Processes Affecting the Geochemical Composition of Wetland Sediment

Geochemical analyses of sediment core and water samples from five wetlands along four major drainages of the Animas River watershed study area were undertaken to characterize wetland chemistry in an acid drainage regime. The objectives of this study were to determine the capability of the wetlands to retain or release metals produced by acid weathering processes that occur in the area, to examine the past and present effect of these weathering processes on the geochemistry of the wetland systems, and to investigate how these processes affect the composition of wetland sediment. Geochemical profiles in wetland cores and dating of cores by 14 C, 210 Pb, and 137 Cs chronology provided records of historical changes in the metal flux to the various wetlands. Analysis of nine sediment cores for major- and trace-element abundances (total and HCl-extractable concentrations), organic and mineral matter content, acid-volatile sulfide, and secondary iron oxyhydroxide revealed that, compared to iron bogs (iron-rich wetlands) in the same region, wetland sediment is characterized by higher organic content (10-90 weight percent) and generally lower concentrations of iron and aluminum. Mean concentrations from all nine cores range from 1.5 to 34.5 weight percent iron and from 3.9 to 6.9 weight percent aluminum. In individual samples, however, maximum concentrations of total iron and aluminum are as high as 50.1 and 12.0 weight percent, respectively. Trace-element content of wetland sediment varies widely. Maximum total concentrations of trace elements in wetland sediment are as follows: Cement Creek, 770 ppm copper, 1,600 ppm lead, 7,000 ppm zinc, and 88 ppm arsenic; Mineral Creek, 330 ppm copper, 700 ppm lead, 2,600 ppm zinc, and 57 ppm arsenic; South Fork Mineral Creek, 760 ppm copper, 190 ppm lead, 3,000 ppm zinc, and 37 ppm arsenic. The ranges of acid-extractable trace-element concentrations are copper, 17-200 ppm; lead, 17-640 ppm; zinc, 70-1,720 ppm; and arsenic, from below detection to 48 ppm. The percentage of an individual element that is acid extractable ranges from 0 to about 15 percent. Based on these iron, aluminum, and trace-element values, the wetlands appear to concentrate some elements within solid phases. Four water samples from South Fork Mineral Creek showed low concentrations of dissolved iron and aluminum (<1 ppm), zinc (<50 ppb), and arsenic (below detection; <10 ppb). The sample from the westernmost locality was highly acidic (pH 3.2) with relatively higher concentrations of dissolved major and trace elements, but sediment from the associated wetland had among the lowest total abundances of solid-phase metals. This results from decreased metal adsorption at lower pH and low availability of metals from surrounding rock units that are not highly mineralized or hydrothermally altered. The easternmost wetland sediment had slightly higher total metal abundances, and copper, in particular, was enriched in the sediment. These two samples illustrate the increasing importance of adsorptive processes with higher pH, and the increased availability of metals from altered or mineralized rocks (or windborne metal-bearing detritus) farther east along South Fork Mineral Creek. A water sample from mainstem Mineral Creek had 2-18 times higher concentrations of elements than South Fork Mineral Creek samples, a result of Mineral Creek draining an acid-sulfate alteration zone. A nearby wetland sediment was also enriched in most elements, probably through deposition of windborne metal-rich detritus. One site, the Forest Queen wetland along the upper Animas River, was cored in detail (five cores) to determine the role of sulfate reduction and resulting sulfide mineral formation in the sequestration of metals from influent acidic, metal-rich water. Acid-leaching (6N HCl) of sediment with recovery of acid-volatile sulfide (AVS) was used to examine associations of metals with authigenic sulfide minerals. High concentrations of extractable metals generally were not associated with the highest AVS yields, indicating that although metals may be sequestered in iron monosulfide phases, the process is unevenly distributed throughout the wetland. The maximum total concentrations of trace elements in Forest Queen wetland sediment are copper, 215 ppm; lead, 1,600 ppm; zinc, 3,250 ppm; and arsenic, 73 ppm. Proximity to the source water and metal flux to core sites are important factors in metal enrichment. Element enrichment in the upper 60 cm and deeper than 80 cm indicate two possible flow paths and (or) sources of metal in the wetland.