Relationship between mineral weathering and groundwater composition

Nancy Hinman

University of Montana

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Initial Publication Date: May 17, 2005 | Reviewed: October 21, 2012 (see revision history: 4 events)

Summary

By observing changes in rocks exposed to chemical weathering, students are to infer resulting changes to the chemical composition of groundwater. Students are required to justify their inferences through the use of chemical reactions involving common minerals. Students must predict outcome of a chemical weathering experiment.

Aqueous Solutions, Acid/Base Reactions, Oxidation/Reduction Reactions | College Lower (13-14), College Upper (15-16)

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Context

Audience

This exercise is used in an undergraduate course in geochemistry or in hydrochemistry.

Skills and concepts that students must have mastered

understanding of major rock-forming minerals
understanding of chemical evolution of groundwater
understanding of the processes of chemical weathering

How the activity is situated in the course

this activity occurs in the first half of the course
this activity is an exercise to be conducted in groups
this activity is a two-part exercise

Goals

Content/concepts goals for this activity

Students will need to understand chemical weathering in order to infer changes to groundwater composition.
They will need to be able to read histograms and plot data.

Higher order thinking skills goals for this activity

Students are expected to analyze published data in order to develop predictions for how compositions will change in experiments.

Other skills goals for this activity

students will work in groups
learn to use a pH meter and basic sampling equipment
acquire and process data
compare observations with predicted results

Description of the activity/assignment

In this pair of activities, students start by using published data to predict what will happen to groundwater composition as a consequence of chemical weathering. The data are provided in a spreadsheet (Hinman_weathering). Students are given the histograms only; both are normalized to 100 %, while one includes silica and the other does not. Students must use resources to predict how groundwater composition will change as a consequence of the observed weathering, and support those predictions using balanced chemical-weathering equations. Afterwards, they conduct a laboratory experiment in which they subject crushed rock to four types of solutions (acid solution, organic-rich solution, rainwater, and alkaline solution). The pH of each solution is measured, and subsequently adjusted after 24 and 48 hours. Solutions are sampled after 14 days. They are analyzed by ICP, and the compositions reported to students for comparison with their predictions.

Determining whether students have met the goals

Students are evaluated on the quality and accuracy of their reports on published data.
They are evaluated separately on the quality and accuracy of their predictions on the laboratory experiment.

More information about assessment tools and techniques.