Nanji Landfill is the biggest landfill site in Seoul Metropolitan Area. It had been landfilled from the year of 1978 to 1992. As this landfill was dumped without any leachate control system, the leachate has been discharged through the underlying sand layer and has polluted surrounding groundwater. Hydrogeological data around the landfill is scarce because it has been operated recently, and the height of landfill is too thick to monitor. Therefore groundwater monitoring along the boundary of landfill was only possible. To investigate the status of groundwater contamination around landfill site, groundwater samples from monitoring wells and existing wells were collected and analyzed. Chemical equilibrium program (MINTEQA2) and chemical mass balances (CMBs) were applied to get more information on pollutant speciation and the leachate behavior in the groundwater basin. From the chemical equilibrium model, it was noticed that most of divalent metal ions in the contaminated groundwater were in the form of metal carbonate precipitates. It was possible to estimate the apportionment of leachate to groundwater pollution in monitoring well where the CMBs were applicable. The results also showed that the fingerprints of groundwater quality of old landfill and currently active site were significantly different. Multi-layer sampler (MLS) was developed to investigate the vertical distribution of pollutants (chloride, COD, and several metals) in monitoring wells. It consisted of glass cell with two sides of 0.45 $mu$m membrane filter, and distilled water was filled within this cell. After sufficient time of MLS settling, concentration of the inside of cell was equilibrated with surrounding concentration. Based on the laboratory experimental results, three days of putting into the monitoring well was sufficient to reach equilibrium state. Also by using MLS, it was possible to estimate horizontal specific velocity of groundwater. From the mass balance equations, the equation for determining the specific velocity of groundwater was derived, and corresponding parameters were gained in laboratory and field experiments. Estimated specific velocity was well compared with the product of hydraulic conductivity from pumping test and hydraulic gradient. Pollutants loading from landfill to surrounding groundwater were estimated. The leachate generation amount was calculated by using HELP (Hydrologic Evaluation of Landfill Performance) model, and seasonal leachate quality was calculated on the basis of the sampling results. By using pollutants source characteristics and hydrogeological data of Nanji Landfill site, groundwater pollution modeling was performed. As the site considered is so close to the polluting source, only the following parameters were important; e.g., groundwater velocity, horizontal and vertical location, and vertical dispersion coefficient. From the CMB analysis and the calibration of groundwater model, a vertical dispersion coefficient for the groundwater of the surrounding landfill site was determined. From the above results, it was known that the effective measure of groundwater pollution control was cover layer capping with low permeable soil. And pump and treat method without any groundwater flow control measure was not easy because the amount of groundwater to be treated was too much.
