Evaluating natural attenuation of groundwater pollution from a coal-carbonisation plant: Developing a local-scale model using MODFLOW, MODTMR and MT3D

The effluent from a former coal-carbonisation plant in Nottinghamshire has contaminated the underlying aquifer with ammonium and organic compounds. In terms of flow and contamination, the site has experienced a complex histor y. MODFLOW has been used to unravel the past flow directions and to provide a basis for solute transport and biodegradation modelling. The modelling h as shown that a fine grid is required to represent local flows to prevent i ncorrect site interpretations. A telescopic mesh technique was essential fo r this study to enable the site features to be adequately represented while including the regional hydrogeological influences. The influence of grid s ize to numerical dispersion was investigated for the MT3D computer program. The 'method of characteristics' and 'hybrid method of characteristics' mod ules of MT3D were found to be relatively free from numerical dispersion for all the grid sizes investigated. However, the 'modified method of characte ristics' suffered extensively, and a linear relationship between grid size and numerical dispersion was demonstrated for this complex model.