Leakage

In one of my blogs I described a few of the historical (and not so historical) sites, where Alum Shale was mined for various purposes (alum, oil and gas extraction, uranium and/or vanadium). I also mentioned that huge piles (and hills) of burnt Alum Shale, so-called red ash are still testimony to gone-by mining activities. Several studies have looked closer into how much of the toxic substances that are contained in the mining waste leak out into the environment and how leakage actually occurs. The summary of these studies, which I give below, is maybe a bit too much on the science side, but the essence of all these studies is that Alum shale, whether it is unprocessed, processed, or burnt leaks toxic elements to the groundwater and soil as a result of weathering.

Falk et al. (2006), and Lavergren et al. (2009a, 2009b) focused on Degerhamn, which is located on the west coast of Öland, and where Alum shale was mined between the years 1723 and 1890. They studied non-weathered Alum shale, weathered Alum shale and burnt Alum shale (also called red ash) to analyze the abundance and mobility of a large suite of elements (Ca, Fe, S, As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, U, V and Zn) before and after weathering. To do this, they conducted a series of controlled leaching experiments in the laboratory. They found that non-weathered Alum shale contains high amounts of As (88–122ppm), Cd (0.4–4.6 ppm), Mo (64–176 ppm), U (27–71 ppm), V (496–1560 ppm), Cu (113 ppm), Ni (100 ppm) and Zn (304 ppm) , while the content of Cd, Mo, Ni, Zn and U is lower in weathered shale. This means that Cd, Mo, Ni, Zn and U are easily leached when in contact with water and during weathering. Highly acid groundwater, which occurred in connection with burnt alum shale in the old Degerhamn mine, showed strongly elevated values for Al, U, Cd, Co, Cu, Ni and Zn. But even in near-neutral waters, the researchers found high amounts of Cd, Mo and U. The results of these studies are very clear: simple weathering of alum shale leads to leakage of toxic elements. Weathering of burnt alum shale however supplies massive amounts of metals to aquatic environments.

In her Bachelor thesis presented at Lund University, Anna Pettersson analyzed the elemental composition of Alum shale and red ash deposits in Andrarum using the XRF technique. Her analyses gave very similar results to those from Öland. Both the shale and the red ash had high concentrations of As, Ba, Cr, Cu, Mo, U, Ni and V.

Much research has focused on Kvarntorpshögen, the 110 m high hill in the province of Närke. The hill consists of Alum shale waste, i.e. the remains of oil extraction and mining for uranium that took place between 1942 and 1966. The waste is made up of burnt and processed Alum shale, which contains high concentrations of for example Co, V, Cr, Zn, Cu, U and Ba.

Red shale ash of which the Kvarntorp hill is composed. Picture by Arvid Rudling. Downloaded from: https://upload.wikimedia.org/wikipedia/commons/thumb/1/11/Röd_skifferaska.JPG/1024px-Röd_skifferaska.JPG

Lovise Casserstedt and Lovisa Karlsson conducted a whole suite of different analyses to compare the geochemical composition of unprocessed, processed and burnt Alum shale and leaching of metals from the different materials under various pH and temperature conditions. One of the results was that if water with extreme low and high pH infiltrates the waste deposit, then a whole suite of metals is being released. However release of Ni, Mb, and Ba occurred in high concentrations even at a close to neutral pH (5.5.-8.5). Water samples analyzed around Kvarntorp show that water draining the waste site had generally high concentrations of most elements as compared to water entering the site, but also that metals are being precipitated and diluted downstream of the deposit. One observation was that heavy leaching of certain metals seems to be concentrated to specific areas. To investigate this further, Åhlgren & Bäckström (2016) analyzed water samples from different localities upstream and downstream of Kvarntorpshögen. They concluded that Kvarntorpshögen is one of the most important contributors of metals, but probably not the only one. To identify the sources for metal leaching, Åhlgren et al. (2017) analyzed water in wells from around Kvarntorpshögen over a longer time period and also conducted controlled leaching experiments on solid shale ash samples. One of their results was the finding of high concentrations of trace metals such as U, V, Ni and Mo in the groundwater wells around the waste deposit, but also that metals in the shale ash and fine particles leach differently. In Åhlgren et al. (2018) focus was therefore on better understanding the difference in the leaching potential of metals in the shale ash and in fine particles. The controlled laboratory analyses showed that low pH led to increased leaching of U, V, and Ni and high pH to leaching of Mb in both fines and in the ash samples. However, at low pH, the ash samples leached less U and Ni as compared to the fine particles. The differences and heterogeneity of the processed and burnt Alum shale in Kvarntorpshögen thus lead to large differences in the leaching of metals to the groundwater and would explain the observed divergence in metal content at different sites around the waste deposit.

It would be really interesting to conduct similar studies around Andrarum, where old heaps still testify to former mining and where the small stream Verkeån enters and drains the old mining area. Precipitation of iron in small ponds close to the historical mining site and highly eutrophic standing water suggest that the sediments and the water contain high concentrations of toxic elements.

Strolling along Andrarum’s historical mining area in August 2019

References

Casserstedt, L. (2014): Chemistry and mineralogy of shale oil mining at Kvarntorp. MSc thesis, Department of Earth Sciences, University of Gothenburg, 39 pp.

Falk, H., Lavergren, U. & Bergbäck, B. (2006): Metal mobility in alum shale from Öland, Sweden. Journal of Geochemical Exploration 90 (2006) 157–165.

Lavergren, U., Åström, M., Bergbäck, B. & Holmström, H. (2009a): Mobility of trace elements in black shale assessed by leaching tests and sequential chemical extraction. Geochemistry: Exploration, Environment Analysis, Vol. 9 2009, pp. 71–79. DOI 10.1144/1467-7873/08-188

Lavergren, U., Åström, M., Falk, H. & Bergbäck, B. (2009b): Metal dispersion in groundwater in an area with natural and processed black shale – Nationwide perspective and comparison with acid sulfate soils. Applied Geochemistry 24, 359-369.

Karlsson, L. E. (2011): Natural weathering of shale products from Kvarntorp. Report 15 hp, Örebro University, 93 pp.

Karlsson, L. E., Bäckström, M., Allard B. (2012): Leaching of sulfidic Alum shale waste at different temperatures. 9th International Conference on Acid Rock Drainage. 12 pp. DOI: 10.13140/2.1.3837.8242

Karlsson, L. E. (2014): The water course at Kvarntorp. Report 30 hp, Örebro University, 63 pp.

Pettersson, A. (2011): Spårämnen i alunskiffer, rödfyrshögar och björkträd vid Andrarums alunbruk, Skåne. Examensarbete i miljövetenskap, Lunds universitet, 25 pages.

Åhlgren K. & Bäckström, M. (2016): Identification of major point sources in the severely contaminated alum shale area in Kvarntorp, Sweden. In Drebenstedt, C., Paul, M. (eds.), Mining Meets Water – Conflicts and Solutions. Proceedings IMWA 2016, Freiberg/Germany, 6 pp.

Åhlgren, K., Sjöberg, V., Sartz, L. & Bäckström, M. (2017): Understanding groundwater composition at Kvarntorp, Sweden, from leaching tests and multivariate statistics. In Wolkersdorfer, C., Sartz, L., Sillanpää, M., Häkkinen, A. (eds), Mine Water and Circular Economy. Proceedings IMWA 2017, Lappeenranta/Finland, 7 pp.

Åhlgren, K. Sjöberg, V., Bäckström, M. (2018): Leaching of U, V, Ni and Mo from Alum Shale Waste as a function of redox and pH – Suggestion for a leaching method. In Wolkersdorfer, C., Sartz, L., Weber, A., Burgess, J., Tremblay, G. (eds), Risk to Opportunity. Proceedings IMWA 2018 Pretoria/South Africa, 6 pp.