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Transport of heavy metals in the Clark Fork River

An original activity by Kathleen Harper, University of Montana Geosciences
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Summary

This is an activity about transport of sediment contaminated by copper, arsenic, and other heavy metals that was deposited into the Clark Fork River channel as the result of historical mining activity. The Clark Fork River between Butte and Milltown, Montana has been the focus of several large superfund projects designed to address the impacts of this legacy of mining in the watershed. This activity is used in an introductory physical geology lab (primarily non-majors) with students who may have limited experience working with quantitative analysis and analyzing graphs.

Learning Goals

In this activity, students work with a practical geoscience problem that requires them to apply their new understanding of how sediment is redistributed by a meandering river. They will also be asked to analyze some simple data about the distribution of heavy metals in sediments along the course of the river and in sediments from tributary streams to look for the pattern of contamination.

In Part A is to estimate how long it will take the Clark Fork River to move a pulse of sediment, consisting of highly contaminated mining waste, through a meandering section of its course.

In Part B, students are asked to interpret a plot of arsenic concentration in river sediment to determine possible sources of contamination in this part of the Clark Fork River watershed.

Methods of Geoscience

The meander activity follows working with a model - a tabletop stream table. This activity brings the insights students have gained from working with the model to application to a real-world geological problem. Students apply their knowledge of how a meandering river channel transports sediment to the question of how long it will take the Clark Fork River to dispose of (or at least move downstream) a large pulse of heavy-metal contaminated sediments. In order to plan for monitoring or cleanup of contaminated sediments in the river, we first need to understand how they will be treated by the natural system. Utilizing a regional example that the students may have heard of - most are not familiar with any of the specific details - captures student interest in the problem.

The arsenic data analysis is basically an exercise in plot interpretation. It is an example of data that isn't entirely straightforward - there are more than one source of contamination in the region that may need to be considered. This analysis also sets the scene for an additional activity regarding arsenic contamination in groundwater surrounding the Milltown reservoir - where the contaminated sediments accumulated following the 1908 floods. Recognition of the contamination of groundwater eventually led to an immense Superfund cleanup effort involving removal of the dam (just a few miles up the Clark Fork River from our campus in Missoula), restoration of the historic river channel, and transport of a large amount of river sediment back to a site near the smelter.

Context for Use

This activity is currently used as part of a lab for a Physical Geology course. The activity takes 30 minutes of a 2-hour lab session.

This activity follows others on river channel morphology and a interactive stream table demonstration. The stream table activity allows students to see how the (smaller-scale) meanders propagate over time.

No special equipment required (in our lab, we include a stream table exercise prior to this, but other methods could be used to teach about meander propagation).

Tracing paper and a ruler are needed.

Description and Teaching Materials

Part A is a practical application of the understanding of how sediment is transported by a meandering river channel over time. In this part, students measure the cutbank erosion of a single meander along the Clark Fork river on aerial photographs from two different years. They use these measurements to estimate the rate of erosion along this stretch of the river. They then extrapolate upward to estimate how quickly a given volume of sediment will move through this section of the river. This problem was analyzed by a graduate student more extensively, utilizing GIS to analyze the sediment transport in multiple well developed meanders in this section of river.

Part B is a data analysis activity. In this part, students are asked to interpret a plot of arsenic concentration in river sediment to determine possible sources of contamination in this part of the Clark Fork River watershed.



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