Heavy Metal Migration Through Clay Below A Domestic Waste Site

Migration of contaminants from a domestic waste landfill overlying a 30 m thick natural clay deposit is presented. Profiles for Na(\u27+), Mg(\u272+), DOC and other soluble species indicate that, at 16 years, contaminants have migrated up to 130 cm from the waste. Profiles for conservative species suggest that Cl(\u27-), (\u273)H, (\u2718)O and (\u272)H fronts are ahead of those of species such as Na(\u27+), K(\u27+) and DOC. In comparison, the heavy metals, Fe, Mn, Cu, Zn and Pb, have migrated only 10-20 cm.;Batch equilibrium studies involving Fe(\u272+), Cu(\u272+), Zn(\u272+) and Pb(\u272+) result in classical adsorption-pH curves which show increasing adsorption at higher pH levels and vice versa. The presence of soil carbonates is shown to significantly increase the mass of Pb and Cu removed from domestic landfill leachate.;Highly reducing conditions in the top 50 cm of the contaminated barrier clay are inferred from laboratory-measured redox potentials. Reduced species of nitrogen (NH(,4)(\u27+)-ammonium) relative to oxidized species (NO(,2)(\u27-)(nitrite) and NO(,3)(\u27-)(nitrate)) is also much more abundant in this zone. Corresponding pH values show little variation in the profile, increasing from about 7.8 at the clay-waste interface to 8.2 in the rest of the profile. Soil bacteriological studies suggest that a higher microbial population exists in the interface region probably due to increased availability of nutrients. It is suggested that a greater depletion of oxygen has, consequently, occurred near the interface.;Combination of E(,h)-pH and selective dissolution data with thermodynamic calculations lead to the evidence that carbonates may be controlling the solubilities of heavy metals in the clay pore water. With the exception of Cu(OH)(,2), the pore water is found to be undersaturated with respect to metal hydroxides. The presence of insoluble metal-organic complexes is also inferred from the selective dissolution analysis.;Volume estimates based on chemical precipitates indicate that soil porosity decreases only slightly from 0.39 at 20 cm and greater depths to 0.375 at the interface. This represents a 4% reduction in porosity. Na(\u27+) and K(\u27+) retardation factors calculated from the porosities, show only slight increases at the interface. From these it is concluded that metal precipitation, higher microbial population and adsorbed Na(\u27+) are possibly all responsible for small decreases in hydraulic conductivity near the interface