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Transport of metals leaching from land-disposed oil field wastes

The potential for groundwater contamination by metals leaching from land-disposed oil field exploration and production (E&P) wastes is an environmental concern. In this study a geochemical transport model is adopted to consider multispecies metal migration. Objectives are to characterize the chemical composition of E&P wastes, to evaluate the mobility and pollution potential of metals in E&P wastes and to investigate the utility of simplified modelling approaches. A chemical transport model coupling a hydrologic submodel is used for the analysis. The hydrologic transport submodel considers one-dimensional advective-dispersive transport: of components in the aqueous phase under steady unsaturated flow conditions. The geochemical submodel considers speciation, precipitation-dissolution and ion exchange reactions assuming local equilibrium conditions prevail. Simulations are performed to evaluate the movement of metals through the unsaturated zone for the base case corresponding to a typical waste pit. Sensitivity analyses are performed for selected input parameters that are expected to have significant effect on metal transport. Breakthrough curves (BTCs) are obtained for metal species As, Ba, Cd, Cr, Cu, Pb and Zn at the water table. Inspection of BTCs reveals multiple peaks and complex interrelationships among model parameters which cannot be reproduced using single species transport models with linear reaction terms. Precipitation plays a significant role in attenuation of metal concentrations in E&P wastes. Calculated vadose zone attenuation ratios indexing the pollution potential of various metals span a wide range of values due to the differences in reactivity of metals. Results for the relative vadose zone concentration with respect to maximum contaminant levels in drinking water suggest that, in general, chloride is likely to to be more critical than trace metals in controlling the groundwater quality. Cadmium and copper are the next most likely to lead to non compliance. Under conditions where Cr(III) is the dominant species in waste, chromium attenuation is greatly increased due to precipitation.