Amendment effectiveness and seasonal variations in a lead contaminated acidic wetland

Three in-situ lead (Pb) soil amendment treatment methods were tested over 26 months in a naturally acidic contaminated wetland with the intent of sequestering high levels of Pb contamination as stable mineral forms. Two of the treatments used Apatite II, a phosphate-based amendment, to promote the formation of pyromorphite. The apatite amendment was surface applied one method and physically mixed to the surface soils in the other. A sulfate-rich compost-based mixture was used for the third treatment, with the goal to promote the formation of galena. Amendment effectiveness was evaluated by comparison of Pb porewater concentrations between treatments, and by changes in the speciation of sediment Pb as determined by X-ray absorption spectroscopy (XAS). Results show that in the two years following application, the three treatments did not make a significant impact on Pb mobility and speciation. Dissolved Pb concentrations did not decrease significantly in the treatment plots in comparison to the control plots. Furthermore, speciation analysis did not show evidence of pyromorphite formation in the phosphate-amended plots. Galena was found in the third treatment with sulfate-rich compost, however the significance of this was limited by sample size and the presence of galena in other areas of the wetland. There was a seasonal variation in the Pb speciation, with galena and Pb-cysteine being more prevalent at the end of the wet season and less prevalent at the end of the dry season, indicating that while the induced formation of reduced Pb-sulfur species is possible, there are challenges to maintaining those species without concurrent control of soil water saturation. This study shows that effective amendment treatments must account for site-specific characteristics such as pH and redox dynamics, and use appropriate preservation methods for accurate speciation results.