Landscape control of uranium and thorium in boreal streams – spatiotemporal variability and the role of wetlands

Abstract

The concentrations of uranium and thorium in ten partly nested streams in the boreal forest region were monitored over a two-year period. The investigated catchments ranged from small headwaters (0.1 km<sup>2</sup>) up to a fourth-order stream (67 km<sup>2</sup>). Considerable spatiotemporal variations were observed, with little or no correlation between streams. The fluxes of both uranium and thorium varied substantially between the subcatchments, ranging from 1.7 to 30 g km<sup>−2</sup> a<sup>−1</sup> for uranium and from 3.2 to 24 g km<sup>−2</sup> a<sup>−1</sup> for thorium. Airborne gamma spectrometry was used to measure the concentrations of uranium and thorium in surface soils throughout the catchment, suggesting that the concentrations of uranium and thorium in mineral soils are similar throughout the catchment. The fluxes of uranium and thorium were compared to a wide range of parameters characterising the investigated catchments and the chemistry of the stream water, e.g. soil concentrations of these elements, pH, TOC (total organic carbon), Al, Si and hydrogen carbonate, but it was concluded that the spatial variabilities in the fluxes of both uranium and thorium mainly were controlled by wetlands. The results indicate that there is a predictable and systematic accumulation of both uranium and thorium in boreal wetlands that is large enough to control the transport of these elements. On the landscape scale approximately 65–80% of uranium and 55–65% of thorium entering a wetland were estimated to be retained in the peat. Overall, accumulation in mires and other types of wetlands was estimated to decrease the fluxes of uranium and thorium from the boreal forest landscape by 30–40%, indicating that wetlands play an important role for the biogeochemical cycling of uranium and thorium in the boreal forest landscape. The atmospheric deposition of uranium and thorium was also quantified, and its contribution to boreal streams was found to be low compared to weathering

Metal Transport in the Boreal Landscape—The Role of Wetlands and the Affinity for Organic Matter

“…The area is up to 94% dominated by metagreywacke, which is overlain by mainly till in the upper parts of the catchment and glaciofluvial silt and sand deposits in the lower parts (Supporting Information, Figure S1). Both airborne gamma spectrometry 13 and analyses of soil cores and soil water throughout the catchment 33 indicate that the chemical composition of the mineral soils is homogeneous. The entire Krycklan catchment covers 68 km 2 , but within the framework of the Krycklan Catchment Study (KCS) 15 subcatchments are regularly sampled and analyzed for a wide range of hydrochemical parameters.…”

“…dissolved organic carbon (DOC) and elements such as Fe, Al, P, Mn, and Se. 6−12 Recently, Lidman et al 13 demonstrated that the fluxes of U and Th in small boreal streams are strongly influenced by wetlands, suggesting that wetlands may reduce the transport of U and Th from the boreal forest landscape by as much as 30− 40%. Investigations of a wetland in the same area also confirmed that the these numbers are consistent with the amounts of U and Th that historically have been accumulated in the peat.…”

“…32 Yet, while the importance of the affinity for organic matter and the ability of many metals to accumulate in peat are wellestablished, much remains unclear concerning the quantitative importance of these processes for the biogeochemical cycling of metals on the catchment scale and beyond. On the basis of the strong control that wetlands exert over the fluxes of U and Th in boreal streams 13 we hypothesized that many other metals might be affected in a similar manner. The objective of this study was therefore to further broaden the perspective by investigating the spatial variability in the fluxes of a wide range of metals with contrasting biogeochemical properties and to try to relate these spatial patterns to catchment characteristics.…”

From soil water to surface water – how the riparian zone controls element transport from a boreal forest to a stream

“…4). As concerns the stream, in turn, it is known from previous research that it is, in many respects, typical for forested catchments in the area (Cory et al, 2006;Björkvald et al, 2008;Lidman et al, 2012Lidman et al, , 2014Köhler et al, 2014). Despite their size, small streams and headwaters are important for generating runoff in the boreal landscape, even in large rivers (Bishop et al, 2008).…”

Spatial patterns of some trace elements in four Swedish stream networks

“…Some studies estimating the spatial variability of trace elements in Scandinavia, at spatial scales from 10-100 km 2 , have been published (Tarvainen et al, 1997;Edén et al, 1999;Andersson et al, 2006;Dahlqvist et al, 2007;Reimann et al, 2009;Lidman et al, 2012). The novel focus in this study is on headwaters, < 2 km 2 , in combination with larger streams in the channel network of complete rivers, including the lakes embedded in those networks.…”