Lawrence D. Lemke

Wayne State University

This exercise is appropriate for an upper division undergraduate or graduate level course in hydrogeology incorporating single phase subsurface flow and transport.

Students should have mastered:
- Concepts of aquifer and aquitard (aquifer heterogeneity)
- Darcy's law and flow in response to equipotential differences in homogeneous aquifers.
- Single phase transport of dissolved constituents.

Students should have been exposed to concepts of:
- Drawdown and radial flow in response to pumping.
- Dispersion.
- Breakthrough curves.
- Monitoring and pumping well fundamentals (screened intervals, pumping rates, etc.)

This activity is used near the end of the semester as a stand-alone exercise to reinforce the concepts of flow and transport in heterogeneous aquifers. It requires students to synthesize a number of course concepts and make predictions about responses to remedial pumping while considering single phase plume migration and groundwater flow in three dimensions.

Students will:
- explore and expand their understanding of aquifer heterogeneity.
- predict changes in contaminant plume migration in response to pumping.
- evaluate the ability of pumping wells and monitoring wells to detect predicted changes.
- consider the efficacy of performance monitoring well networks in the face of changing stresses to the hydrologic system.

During this exercise, students will:
- synthesize information
- formulate hypotheses
- evaluate hypotheses based on data
- recommend alternative actions

In this exercise, students predict changes in the movement of a dissolved plume in response to remedial pumping in an unconfined aquifer. The underlying conceptual model for the distribution of aquifer and aquitard materials is not known with certainty. Consequently, two alternative end-member conceptualizations are presented to students who are then asked to hypothesize differences in predicted responses at the pumping wells and nearby monitoring wells for each conceptual model. Predictions are compared to actual field data, and students discover that contaminant concentration measurements depend not only on the location of the observation point (in three dimensions), but also on the length of the screened interval through which water samples are collected. The activity is divided into three parts: (1) site/problem description, (2) formulation and testing of hypotheses for pumping wells, and (3) formulation and testing of hypotheses for monitoring wells. The activity gives students practice in three dimensional thinking and reinforces their intuitive understanding of contaminant plume migration in response to natural gradients and engineered stresses.

Student learning can be assessed through responses given during class discussions (Are predictions reasonable? Do interpretations of the data make sense? Can alternative working hypotheses be developed?). Subsequent evaluation of concept retention can be conducted using true/false or short answer questions on subsequent quizzes or exams. Example questions are provided in the full activity description.

Lemke, L.D., and J.A. Cypher, 2006, Use of Alternative Conceptual Geologic Models to Evaluate Contaminant Transport Modeling Uncertainty in a Glacial Aquifer System, in E. Poeter, M. Hill, and C. Zheng, eds., Proceedings of MODFLOW and More 2006: Managing Ground-Water Systems, Golden, CO, May 21-24, 2006, p. 41-45.