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Abstract
Contaminant transport models often use a velocity field derived from a MODFLOW flow field. Consequently, the accuracy of MODFLOW in representing a ground water flow field determines in part the accuracy of the transport predictions, particularly when advective transport is dominant. We compared MODFLOW ground water flow rates and MODPATH particle traces (advective transport) for a variety of conceptual models and different grid spacings to exact or approximate analytic solutions. All of our numerical experiments concerned flow in a single confined or semiconfined aquifer. While MODFLOW appeared robust in terms of both local and global water balance, we found that ground water flow rates, particle traces, and associated ground water travel times are accurate only when sufficiently small cells are used. For instance, a minimum of four or five cells are required to accurately model total ground water inflow in tributaries or other narrow surface water bodies that end inside the model domain. Also, about 50 cells are needed to represent zones of differing transmissivities or an incorrect flow field and (locally) inaccurate ground water travel times may result. Finally, to adequately represent leakage through aquitards or through the bottom of surface water bodies it was found that the maximum allowable cell dimensions should not exceed a characteristic leakage length lambda, which is defined as the square root of the aquifer transmissivity times the resistance of the aquitard or stream bottom. In some cases a cell size of one-tenth of lambda is necessary to obtain accurate results.
Introduction
Numerous studies have addressed the accuracy of solutions to the dispersion/advection equation. Moltyaner et al. (1993), for instance, describe the performance of transport models when compared to field experiments at the Twin Lakes site in Ontario, Canada. They found that the method of characteristics MOC (Konikow and Bredehoeft 1978) or random walk (RW) procedure (Kinzelbach 1986) provided much better solutions than a direct finite element solution to the transport equation. The two recommended methods use an externally supplied velocity field (flow field) as the bases for solving the dispersion/advection equation. Moltyaner et al. found that transport model results compared well with field measurements only if measured ground water velocities were used. In practice, however, the velocity field is derived from a ground water flow...