Precipitates forming at various stages of acid mine drainage treatment in a high metal load (≈1000 mg L−1 Fe) and low pH (≈3) constructed wetland were characterized by chemical dissolution, x‐ray diffraction, thermal analysis, and scanning electron microscopy. Minerals precipitating in flumes and in entry wetland cells lacking vegetation included poorly crystalline ferrihydrite, lepidocrocite, goethite, possibly an akaganéite‐like mineral, and high Fe/S ratio Fe oxyhydroxysulfates (similar to schwertmannite). Within vegetated wetland cells lined with crushed limestone, well‐crystallized gypsum, lepidocrocite, and Fe‐oxyhydroxysulfate minerals with low Fe/S ratios were accompanied by gradual reductions in ferrihydrite and akaganéite. The Fe/S molar ratios of Fe oxyhydroxysulfates in flume precipitates averaged 5.2 ± 0.3, while those of cell precipitates averaged ≈3.5 ± 0.5. The oxalate‐extractable (Feox) to total (Fet) Fe fraction of the precipitates was considerably higher in wetland cells (1.1 ± 0.3), where organic C was 10‐fold higher than in entry flumes (0.7 ± 0.1). Scanning electron micrographs of flume precipitates showed a fiber‐like morphology of densely aggregated spherical particles, 1.5 to 2.0 mm in diameter, with a closely packed microcrystalline matrix. Precipitates collected from vegetated wetland cells formed aggregates of somewhat smaller diameter spherical particles with grassy surfaces or finger‐like projections entangled with bacterial cells. The overall composition of the precipitates suggested that the Fe chemistry is controlled primarily by the solubility of Fe oxyhydroxides in flumes and by S‐enriched Fe oxyhydroxysulfates inside the wetland cells. Although jarosite and goethite are thermodynamically favored in the wetland cells, their formation appeared to be inhibited by the presence of organics and the precipitation of Fe oxyhydroxysulfates and gypsum.
The composition of soil solutions and surface waters emanating from unreclaimed or partially reclaimed stripmined watersheds with low buffering capacity in Kentucky were compared with soil solution compositions of unaffected strata in the watershed. The data suggest that almost 20 yr after mining, most soil solutions and surface waters of the disturbed areas still contain high levels of dissolved Al, controlled primarily by the solubilities of a jurbanite‐like mineral (upper limit) and alunite (lower limit). Soluble Al in solutions of undisturbed areas was consistent with the solubility of kaolinite or gibbsite. The absence of jurbanite x‐ray diffraction peaks suggested the presence of an amorphous or a mineral stoichiometrically similar to jurbanite. Despite greater contact times of soil solutions with the soil compared to surface waters, their compositional differences were insignificant. The control of soluble Al by basic aluminum sulfate minerals was not affected by the variable mineralogical and textural composition of soil and geologic strata in the watershed. Apparently, this is the result of low buffering capacity. At pH < 4, pH and sulfate activities can be used to accurately predict the levels of soluble Al3+ in surface and groundwaters of the watersheds. Similar predictions from pH and SO2−4 activities can also be made for dissolved Fe3+ levels, supporting the stoichiometry but a much higher solubility than that of jarosite.
Redox potentials (Eh) were monitored bimonthly and porewater chemistry was analyzed seasonally at three slightly-acidic, high-elevation Kentucky wetlands that differed in hydrology, parent materials, and vegetation. At all sites, Eh values were below 300 mV, which indicated that reducing conditions persisted within the upper 90 cm and fluctuated mainly within the range of iron and sulfate reduction. Significant relationships of Eh values with depth were observed only at the Martins Fork wetland, where precipitation was the primary water source. The strongest and most stable reducing conditions, observed at the Kentenia site, reflected consistently high water levels, which were sustained by ground water. The third wetland (Four Level) was distinguished by irregular Eh fluctuations coinciding with strong seasonal ground-water upwelling. Although Fe 3+ and SO 4 2-were the primary terminal electron acceptors in all wetlands, porewater chemistry also varied significantly by season and soil depth in response to piezometric water level fluctuations. Additional factors that influenced porewater chemistry included: (1) the presence of limestone parent materials that affected porewater pH, Ca 2+ , and Mg 2+ ; and (2) the prevalence of sphagnum moss or graminoid species that influenced dissolved organic carbon, CO 2 , and CH 4 . Results from this study indicated the diverse range and importance of multiple factors in controlling biogeochemical processes and properties in small, highelevation Appalachian wetlands.
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