Temporal and spatial trends for trace metals in streams and rivers across Sweden (1996–2009)

Abstract

Abstract. Long term data series (1996 through 2009) for trace metals were analyzed from a large number of streams and rivers across Sweden varying in tributary watershed size from 0.05 to 48 193 km 2 . The final data set included 139 stream sites with data for arsenic (As), cobalt (Co), copper (Cu), chromium (Cr), nickel (Ni), lead (Pb), zinc (Zn), and vanadium (V). Between 7 % and 46 % of the sites analyzed showed significant trends according to the seasonal Kendall test. However, in contrast to previous studies and depositional patterns, a substantial portion of the trends were positive, especially for V (100 %), As (95 %), and Pb (68 %). Other metals (Zn and Cr) generally decreased, were mixed (Ni and Zn), or had very few trends (Co) over the study period. Trends by region were also analyzed and some showed significant variation between the north and south of Sweden. Regional trends for both Cu and Pb were positive (60 % and 93 %, respectively) in the southern region but strongly negative (93 % and 75 %, respectively) in the northern region. Kendall's τ coefficients were used to determine dependence between metals and potential in-stream drivers including total organic carbon (TOC), iron (Fe), pH, and sulphate (SO 2− 4 ). TOC and Fe correlated positively and strongly with As, V, Pb, and Co while pH and SO 2− 4 generally correlated weakly, or not at all with the metals studied.

Long-term patterns in dissolved organic carbon, major elements and trace metals in boreal headwater catchments: trends, mechanisms and heterogeneity

“…We expected that most metals should increase with DOC at least in autumn (except iron) but we observed the opposite. Though the declining trend that we observed in zinc is understandable as the element has weak binding capacity with organic matter (Hernandez et al, 2006;Huser et al, 2012) and its deposition is generally declining in Northern Sweden (Huser et al, 2011). However, stream copper concentration showed a similar declining trend on both an overall and seasonal basis despite its stronger affinity to DOC.…”

“…However, the interpretation of copper behaviour appeared to be more complicated than the complexation hypothesis stated above, as seasonal declining trends were observed in January, March, and June when there was a corresponding increase in pH. The recent study by Huser et al (2011) reported a similar declining pattern in copper trends across Sweden. Landre et al (2009) also noted that little or no relationships existed between copper and DOC, suggesting that there are more subtle processes controlling copper biogeochemistry in headwater catchments that are yet to be understood.…”

“…However, stream copper concentration showed a similar declining trend on both an overall and seasonal basis despite its stronger affinity to DOC. The significant declining trend in copper that we observed can be attributed more to regional controls such as non-alkaline nature of soils in the northern boreal catchments (Huser et al, 2011) as pH increase was not significant in most months. However, the interpretation of copper behaviour appeared to be more complicated than the complexation hypothesis stated above, as seasonal declining trends were observed in January, March, and June when there was a corresponding increase in pH.…”

“…Atmospheric deposition of trace metals decreases significantly in Sweden toward the northern part of the country (Huser et al, 2011) where our study catchment is located. We expected that most metals should increase with DOC at least in autumn (except iron) but we observed the opposite.…”

Spatial patterns of some trace elements in four Swedish stream networks

“…We used data, covering the period October 1996-2009, from 132 reference sites at the Swedish national environmental monitoring programme for watercourses in southern Sweden for comparison with observed concentrations in the present study (same sites as in Huser et al, 2011). Concentrations of Co, Fe and TOC from this study were usually higher than the median of the national programme, while this study's values were lower for Cu, Mo and Zn.…”