Transport of metals leaching from land-disposed oil field wastes

Abstract

The potential for groundwater contamination by metals leaching from land-disposed oil field exploration and produc tion (E&P) wastes is an environmental concern. In this study, a geochemical transport model is adopted to consider multi- species 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 inves tigate the utility of simplified modelling approaches. A chem ical transport model coupling a hydrologic submodel is used for the analysis. The hydrologic transport submodel considers one-dimensional advective-dispersive transport of compo nents in the aqueous phase under steady unsaturated flow con ditions. The geochemical submodel considers speciation, pre cipitation-dissolution and ion exchange reactions assuming local equilibrium conditions prevail. Simulations are per formed 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 inter relationships among model parameters which cannot be reproduced using single species transport models with linear reaction terms. Precipitation plays a significant role in attenu ation of metal concentrations in E&P wastes. Calculated vadose zone attenuation ratios indexing the pollution poten tial of various metals span a wide range of values due to the dif ferences 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.