THE GEOCHEMISTRY OF HUMIC ACID AND IRON DURING ESTUARINE MIXING

Field measurements of dissolved humic acid and iron were made along the length of seven different coastal plain estuaries in the middle Atlantic United States. Dissolved humic acid carbon represented a maximum of 20% of the dissolved organic carbon in these estuaries. One hundred percent of the dissolved humic acid (HA) and 50 to 90% of the dissolved iron were shown to be removed in these estuaries.; Laboratory experiments carried out by mixing river water with sea water showed iron removal follows second order kinetics with rates that increase linearly as the square of the salinity. Also, from these mixing experiments, no significant salt induced removal of dissolved humic acid was observed in two rivers (the Broadkill and St. Jones), but 100% humic acid removal was observed in the third (Mullica). In addition, no consistent removal kinetics were observed. These results suggest that humic acid removal is not universally dependent on salinity. The differences between dissolved iron and humic acid removal kinetics, the inability of extracted humic acid to stabilize laboratory preparations of iron hydroxide colloids, and the failure of field measured C/Fe ratios to fit proposed iron-humic acid association models leads to the conclusion that the estuarine removal of dissolved iron and humic acid may be chemically independent events.; A model of the estuarine distribution of iron as a function of salinity was derived from observed laboratory removal kinetics and the continuity equation for the conservation of mass. Predictions of estuarine iron distributions made from the model fit field data in 5 of 6 estuaries studied within the error of measurement.; Aquatic humic acid, high molecular weight organic matter, and salt precipitated organic matter were extracted from the dissolved phase of the Mullica and Broadkill estuaries and chemically analyzed. Aquatic humic acid had an elemental composition and Proton nmr spectra indicating that 30 to 60% of the hydrogen from the organic extracts mentioned above may be associated with acid mucopolysaccharides; an interpretation supported by Alcian Blue staining. Failure to find significant concentrations of monosaccharides after acid hydrolysis of these organic fractions suggests that much of the carbohydrate is composed of polyuronic acids and aminoglucans. The organic extracts contained less than 15% protein.; From 30 to 60% of the high molecular weight organic carbon was shown to be humic acid carbon. One hundred percent of the humic acid carbon found in each river could be extracted from the high molecular weight organic matter taken from the same river. Thus, aquatic humic acid originates from the high molecular weight portion of these rivers.; Less than 30% of the salt precipitated organic carbon from the Mullica and Broadkill rivers was shown to be humic acid carbon, and less than 20% of the humic acid carbon found in these rivers could be extracted from the salt precipitated organic matter fraction. Therefore, greater than 70% of the salt precipitated organic matter was different from aquatic humic acids.