Porewater transport and diagenetic reactions strongly regulate the mobility of metals in sediments. We executed a series of laboratory experiments in Gust chamber mesocosms to study the effects of hydrodynamics and biogeochemical transformations on the mobility and speciation of Zn in contaminated sediments from Lake DePue, IL. X-ray absorption spectroscopy (XAS) indicated that the oxidation of surficial sediments promoted the formation of more mobile Zn species. Bulk chemical measurements of porewater, overlying water, and sediment also suggested that this process liberated aqueous metals to porewater and facilitated Zn efflux to the overlying water. In addition, sediment resuspension events increased the release of aqueous metals to both surficial porewater and the overlying water column. XAS analysis indicated that resuspension increased dissolution of Zn-sequestering mineral phases. These results show that both steady slow porewater transport and rapid episodic resuspension are important to the release of metal from fine-grained, low-permeability contaminated sediments. Thus, information on metals speciation and mobility under time-varying overlying flow conditions is essential to understanding the long-term behavior of metals in contaminated sediments.

X-ray absorption spectroscopy is often used for assessing the chemical speciation of metals in complex environmental matrices. The approach consists of performing spectral decompositions that rely on the selection of a suite of reference spectra representing the various coordination environments of the metal in the sample. From an experimental perspective, the quality of the data collected (that is the level of noise compared to the number of counts from the selected element) plays an important role in assessing the confidence of the speciation results. We present a method for the selection of an appropriate set of reference spectra based on the quantitative assessment of their interdependency. This method is illustrated in the case of the selection of reference spectra for performing nickel speciation.

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