USING PHYSICAL MODELS (“ANT FARMS”) OF HETEROGENEITY WITH STUDENTS AT ALL LEVELS

A key concept for a hydrogeologist’s success is that of the influence of heterogeneity on groundwater velocities and contaminant dispersion. Though we can describe how heterogeneity influences groundwater flow in lectures, models, and from tracer test results, a tangible physical model of transport processes significantly aids our students’ ability to visualize how heterogeneity affects transport. To accomplish this, we use “ant farm” models, repacked in a heterogeneous configuration, for exercises in a wide range of classes. At the introductory level, we use food coloring as dye tracers through the ant farms in order to reach the objective that students are able to evaluate issues related to groundwater cleanup, comparing processes observed in the ant farm model to processes occurring at a local large fuel spill. This was derived from similar labs posted on the SERC site (e.g., http://serc.carleton.edu/NAGTWorkshops/intro/activities/23416.html). For upper division undergraduate students and entry level graduate students, we use a fluorescein dye tracer at a constant concentration in the up-gradient input, measuring the breakthrough curve from samples taken at the down-gradient end of the model with a fluorimeter or spectrophotometer (other tracers may be used, too). Students sketch the tracer progress, make observations of how high-K and low-K zones affect transport, and estimate effective K and dispersion parameters from two parts of the ant farm model. Focused questions ask the students to evaluate the Fickian assumption used in most transport modeling. For advanced graduate students, we discretize the ant farm (with crepe tape) and model the system using GMS and MODFLOW. Prior to running the experiment, we evaluate K for all the facies types. We run the ant farm experiment with a fluorescein dye tracer, as described above, and then construct models of the ant farm system in GMS. We calibrate the numerical model using the dye tracer results, and students evaluate uncertainty in their models even given such complete information about the groundwater system. In all cases, the students gain an appreciation for the importance of capturing heterogeneity in their evaluations of aquifers and contaminant transport at a level appropriate for their experience in the field.