Iron as a source of color in river waters.

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 490 nm. 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.