CONCENTRATION DATA AND DIMENSIONALITY IN GROUNDWATER MODELS - EVALUATION USING INVERSE MODELING

A three-dimensional inverse groundwater flow and transport model that fits hydraulic-head and concentration data simultaneously using nonlin ear regression is presented and applied to a layered sand and silt gro undwater system beneath the Grindsted Landfill in Denmark. The aquifer is composed of rather homogeneous hydrogeologic layers. Two issues co mmon to groundwater flow and transport modelling are investigated: 1) The accuracy of simulated concentrations in the case of calibration wi th head data alone; and 2) The advantages and disadvantages of using a two-dimensional cross-sectional model instead of a three-dimensional model to simulate contaminant transport when the source is at the land surface. Results show that using only hydraulic heads in the nonlinea r regression produces a simulated plume that is profoundly different f rom what is obtained in a calibration using both hydraulic-head and co ncentration data. The present study provides a well-documented example of the differences that can occur. Representing the system as a two-d imensional cross-section obviously omits some of the system dynamics. It was, however, possible to obtain a simulated plume cross-section th at matched the actual plume cross-section well. The two-dimensional mo del execution times were about a seventh of those for the three-dimens ional model, but some difficulties were encountered in representing th e spatially variable source concentrations and less precise simulated concentrations were calculated by the two-dimensional model compared t o the three-dimensional model. Summed up, the present study indicates that three-dimensional modelling using both hydraulic heads and concen trations in the calibration should be preferred in the considered type of transport studies.