Peter Lea

Geology Department, Bowdoin College

Introductory to upper-level courses in geosciences that deal with the water cycle, stream flow and water quality

This exercise is typically used to construct an introductory conceptual model with which to investigate variations in surface-water quality, runoff mechanisms, and sources of stream flow. It can immediately precede field investigations of how the selected water-quality parameters vary in space and/or in time.

Students recognize that water acquires certain biogeochemical fingerprints as it moves through the hydrologic cycle, creating tracers that can be used to infer processes and flow paths. Water-quality characteristics are seen to reflect physical, chemical and biological processes operating simultaneously on multiple parameters. As a side benefit, students are introduced to some basic water-quality parameters in the context of watershed processes, and the stage is set for later investigations of tracers and mixing models within watershed studies.

This activity requires formulation of hypothesis when students are working in the first phase of the jigsaw, followed by synthesis of diverse lines of evidence into a process-based model during the second stage.

Using a jigsaw approach, students investigate biogeochemical transformations of water as it moves through the hydrologic cycle. In the first phase, student groups are given a schematic representation of the hydrologic cycle with representative concentrations of a single variable (nitrate, silica, pH and conductivity) provided for oceans, precipitation, streams, and shallow and deep groundwater. After each group has achieved a satisfactory explanation of its own variable, students are recombined to explain and compare the processes that control each variable, and to look for common themes (e.g., weathering reactions in subsurface increase conductivity, silica and pH). The resulting conceptual framework facilitates use of water-quality variables as tracers to interpret runoff processes and stream-flow sources.

The exercise is usually done informally, without an immediate explicit evaluation. I typically have each group summarize their interpretations after each stage of the jigsaw, providing opportunities for clarifying questions and/or suggestions.