What group of humic substances determines colour of unpolluted river water? Case study from two different rivers of North-Western Russia

Purpose There is no clear answer to the question as to what kind of humic substances (HSs) (fulvic acids (FAs) or humic acids (HAs)) is primarily responsible for colour of water in water bodies. To resolve this problem, the purpose of this paper is to compare optical properties of the samples of river water with solutions (considered as standard) containing HAs and FAs in different concentrations. Design/methodology/approach The author sampled two typical unpolluted rivers of Karelian Isthmus (Leningrad Region, North-Western Russia) with different water colours - Vyun with light-brown water and Kivioja with dark-brown water. Findings It is revealed that the shape of the light absorption curve for both rivers is similar to that of FAs. Among the coefficients, is unsuitable for differentiation between HAs and FAs due to its wide transgression. In contrast, the E-540 coefficient allows one to separate these groups of HSs very well. The E-540 values of the river water samples categorise them as FAs. The values of lly from Kivioja, contain FAs with a large molecular weight in comparison with soil FAs (up to 1.75-fold higher). It shows a distinct character of river water HSs, which is consistent with the published data. Practical implications The results outline a more direct approach to decision support in the sphere of ecological monitoring of river water. Originality/value The author has concluded that river waters are coloured by FAs.