Heavy metal migration at a landfill site, Sarnia, Ontario, Canada—2: metal partitioning and geotechnical implications

Heavy metal profiles below a 15-year old sanitary landfill overlying a 30 m thick natural clay deposit are presented. Results indicate that unlike soluble species such as Cl⁻ and Na⁺ which have migrated distances up to 130 cm, Cu, Zn and Pb have migrated only up to 10 cm. The extent of Fe migration is estimated to be 20 cm. Highly reducing conditions at the interface (E_h= −130mV), coupled with the alkaline nature of the clay pore waters, have resulted in the precipitation of migrated heavy metals as carbonates. At the clay/waste interface, 88, 84 and 80% of the excess Fe, Zn and Pb, respectively, are present as secondary carbonates. This is confirmed by selective chemical dissolution analyses which also show that Fe, Zn, Pb and, to a greater extent, Cu are present in solid organic forms at the interface. Batch equilibrium studies clearly show that Cu and Pb removal from leachate is significantly increased by the presence of carbonates in the soil. For example, 75% more Pb is removed by the carbonate-rich bulk soil than the carbonate-free soil. The batch studies also show that when thepH> 5.2, removal of metal increases significantly due to precipitation as carbonates. From the results it is concluded that the presence of metal sludges in landfills lined naturally or artifically by a carbonate-rich clayey barrier reduces the rate of migration of numerous toxic transition metals and may also decrease the barrier porosity by precipitation. The decreases in porosity will be beneficial to the performance of the barrier due to reductions in both advection and diffusion.