Spatial Heterogeneity of Geochemical and Hydrologic Parameters Affecting MetalTransport in Ground Water

摘要

Reliable assessment of the hazards or risks arising from groundwatercontamination requires the capability to predict the movement and fate of dissolved solutes in groundwater. The modeling of metal transport in groundwater requires adsorption coefficients to describe ion adsorption. Metal-ion sorption in natural systems is usually thought to be controlled by surface reactions with Fe and Al oxyhydroxides and organic coatings on particles. The importance of surface coatings makes it difficult to relate the bulk mineralogical composition of a sample to its adsorptive reactivity. Determining the predominant adsorbing surface in a mineral assemblage can be a useful approach to modeling adsorption with a surface complexation model. One objective of this study was to search for a geochemical 'indicator' of the mineral surface(s) controlling lead (Pb) and zinc (Zn) sorption on the aquifer sand. The spatial variability of parameters used in models for solute transport in groundwater are of significance in the simulations of solute movement. Variability of hydraulic conductivity has been shown to be a key cause of observations of macroscale dispersion in sand and gravel aquifers. Like hydraulic conductivity, adsorption coefficients and other geochemical properties of the porous medium are expected to be spatially variable. In this study, 14 continuous cores of subsurface material were collected from a shallow sand and gravel aquifer where the spatial variability of hydraulic conductivity had been previously studied. Each core was sectioned into subsamples. Measurements were made of: (1) grain size distribution to estimate hydraulic conductivity and (2) lead and zinc adsorption for a constant set of experimental conditions.