Mobilization and Transport of Selected Metals in Landfill Leachates: Case Study in Central Massachusetts, USA

Most present day landfills are recently decommissioned municipal waste/construction & demolition waste facilities that stopped receiving waste materials during the 1980's through the 1990's. These landfills are not lined and are, due to their original siting in local terrain depressions, also partially submerged in ground water. Composition of groundwater during its through-flow and/or by mixing with the percolating recharge from the above will become modified by addition of organic as well as inorganic secondary dissolved substances. Microbial decomposition of DOC's and its need for oxygen acceptors subsequently forms a downstream zoned leachate plumes. In this presentation we will compare metal equilibria in an aquifer downstream from wetlands with equilibria downstream from landfills. Wetland influenced aqueous equilibria are near the stability field of ferric oxi-hydroxides whereas landfill influenced environments reach toward the Fe-sulfide zone.

Metals, including Fe, As, Cr, Co, Cu, Pb, Mn, Ni, and Zn partition between 3 states: (1) sulfides; (2) adsorption onto ferric precipitates; and (3) dissolved aqueous state. Zones of groundwater discharge toward the surface water reservoirs have strong oxidation gradient, contain oxidative re-precipitation of ferric hydroxides, and adsorptive metals enrichment. Multi-elemental correlations of Fe, As, and Ni, Co, Cu, Zn, Pb, Mn within this zone suggest a similar source mobilization as landfill exogenic substances and simple transport mechanism within the wetland influenced aquifer. Lack of such correlations in the area of known seepage points from a discharging landfill plume is either further modified by sulfide precipitations and/or by additions of dissolved metals originating as landfill endogenic components. Comparison with other landfills outside the Central Massachusetts zone indicate that metals within the landfill substrate are the principal source of mobilized metals in the plumes.