient streams, characterized by silt deposits in the near-stream zone. In the small forested streams underlain by till, DOCwas of greater importance for the observed concentrations, as indicated by the positive correlation of both Fetot and Fe<0.4 with DOC (r 2 = 0.77 and r = 0.76, p < 0.001) at the smallest headwater forest site. In conclusion, wetland area and DOCwere important for Fe concentrations in this boreal stream network, whereas silt deposits strongly influencedMn concentrations. This study highlights the importance of studying stream water chemistry from a landscape perspective in order to address future environmental issues concerning mobility of Fe, Mn and associated trace metals. 2008 Elsevier Ltd. All rights reserved.