Prospects for Future Remediation of the Abandoned Folldal Mines Physico-Chemical Interpretation and Modeling of Leachates After Capping
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Prospects for future remediation of the abandoned Folldal mines physico-chemical interpretation and modeling of leachates after capping Anna M. Va rheim Master Thesis in Geoscience Environmental Geology 60 credits Department of Geoscience Faculty of Mathematics and Natural Sciences THE UNIVERSITY OF OSLO June 2019 II Prospects for future remediation of the abandoned Folldal mines physico-chemical interpretation and modeling of leachates after capping III Illustration representing the different aspects affecting sulfide weathering and change the production, migration and dilution of AMD. Inspired by illustration by Favas et al. (2016). © Anna M. Vårheim 2019 Anna M. Vårheim http://www.duo.uio.no/ Print: Reprosentralen, University of Oslo IV Abstract The abandoned sulfide ore mines located at Folldal, in central Norway, have been closed since 1993. However, centuries of mining activities have resulted in waste rock and tailings exposed to the natural environment. All the three ingredients are present to generate acid mine drainage (AMD), pyrite, water and oxygen. This has severely impacted the quality of the natural waters and the associated ecosystems at Folldal. The tailings are spread over a large area, and capping is therefore being considered by the Norwegian Directorate of Mining, as a way of reducing the oxidation of sulfide minerals and infiltration of AMD into the groundwater and associated streams and rivers. To test how successful a potential capping would be at Folldal two column experiments have been set up to test two different capillary barrier caps covering reactive tailings. In addition to the two columns with capped reactive material, two reference columns were set up to see how the acid generating tailings and the pre-oxidized material (that made up one of the caps) developed on their own. Previous studies have shown how successful the two capping options were at reducing oxygen diffusion into the tailings. It was clear that the physical properties of the two different caps resulted in different physical properties, even as the effect on the AMD formation was yet to be investigated. The grain size difference between the coarse and fine material of the pre-oxidized cap option was more ideal and succeeded to a greater extent in creating the desired capillary effect. The aim of this Master Project was to analyze and interpret the experimental results obtained, with a special focus on the interpretation of the geochemical composition and changes in the collected leachates. Leachates were collected and chemically analyzed over time to characterize their physico-chemical composition and changes. The geochemical modeling software PHREEQC was used to interpret the chemical data collected and gain insight into the expected future development of the leachate composition in the area surrounding the mines. This thesis works chemical investigations show that the distinct chemistry of the two caps results in different physio-chemical responses. The cap made up of fine-sand had more alkaline minerals to begin with, but the leachate development of the two columns was very similar when it came to their effectiveness in reducing the concentration of heavy metals. V Both the reference columns were acid generating and thus the total content of acid generating tailings in the column with the pre-oxidized cap was greater. Thus, when it is taken into account that the pre-oxidized material would be acid generating in itself it seems clear that using the pre-oxidized material in a cover in order to reduce the heavy metal leaching and acid generating potential of the more concentrated reactive tailings is a good purpose to put them to. The physical measurements, the dissolved oxygen, pore water pressure and water saturation measurements suggested that the pre-oxidized capping was better suited than the sand capping because of the more suited physical properties. VI Acknowledgements A project like this is not accomplished by one person alone and there are many people that have impacted the outcome of this master thesis. I would first like to thank my supervisors Prof. Gijs D. Breedveld and D.Sc. Clara Sena for great advice and guidance throughout this thesis work. Without their unwavering belief in me, and their impassioned speeches on how; “no science is without uncertainties” this thesis work would not have gone as smoothly as it has. Furthermore, the results used in this thesis would not have come about without the great effort put into installing the columns at NGI by Mari Tvedten and Thomas Pabst. The great cheer and spirit of the master-students in room 210, including my study-buddy and best friend Maria Uglum, has made this thesis work more enjoyable than I could have imagined. I don’t know what I would have done without our great lunch discussions, both scientific and otherwise. It’s been a joy commiserating with all of you, in an exciting, fun, all be it stressful time of our life. Much gratitude also goes to my love, Carl-Johan, who has stuck with me through all the long days that go into an undertaking of this size. And finally, I would like to thank my family for their love and support, without which I would never have made it to University in the first place. VII VIII Table of contents 1 Introduction ........................................................................................................................ 1 2 Theoretical framework ....................................................................................................... 4 2.1 Water flow in porous media ........................................................................................ 4 2.1.1 Saturated zone ...................................................................................................... 5 2.1.2 Unsaturated zone .................................................................................................. 7 2.1.3 Contaminant transport .......................................................................................... 8 2.1.4 Oxygen transport ................................................................................................ 11 2.1.5 Capillary barrier ................................................................................................. 12 2.1.6 Water retention cover ......................................................................................... 15 2.2 Chemical processes .................................................................................................... 17 2.2.1 Formation of acid mine drainage ....................................................................... 17 2.2.2 Microbiological processes .................................................................................. 20 2.2.3 Thermodynamics ................................................................................................ 22 2.2.4 Stoichiometry ..................................................................................................... 23 2.2.5 Redox potential .................................................................................................. 24 2.2.6 Sorption .............................................................................................................. 26 2.2.7 Effects of Acid mine drainage ............................................................................ 27 3 Study site .......................................................................................................................... 32 3.1 Mining history ........................................................................................................... 33 3.2 Geological setting ...................................................................................................... 35 3.3 Environmental Monitoring ........................................................................................ 37 3.4 Current situation ........................................................................................................ 40 4 Method and material ......................................................................................................... 44 4.1 Pressure plate test ...................................................................................................... 45 4.2 Water retention curves ............................................................................................... 46 4.3 Hydraulic conductivity .............................................................................................. 49 4.4 Column experiments .................................................................................................. 50 4.4.1 Column installation ............................................................................................ 52 4.5 Chemical Speciation .................................................................................................. 61 4.5.1 Chemical material characterization .................................................................... 61 4.5.2 Leachate characterization ................................................................................... 65 IX 4.6 PHREEQC input data and modeling ......................................................................... 67 4.6.1 Aqueous solutions .............................................................................................. 68 4.6.2 Inverse geochemical modeling ........................................................................... 70 5 Results .............................................................................................................................