[en] An ongoing series of natural gradient tracer tests are being conducted in Wisconsin's central sand plain, a region of thick sandy glacial outwash. The initial motivation for the tracer tests was to determine the flow path around a drainage ditch in order to evaluate the role of ditches in limiting the spread of agricultural contamination. The tests were also designed to permit a detailed evaluation of the tracer movement within the aquifer. These tracer tests involve the simultaneous introduction of bromide and iodide tracers, each at a different depth, up-gradient of the ditch. The path of the tracer is monitored by frequent synoptic sampling from a dense three-dimensional array of multilevel sampling wells. The overall movement of the tracer plume suggests that the drainage ditch acts as a barrier to the shallow flow within the aquifer. A more detailed examination of the groundwater flow pattern reveals that it is quite complex. The direction and velocity of the flow varies spatially and includes a significant component of vertical flow across aquifer stratification. A variation in groundwater velocity from an average rate of 0.4 ft./day at a depth of 20 ft. to more than 1 ft./day near the water table clearly affected the movement of the tracer plume. The transient nature of the flow system and small-scale aquifer heterogeneities also appear to have affected the shape and path of the tracer plume. A detailed examination of breakthrough curves along with plume and water table maps calculated for over 40 different days during the test were used to assess the combined effects of aquifer heterogeneity, flow across stratification, and fluctuating gradients on macro scale dispersion. Additional tracer tests are being conducted in order to refine our interpretations. (Author) (28 refs., 2 tabs., 8 figs.)