An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark 1. Experimental setup, tracer movement, and fate of aromaticand chlorinated compounds

A continuous, natural-gradient field injection experiment, involving 18 xen obiotic compounds and bromide as tracers, was performed in the anaerobic re gion of a leachate plume downgradient from the Grindsted Landfill, Denmark. The injection lasted for 195 days, and within this period a continuous clo ud was established. Over a period of 924 days the cloud movement was monito red in approximately 70 discrete sampling points in the central part of the cloud, and the spatial distribution was described by seven cloud snapshots involving 400-700 sampling points. The bromide cloud movement closely foll owed the varying flow direction predicted by the water table measurements. Moment analysis showed decreasing tracer flow velocities and reduced captur e of bromide mass with time, which may be explained by varying flow conditi ons (direction, hydraulic gradient) and the heterogeneous geological condit ions in the sandy aquifer. Naphthalene, having the highest log K-ow value, was the most retarded compound, with a retardation of less than 10%. Theref ore sorption was not considered to be a significant attenuation process for any of the compounds studied. Transformation under iron-reducing condition s was observed for toluene, o-xylene, TeCM, 1,1,1-TCA, PCE, and TCE, while transformation of benzene and napthalene was not detected in the aquifer wi thin the time frame of this study. First-order transformation rates were in the range of 0.028-0.039 d(-1) and 0.0014-0.0028 d(-1) for the aromatic co mpounds toluene and o-xylene, respectively. The rates for the chlorinated a liphatic compounds, tetrachloromethane, 1,1,1-trichloroethane, tetrachloroe thylene, and trichloroethylene, were >0.7 d(-1), 0.0044-0.0054 d(-1), 0.001 2-0.0038 d(-1), and 0.0003-0.001 d(-1), respectively. Long lag periods and slow transformation rates were observed for some of the compounds, suggesti ng that lack of transformation reported in the literature may be attributab le to short experimental periods in those studies.