IS THERE ANY DISSOLVED INORGANIC IRON IN STREAMS?

Iron plays an important role in the fluvial transport of phosphate, carbonates and sulfides, as well as contaminants such as arsenic. Therefore, understanding whether fluvial iron is transported dominantly in particulate or dissolved form can help explain the contribution of iron to contaminant transport and eutrophication. The term “dissolved” is operationally defined as less than 0.45 microns. Although this definition is applicable for most solutes, we hypothesized that the majority of iron in piedmont streams should occur in particulate form because of near neutral pH and oxidizing conditions in the streams. Thirty-two water samples were collected in both urban and rural streams in the piedmont of northwestern South Carolina under baseflow conditions. The samples were passed sequentially through 0.45, 0.22, 0.10, 0.050, and 0.025 micron filters and iron concentrations were measured by ICP-AES. Iron concentrations for the 0.45 micron filtering ranged from 0.0146 mg/L to 1.035 mg/L while concentrations for the 0.025 filtering ranged from 0.003 mg/L to 0.091 mg/L, for an average decrease in concentration of 87%. The largest decreases occurred between 0.45 and 0.10 micron, with little change below 0.10 micron. Speciation models using Visual MINTEQ suggested that samples above 0.10 micron were supersaturated with respect to various iron minerals such as ferrihydrite and hematite. Below 0.10 micron, the water was undersaturated with respect to iron minerals and nearly 100% of the dissolved iron was complexed with dissolved organic carbon. In contrast to the speciation models, plotting individual samples on a ferrihydrite stability diagram suggested that many samples were still saturated with respect to ferrihydrite after filtering through 0.025 micron. The comparison between the results of the iron concentration data, speciation model, and stability diagram led us to conclude that little, if any, of the iron in streams was in the inorganic dissolved form. Instead, we suggest that most of the iron was in the form of colloidal particles, most likely ferrihydrite. Furthermore, any dissolved iron was kept in solution by complexing with dissolved organic carbon, which itself may be colloidal. Therefore, we suggest that the common assumption that dissolved inorganic iron exists in streams is likely incorrect.