Acid Mine Field-Lab Experience:

Greg Druschel, University of Vermont

Intended Audience: This activity was used for an introductory Geochemistry class, which contained students from sophomore level to master's students. We took the trip in mid-October, about as late as possible in Vermont, after they had gained some previous field experience and lab work in addition to an in introduction to these principles in class.

Location:

Ely Mine, east-central Vermont.

Summary:

Context:

In this field-based exercise, students were given a tour of an abandoned mine site and asked to utilize a set of field measurements to describe the spatial details of metal transport. This drew on previous experience with field measurements and prompted them to work together to solve an observed chemical evolution of a contaminated brook. The exercise is an excellent example for students to bring together acid-base chemistry, redox chemistry, and thermodynamic concepts together to describe a reasonably complex system. The field component was completed in a full day, but we will be expanding this next fall for a 2 day exercise (weekend). The exercise was coupled with subsequent labs to complete analysis of the water samples and accomplish speciation calculations using PHREEQCI.

Goals:

Goals for this activity include:

• recognize and describe chemical changes resulting in mineral oxidation and precipitation
• be able to reconstruct those changes in a thermodynamic framework (speciation modeling, use of Eh-pH diagrams)
• identify significant changes in redox state and how that impacts metal transport, microbial ecology
• importance of keeping a detailed field book and appreciation for proper field sampling techniques (in situ measuremnts, filtration, etc.)
• assessment of changing geochemical conditions and attempts to trace sources
• adapting their sampling strategy in response to observations/ field results
• testing hypothesis-driven sampling to understand a complex system
• bringing together water(chemical and hydraulic)-rock-atmosphere-microbe interactions
• discussion/ problem solving as a group
• evaluating sampling strategies
• computational component to understanding a field observation

Notes and Tips:

Assuming that everyone will not be descending on the Ely Mine (note that here, as with most mine sites, it is respectful and likely necessary for your own safety to secure permission from the owner (which can take time, patience, and occasionally a lawyer), this exercise is likely portable to other AMD sites around the world. I found that the students in my class were quite capable and they discovered some aspects of the site which turned out to be interesting. I found that letting them probe and do this as a group with me being there to correct mistakes or answer specific questions was fruitful. Occasional breaks (lunch for instance) proved to be good times to break out copies of Eh-pH diagrams, mine maps, and mineral info (USGS Open File 03-260 or Piatek et al., 2004 Applied Geochemistry V. 19 p. 1039-1064 for example at Ely) for discussion. My students all had completed a year of introductory chemistry (prerequisite for the course) and half a semester of the geochemistry class and were reasonably competent with spectrophotometric analyses and calibration curves. I think that the previous lab and field work previous to this trip, however, made it easier for them to focus on the geochemical processes instead of the analytical techniques. While not everyone would have voltammetric probes for Fe²⁺ and H₂S, an Eh probe should suffice if areas where significant difference w.r.t. Eh can be located. I would be happy to have individuals interested in a trip to the Ely Mine with their own class join us for these trips - I can also provide the specific materials for this mine to them directly.

Assessment and Evaluation:

I was quite impressed with the student's performance in these activities, and if anything the first trip showed me that even more could be done. Next Fall we will additionally include an exercise specifically on filtration and possibly more sediment characterization and mineral surface chemistry.

Materials and Handouts:

• Activity Description/Assignment (Microsoft Word 61kB May4 05)
• Instructor's Notes (Microsoft Word 25kB May3 05)
• Photos of class field trip (Microsoft Word 1.4MB May4 05)

References:

SEG Guidebook Series Volume 35 includes quite a lot about the mine waste materials and water chemistry (Bob Seal, Jane Hammerstrom, and Nadine Piatek have done a large volume of great work there) as well as a chapter on the mine deposit (Slack et al.) and history (Kierstad) for this mine and the Elizabeth mine (which is now a Superfund site).