Total organic carbon in streamwater from four long-term monitored catchments in Norway

Abstract

By linear regression analyses, flux, concentration, and net charge (NC in μeq/mg C) of total organic carbon (TOC) have been related to different physico-chemical parameters present in air/precipitation and streamwater at four long-term monitored catchments in Norway, during the period 1986–1992. The catchments vary a lot with respect to annual water input, acid rain, and the streamwater concentration of TOC. Thus, relationships between concentration/NC of TOC and chemical compounds in precipitation and streamwater were often catchment-specific. However, seasonal climatic changes, like air temperature and hydrology, were found to be important for the concentration variations of TOC. The correlation between concentration of nonmarine base cations (ΣBC ∗) and concentration of TOC at the four sites was far more significant (r=0.86) than the corresponding correlation between nonmarine sulphate (SO 4 ∗) and ΣBC ∗ (r=0.57). This indicates the important role of organic matter in weathering reactions in the catchment. Concerning weathering and cation leaching of aluminum, the correlation between concentration of total aluminum (RAL) and SO 4 ∗ was far more significant (r=0.93) compared with corresponding correlation between RAL and TOC (r=0.40). This should be expected, because a high concentration of SO 4 ∗ means a low pH, which is needed to dissolve substantial amounts of Al from soils. The NC of TOC was found to be most affected by compounds in precipitation, primarily the inputs of sea salts. In the most acidified areas, the influence from strong acid inputs also affects the NC of TOC, but less significantly. The concentration of Na + in precipitation is much higher and the variations much larger compared with the concentration of H + in precipitation in the coastal areas, like Birkenes and especially Kaarvatn. Thus, it is reasonable that the variations in sea salt inputs (primarily NaCl) are more important for the temporary variation in NC of TOC compared with H + ions at these sites. Only at Langtjern, the most TOC-influenced site, a significant and positive correlation was found between H + and the concentration of TOC in streamwater. This indicates that at a certain TOC level, the weak organic acids may affect the streamwater pH. A comparison between measured and calculated concentrations of organic Al indicated that the dissociation constants (pK-values) of the organic acids present at Birkenes must be higher and/or the Al-complexing constants lower compared with the constants given by the programme. At the three other sites, it was the other way around. This may explain why a negative correlation between concentrations of TOC and labile Al was present at Birkenes, while the other sites exhibited a corresponding positive correlation with respect to labile Al or ΣAl n+. The comparison also documented larger differences in these constants between the sites, compared with monthly variations in the constants at one single site.