GEOCHEMICAL EQUILIBRIA OF IRON IN SEDIMENTS OF THE ROARING RIVER ALLUVIAL-FAN, ROCKY-MOUNTAIN-NATIONAL-PARK, COLORADO

Iron geochemistry of the sediments and interstitial waters of the Roaring River alluvial fan in the Rocky Mountain National Park, Colorado was studied using a chemical equilibria approach. Large concentrations of colloidal Fe in the filterable Fe of the interstitial waters results in considerable overestimation of Fe2+ activity. The overestimation of Fe(II) seriously hampered the usefulness of chemical equilibria approach in the study of Fe geochemistry. Buried soil organic matter and a high water table within the alluvial fan has produced a highly reduced environment which results in elevated colloidal and filterable iron concentrations in the interstitial waters. The source of the iron is probably iron oxides within the buried soil and primary iron-bearing minerals in the sediments. When the iron-rich solutions reach the surface, the water rapidly oxidizes and amorphous ferric hydroxides precipitate. Between 190 to 370 mmol of iron per kg of surficial material were precipitated at the surface within two months. Noticeable decreases in the total amount of C and extractable iron (20 per cent and 30 per cent respectively) in the buried soil were observed during the study period (1985-1987) which implies a significant reduction in iron dissolution and subsequent deposition across the alluvial fan.