RICK HEALY’S INFLUENCE AT THE INTERFACE BETWEEN GROUNDWATER AND SURFACE WATER

Rick Healy’s scientific contributions extended well beyond his primary research foci and included studies of exchange between groundwater and surface water. Highly unusual but confirmable field data indicated that sediments beneath a 480-ha lake in northern Minnesota were unsaturated at least 20 m offshore. Colleagues would have a hard time believing this seemingly impossible situation unless similar results could be generated with a numerical model. Rick Healy was the recognized expert for the 2-d, variably saturated flow model, VS2DI. When asked if this model could simulate such a strange condition, Rick thought for nearly a minute before replying that he couldn’t think of why it could not. With Rick’s expert guidance, model results confirmed that sediments indeed could be unsaturated beneath the lakebed due to: (1) a nearby lake that was 14 m lower in stage and (2) a thin layer of lower-permeability sediment on the lakebed. Modeled hydraulic conductivity of the lakebed sediments needed to be 70 times smaller than the underlying coarse, clean outwash to match the measured thickness of unsaturated sediments at the shoreline of the lake. Years later, at the South Platte River north of Denver, CO, Rick was consulted once again to help determine whether discharge of groundwater would be focused near the riverbank, as would be expected but for a drape of low-permeability sediments on the bed surface, or at the thalweg near the opposite bank, where no fine-grained sediments were present. This time, the VS2DI model needed to be modified to do a better job of accounting for inter-cell flow volumes of groups of model cells at the simulated riverbed. Also, when the model grid was made substantially finer to allow high-resolution simulation of groundwater discharge with distance from the riverbank, Rick’s expertise was critical to successful model convergence. Model output clearly showed that nearly all groundwater discharge to the river occurred very close to the riverbank, indicating that measured discharge to the river at the thalweg was virtually all the result of hyporheic-scale exchange. In both examples, Rick’s expertise and expert knowledge were critical to understanding the importance of local-scale contrasts in bed-sediment permeability on distribution of exchange between groundwater and surface water.