Acid Mine Drainage Field-Lab Experience Round I

Greg Druschel

Department of Geology

University of Vermont

Ely Mine Field ExperienceEly Mine Field Experience

• Fall 2004 – first try - Round I…Fall 2004 – first try - Round I…

Goals:-recognize and describe chemical changes associated with 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 measurements, filtration, etc.)

Ely Mine

• One of 3 massive sulfide mines in central-eastern Vermont

• Besshi-type massive sulfide primarily mined for Cu (at Ely)

• Historically and archeologically rich – site of Ely’s War and several technological advances in ore processing

• Site is essentially untouched since closure, timbers and foundations of mine buildings still visible in places

Ely Mine Tour

• Start at 5A, parking on an old waste site

• Hike up to the main shaft and discuss the mine site, ore deposits

• Get an overview of the system – mineral transformation, hydrologic cycle

Materials at ElyMaterials at Ely• Wealth of detailed information here from the Wealth of detailed information here from the

efforts of Bob Seal, Jane Hammerstrom, and efforts of Bob Seal, Jane Hammerstrom, and Nadine Piatek at the USGS on the primary ore Nadine Piatek at the USGS on the primary ore minerals, and secondary materials resulting from minerals, and secondary materials resulting from ore roasting, smelting, weatheringore roasting, smelting, weathering

• Massive sulfide is primarily pyrrhotite, Massive sulfide is primarily pyrrhotite, chalcopyrite, sphalerite, and minor pyritechalcopyrite, sphalerite, and minor pyrite

• Roast materials are primarily hematite and Roast materials are primarily hematite and magnetite (product of over-roasting)magnetite (product of over-roasting)

• Sulfosalt minerals often coat tailings and can be Sulfosalt minerals often coat tailings and can be dug out in some of the tailingsdug out in some of the tailings

Ely Brook Field Assessment

• From mine shaft, hike down and survey Ely Brook, starting at the headwater pool and ending at the confluence with schoolhouse Brook

• With students, have them pick out sites which undergo demonstrable change

Choices:1. Headwaters – pond (really

an old beaver dam), but relatively unimpacted

2. Sediment color change – white, vegetation more sparse

3. Sediment color change – red, vegetation absent

4. Anoxic spring feeding into Ely Brook – Black precipitate turning to red

5. Confluence – White and red strips flowing into Schoolhouse Brook

DETAILED Look at each SiteDETAILED Look at each Site

• Split students into several working groups to take field measurements and do analyses:

• Group 1 Water sample collection, filtration, and acidification; sediment/rock samples collection; alkalinity kit (Hach titration)

• Group 2: pH, temperature, conductivity meter• Group 3: Spectrophotometer (Hach portable)

Al, Fe2+, FeT

• Used voltammetric probes at one site

Site 1

• Unimpacted headwater• Discussed where a representative sample might

come from, how to get it…• Used pH, T, Cond meter to survey – checked if

edges vs. middle were different• pH 6.54 at edge, 7.01 nearer the middle

(shallow, ~1 meter deep)• Site Alkalinity measureable here 20.6 mg/l as

CaCO3, no measurable Al3+, but Fe3+ close to 0.2 mg/l

Site 2

• Appearance of fluffy white precipitate

• pH 5.97, Alkalinity now 2 mg/l as CaCO3 (10-fold decrease)

Students asked what the white stuff was, so we did a little field experiment and acidified a sample with some of it in suspension, measured it for Al and Fe and analyzed it with the spectrophotometer…Collected several samples for later analysis in lab

Site 3

• One site evolved…

4.81

5.74

4.76

5.56

4.35 5.45

4.364.36 5.10

3.7

4.24.2

Invisible inputs…

• Students had identified a spatial problem with respect to a contaminant source…

• Let them poke around for a while to figure it out, followed by a good discussion of groundwater flow

• After constructing a crude well, we sampled it and analyzed the water

• pH 2.86, Al3+=0.9 mg/l, FeT=1.5 g/l

• After lab analysis of sulfate, this turned into a good example on Fe hydrolysis!

• Application of microelectrodes to look at presence of sulfide and formation of FeS minerals at anoxic site

• Discussion of oxidation and closed systems

Confluence – mixing and FeOOH, AlOOH precipitation

Lab/ Computational

• Samples brought back to the lab were run on IC for anions

• Provided cation data from Seal et al., for modeling

• PHREEQCI modeling lab to determine saturation indices for AlOOH and FeOOH, used activity diagrams for both systems they constructed as a homework to supplement reports on this

Student Impact

• Every one of the students commented on this in course evaluation

• Brought together acid/base. Redox, hydrolysis reactions as well as mineral/water reactions, microbial interactions, and gas exchange (O2 availability, SO2/H2S loss). Provided a concrete example of thermodynamic calculations they had been doing in class and a basis for more detailed analyses using PHREEQCI

• 3 of them wrote a final paper on an individual aspect from the trip (SO2 degassing, Cu speciation and sorption, microbial pyrite oxidation)

Logistics/ Portability to other sites

• This mine, like many around the U.S. and the world, is privately owned – negotiated access with the owner

• Many AMD sites have characteristics similar to Ely, and are great examples of redox and acid/base geochemistry that students can actually ‘see’ and get their hands on

• Most measurements are fast, cheap, and easy – turns out students are pretty good at working with field data and working out process control…

Next Year

• Expanding this to 2 days

• Bringing piezometers

• Test a study on filtration and sampling for Al3+ concentrations

• Collect samples and do a separate lab on sorption

OTHER THOUGHTS???