Iron as a source of color in river waters

• Published in: The Science of the total environment Vol. 536; pp. 914 - 923
• Main Authors: Xiao, Yi-Hua, Räike, Antti, Hartikainen, Helinä, Vähätalo, Anssi V.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.12.2015
• Subjects: Absorption coefficient; Base flow; Carbon; Catchments; Color; Hydrology; International trade; Iron; Rivers; Total organic carbon; Water color; Iron; Absorption coefficient; Total organic carbon; Water color
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2015.06.092

Organic chromophores of total organic carbon (TOC) and those of iron (Fe) contribute to the color of water, but the relative contributions of colored organic carbon (COC%) and Fe (Fe%) are poorly known. In this study, we unraveled Fe% and COC% in 6128 unfiltered water samples collected from 94 Finnish river sites of contrasting catchment properties. According to regression analysis focusing on TOC alone, on average 84% of the mean TOC consisted of COC, while 16% was non-colored or below the color-detection limit. COC and Fe were much more important sources of color than phytoplankton (chlorophyll a as a proxy) or non-algal particles (suspended solids as a proxy). When COC and Fe were considered as the only two sources of color, COC% ranged from 16.8% to 99.5% (mean 71%) and Fe% from 0.5% to 83.2% (mean 29%). Similar Fe% and COC% values were obtained when color was estimated from the absorption coefficients of COC and Fe at 490nm. Fe% increased as a function of the concentration of Fe and was well predicted by the TOC-to-Fe mass ratio. In 608 samples with TOC-to-Fe ratios of <4.5, Fe dominated the color. TOC-to-Fe ratios varied widely within most sites, but in relation to hydrology. In catchments with a peatland coverage of >30%, peak flow exported elevated amounts of TOC relative to Fe and resulted in a high COC%. Base flow, instead, mobilized elevated amounts Fe relative to TOC and resulted in a high Fe%. In a catchment covered with 31% of agricultural fields, peak flow transported eroded soil particles high in Fe and thus resulted in a high Fe%, while during base flow the water was high in COC%. This study demonstrated that Fe% and COC% vary widely in river water depending on the catchment properties and hydrology. [Display omitted] •The quantitative contribution of Fe to water color was examined.•Fe and TOC explained 86% of the color variability in 6128 water samples.•On average one-third of the color in the examined Finnish rivers comes from Fe.•Fe is a dominant source of color in Finnish rivers with TOC-to-Fe mass ratio <4.5.