Origin and nature of humic substances in the waters of the Amazon River Basin

Abstract Humic substances dissolved in "black water, white water, and clear water" rivers of the Amazon River basin were compared were humic substances found in podzolic (spodic) and latosolic (oxic) soils of the Amaon region. Humic-type organic solutes in black water originated from organic matter decomposition and ground water drainage from podzolic soils; clear water drains from latosols because colored humic solutes are adsorbed on clay minerals contained in latosols. Comparisons of organic solute distributions in tropical surface waters versus temperate surface waters by dissolved organic carbon fractionation analysis chow that the only significant difference was a lower hydrophilic neutral content of tropical surface waters. This finding is indicative of more rapid decay processes In tropical waters. Black-water rivers of the Amazon contained cndy one-third to one-half the organic solute content contained in black-water rivers of the southeastern United States. About 50 percent of the organic carbon In black waters of the Amazon consisted of colorless non-humic organic solutes. Humic solutes contained in Amazon black water differed from humic solutes found In the Suwannee River in the southeastern United States by having a higher E4/E6 ratio in the visible absorbance spectrum which resulted in water of a more reddish coloration. Both the high E4/E6 ratio and low-angle x-ray scattering data suggest relatively small molecular dimensions for humic solutes in Amazon black water. Organic elemental analyses and infrared analysis of humic substances in Amazon waters and soils were typical for humic substances except for the low content of nitrogen for humic solutes in black water.