Tracer test evaluation of a drainage ditch capture zone

Recent analytical and numerical modeling studies (Zheng et al., 1988a, 1988b) suggest that drainage ditches within the central sand plain of Wisconsin may serve to limit the spread of agricultural chemicals. In this study a natural‐gradient tracer test was conducted to determine flow paths around a sand plain drainage ditch. The flow paths define a capture zone for the ditch. These results were used to evaluate the predictive capabilities of an analytical model (Zheng et al., 1988a). The natural‐gradient tracer test began with injections of iodide at two depths and bromide at an intermediate depth. Movement of these tracers reveals that average ground‐water velocity ranged from 0.4 ft/day at a depth of 20 ft to greater than 1 ft/day near the water table. The delineated capture zone was deeper than 12 ft below the water table. These results are encouraging because they indicate that the capture depth of the ditch should allow for the effective removal of shallow agricultural contaminants. The observed capture depth falls within the range predicted by the analytical model of Zheng et al. (1988a). However, a sensitivity analysis of the analytical method reveals that the predicted range of capture depths is large. This range results from the variability and uncertainty of the model parameters. The analytical solution appears best suited for estimating the minimum capture depth, which is a conservative estimate of contaminant capture.