GROUNDWATER POLLUTION SOURCE CHARACTERIZATION OF AN OLD LANDFILL

Only a few landfill investigations have focused on both the quantity and the quality of leachate as a source of groundwater pollution. The investigation of Vejen Landfill in Denmark included an introductionary historical survey (old maps, aerial photographs, interviews, etc.), leachate quality analysis, potential mapping of the groundwater surface below the landfill and leachate flow to surface waters and groundwater. The historical investigation showed that the original soil surface beneath the waste was a relatively heterogeneous mixture of boggy ground and sand soil areas. This indicated that the leaching from the landfill could be unevenly distributed. The main specific organic compounds observed in the leachate were aromatic hydrocarbons (mainly xylenes), phenols and the pesticide MCPP. Preliminary investigations of the leach from the landfill indicated, that both a northerly leach to a drainage ditch and a southerly leach to the secondary aquifer were taking place. To evaluate the proportion of leachate discharging to the drainage ditch, piezometers were installed in the shallow leachate-affected aquifer. On the basis of several soundings, the groundwater surface was mapped and the expected groundwater divides were located. These measurements indicated that approximately 50% of the leachate from the mixed waste discharged to the drainage ditch. This was supported by directly measuring the flux of leachate (as kilograms chloride per year) carried out by continuous gauging of water flow and chloride concentrations in the drainage ditch. Wells were driven into the aquifer at the borders of the landfill area. These proved that the leaching from the landfill was very unevenly distributed. By measurement of present, and estimation of the past, leachate quality and quantity, an evaluation of the history of leachate recharge to the groundwater is given, including time of recharge start and recharge quantities in cubic metres and kilograms of chloride per year.