Environ. Sci. Techno/. 1995,29, 1395-1400
Distribution of Organic Compounds from Municipal Solid Waste in the Groundwater Downgradient of a Landlill (Grindsted, Denmark) KIRSTEN RUGGE, P O U L L. BJERG,* A N D THOMAS H . CHRISTENSEN Institute of Enuironmental Science and Engineering, Groundwater Research Centre, Technical Uniuersity of Denmark, Building 115, DK-2800, Denmark
The distribution of organic compounds in the leachate plume downgradient of the Grindsted Landfill was mapped along two 300 m long transects (285 groundwater samples). At the border of the landfill, elevated concentrations of dissolved organic matter 30-1 10 mg of C L-' (measured as nonvolatile organic carbon, NVOC) were found. In a distance of 130 m downgradient of the landfill, the NVOC had decreased to background level, which is 1-3 mg of C L-l. More than 15 organic compounds were identified in the groundwater at the downgradient border of the landfill with benzene, toluene, ethylbenzene, and xylenes as dominating. No pesticides were identified, but some phenoxy acids, which could be metabolites of known pesticides, were found. In a distance of approximately 60 m from the landfill, most of the specific organic compounds were no longer detectable. Since dilution and sorption apparently cannot account for the disappearance of the specific organic compounds within the first 60 m of the plume, it is proposed that the majority of the specific organic compounds were degraded in the anaerobic plume under metha nogeni c/su Ifate- red u c ing or ironreducing conditions. The investigation indicates that the aquifer has a substantial natural attenuation capacity.
0013-936X/95/0929-1395$09.00/0
Q 1995 American Chemical Society
Introduction Organic compounds, originating from household waste and waste from small businesses, are found in leachates of most municipal waste landfills. These compounds often include benzene, toluene, ethylbenzene, xylenes, (e.g., refs 1-31, chlorinated aliphatic hydrocarbons (e.g., ref 11, and pesticides (e.g., refs 3-51, These organic compounds only account for a few percent of the organic matter in the leachate, but much concern is related to the compounds due to their potential health risk. If the leachate enters the groundwater, these compounds may dramatically affect the groundwater quality unless they are substantially attenuated in the leachate plume. Attenuation of such organic compounds is primarily due to dilution, sorption, and chemicallmicrobial degradation. From laboratory studies, much has been learned during recent years about the sorption and degradation of organic compounds in aquifers in general. However, the information related to leachate-affected aquifers is stillrather limited [see the review by Christensen et al. (@I. In particular, field studies on the fate of organic compounds in actual leachate plumes are too few (1-3) to provide a basis for any generalization on this important issue. This paper presents detailed information on the distribution of dissolved organic carbon and specific organic compounds in the groundwater downgradient of an old landfill (Grindsted, Denmark) and discusses the apparent attenuation processes in the plume. The leachate plume has previously been described in terms of the distribution of redox-sensitive groundwater quality parameters and dominating redox environments (7). The discussion on the distribution of the organic compounds in the plume is related to these redox environments. Also the distribution of pharmaceutical compounds in the plume is reported (8).
Materials and Methods Landflll Site. The Grindsted Landfill covers approximately 10 ha and has an average depth of 5 m. Landfilling started in the 1930s and terminated in the 1970s. The leachate from the landfill sampled in the upper part of the aquifer can be characterized as dilute methanogenicphase leachate. The landfill is described in detail in ref 9. The landfill is located on a glacial outwash plain. The investigated aquifer has a depth of approximately 10-12 m and consists of two sandy layers. The average groundwater pore flow velocity is estimated from hydraulic gradients and hydraulic slug tests to 50 m yr-' in the upper part and 10 m yr-' in the lower part of this sandy aquifer (7). The plume is recognizable in terms of inorganic compounds for about 200-250 m downgradient of the landfill. The redox conditions in the plume were identified on the basis of a detailed characterization of redox-sensitive groundwater quality parameters (7). Sampling. The total number of groundwater samples in the two transects was 285. The sampling points of transect I are shown in Figure 1. The groundwater samples for the organic analysis were obtained from wells with 10cm screens by pressurizing (nitrogen) the sample to the surface through a Teflon tube (7,lO). The Teflon tube was lowered to the bottom of a volumetric flask, which was
VOL. 29, NO. 5, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
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