CREATING AND EVALUATING A SIMPLE FIELD-BASED CONCEPTUAL MODEL

A conceptual model is a simplified pictorial representation of a field situation that is very useful for understanding and analyzing a hydrogeological problem. Creating a conceptual model is a valuable skill for hydrogeology students because it requires them to acquire information from different sources (outcrops, wells, surface water features, borings, etc.) and integrate it into a cross section that is sufficiently complete to reproduce the system. Conceptual models are often used as the basis for more advanced studies of a problem, such as a flow net or a numerical model.

I use a field-based exercise in which students create a conceptual model of a small field site that consists of a sand dune and adjacent peat bog. Students are responsible for collecting all the data, including measuring water levels in several wells (installed earlier), digging shallow excavations and hand-augering boreholes, and examining aquifer materials in outcrops. In addition, they incorporate observations of vegetation and surface water features in the area. The field component of the exercise takes place during an afternoon lab period, with a total field time of about two hours. The finished product is a cross section that incorporates hydrostratigraphy and qualitative water levels. The cross section includes stratigraphic boundaries, schematic equipotentials and flow lines, and the surface water features in the study area. An associated write-up requires a discussion of the hydrogeology of the area, the direction of groundwater flow, an explanation of the origin of vertical head gradients, an explanation for a flowing well, and a discussion of the hydraulic conductivity of the sand and peat. Evaluation of their models includes how well their hydrostratigraphy agrees with the borehole and excavation results, how well their schematic equipotentials and flow lines fit the water level data from wells, and how they incorporate surface water features. Variations of this exercise can be used in virtually any field setting in which there is a reasonable amount of geological and hydrogeological information.