Attenuation of landfill leachate by UK Triassic sandstone aquifer materials 2. Sorption and degradation of organic pollutants in laboratory columns

The sorption and degradation of dissolved organic matter (DOM) and 13 organ ic micropollutants (BTEX, aromatic hydrocarbons, chloro-aromatic and -aliph atic compounds, and pesticides) in acetogenic and methanogenic landfill lea chate was studied in laboratory columns containing Triassic sandstone aquif er materials from the English Midlands. Solute sorption and degradation rel ationships were evaluated using a simple transport model. Relative to predi ctions, micropollutant sorption was decreased up to eightfold in acetogenic leachate, but increased up to sixfold in methanogenic leachate. This behav iour reflects a combination of interactions between the micropollutants, le achate DOM and aquifer mineral fraction. Sorption of DOM was not significan t. Degradation of organic fractions occurred under Mn-reducing and SO4-redu cing conditions. Degradation of some micropollutants occurred exclusively u nder Mn-reducing conditions. DOM and benzene were not significantly degrade d under the conditions and time span (up to 280 days) of the experiments. M ost micropollutants were degraded immediately or after a lag phase (32-115 days). Micropollutant degradation rates varied considerably (half-lives of 8 to > 2000 days) for the same compounds (e.g., TeCE) in different experime nts, and for compounds (e.g., naphthalene, DCB and TeCA) within the same ex periment. Degradation of many micropollutants was both simultaneous and seq uential, and inhibited by the utilisation of different substrates. This mec hanism, in combination with lag phases, controls micropollutant degradation potential in these systems more than the degradation rate. These aquifer m aterials have a potentially large capacity for in situ bioremediation of or ganic pollutants in landfill leachate and significant degradation may occur in the Mn-reducing zones of leachate plumes. However, degradation of organ ic pollutants in acetogenic leachate may be limited in aquifers with low pH buffering capacity and reducible Mn oxides. Contaminants in this leachate present a greater risk to groundwater resources in these aquifers than meth anogenic leachate. (C) 2000 Elsevier Science B.V. All rights reserved.