6.n. Alkalisation

In contrast to the well-known effects of acidic precipitation, in some particular regions anthropogenic alkaline impact occurs. Largest areas of airborne alkalisation are related to the combustion of solid fuels rich of alkaline minerals (oil shale, lignite), production of cement and extracting lime from limestone deposits. Sensitivity of ecosystems is very different: plant communities growing on alkaline soils are very tolerant to such type of pollution. Most vulnerable are the boreal and sub-boreal forest ecosystems based on the podzolic soils and especially bog, where the natural reaction of soil is strongly acidic. Airborne alkaline particles depositing to the underlying surface change the reaction of soil to alkaline and carry excess nutrients to the ecosystem. As a result, several acdiophilous plant species (Sphagnum moss in first order) die off and numerous non-typical species invade. Natural process of peat accumulation in the bog is severely disturbed.

An exceptional region suffering alkalisation is the world’s largest oil shale mining area in the north-eastern part of Estonia. About 12 megatons of Kukersite oil shale was mined in recent years annually, top level was in early 1980’s: over 30 Mton per year. Most of this amount is combusted in two large thermoelectric power plants (total maximum capacity 3 GW, about 40% of it is in use nowadays), minor part used to produce cement and shale oil. The limestone-like mineral part of Kukersite constitutes about 60% of its total mass. At high temperatures in the furnace carbonates are decomposed. As a result, fly ash contains 30% CaO, 9% Al2O3, 7% K2O, 2% MgO and other alkaline oxides (Pets et al., 1985). The cement dust emitted to the atmosphere contains 42% CaO (Mandre, 1995) Emissions from the entire region (nearly 100 km long and 20 km wide stripe along the coastline of Gulf of Finland, Balti Sea) are given in Table 6.m.

TABLE 6.m. Atmospheric emissions from the Estonian oil shale complex,

kilotons.

Pollutant / 1990
(Kallaste et al., 1992) / 2001
(Kohv et al., 2002)
Alkaline particles / 270 / 48
SO2 / 146 / 81
NOx / 14 / 11

The emitted amounts of fly ash are much higher than needed to neutralise the acidic substances emitted from the same sources. In this connection it is important that particles in the size order of 5 – 10 mm (Aunela et al. 1995) are deposited at shorter distances than gaseous substances. But due to tall stacks (80 – 250 m) the alkaline deposition is rather a regional than local phenomenon (Sofiev et al., 2003). Studies on precipitation chemistry show high alkaline impact: pH of snowmelt water is 7 – 8 (extreme value 10), concentration of Ca ion 10 – 25 mg/l. The alkaline impact is obvious at distances up to 30 – 50 km from pollution sources (Kaasik & Sõukand, 2000). As shown by both field measurements and model estimations (Kaasik et al., 1999, 2000, Voll et al., 1989) the deposition loads of Ca in the region nowadays are up to 5 tons per km2 per year and have been in 1980’s up to 20 tons per km2 per year. The cumulative Ca deposition flux (considering 22% Ca in the fly ash) estimated for the industrial era in this region (1960 – 2000) is up to 300 – 500 t/km2 (Kaasik, 2001).

Unfortunately, the ecosystems in the mentioned zone are extremely sensitive to the alkaline deposition. Dramatic altering of bog plant community (both look and functioning) is observed (Karofeld, 1996, Kaasik et al., 2001): most of typical bog species disappeared, several non-typical species (e.g. reed and) are covering large areas, growth of trees (mainly Pinus sylvestris) intensified, pH of the bog water is up to 7 instead of natural values 3 – 4. Resulting from the disappearing of Sphagnum species, peat accumulation is stopped or seriously disturbed at a few hundreds of km2, atmospheric CO2 sink due to carbon binding in the peat layer has diminished by tens thousands tons annually. Existing peat layer is gradually decomposed in most heavily polluted bog sites (Kaasik et al., 2003). The critical fly ash load for bog ecosystem is estimated as a few tons per km2 per year. The ecological effects of alkalisation are found well outside the territory of Estonia, most evidently in the northwest of Russia (Haapala et al., 1996).

References

Aunela, L., Häsänen, E., Kinnunen, V., Larjava, K., Mehtonen, A., Salmikangas, T, Leskelä, J, Loosaar, J. (1995) Emissions from Estonian oil shale power plant, Oil Shale, 12, 5, 165-177.

Haapala, H., Goltsova, N., Pitulko, V. and Lodenius, M. (1996) The effects of simultaneous large acidic and alkaline airborne polutants on forest soil, Environmental Pollution, 94, 2, 159-168.

Kaasik, M., Liblik, V., Kaasik H. (1999) Long-term deposition patterns of airborne wastes in the north-east of Estonia, Oil Shale, 16, 4, 315-329.

Kaasik, M., Sõukand, Ü. (2000) Balance of alkaline and acidic pollution loads in the area affected by oil shale combustion, Oil Shale, 17, 2, 113-128. Available at http://rubiin.physic.ut.ee/~mkaasik/publmk.htm .

Kaasik, M., Rõõm, R., Røyset, O., Vadset, M., Sõukand, Ü., Tõugu, K., Kaasik, H. (2000) Elemental and base anions deposition in the snow cover of north-eastern Estonia. The impact of industrial emissions, Water, Air, and Soil Pollution, 121, 324-366.

Kaasik, M., Ploompuu, T., Sõukand, Ü., Kaasik, H. (2001) The impact of long-term air pollution to the sensitive natural ecosystems: a case study. Lekkas, T. D. (Ed.), Proc. of the 7th International Conference on Environmental Science and Technology, A, 381-387. Available at http://rubiin.physic.ut.ee/~mkaasik/publmk.htm .

Kaasik, M., Ploompuu, T., Sõukand, Ü. (2003) The quantitative effects of long-term cumulative loads of oil-shale fly ash to the bog ecosystem, Oil Shale (in press).

Kallaste, T., Roots, O., Saar, J., Saare, L. (1992) Air Pollution in Estonia 1985 – 1993. Environmental Report 3, Environmental Data Centre, National Board of Waters and the Environment, Helsinki, 61.

Karofeld, E. (1996), The Effects of Alkaline Fly Ash Precipitation on The Sphagnum Mosses in Niinsaare Bog, NE Estonia, Suo, 47(4), 105-114.

Kohv, N., Mandel, E., Ljamtsev, A. (2002) Emitted amounts of pollutants to the air in Estonia 2001 and final report of 2000. Proc. 2-02, Information-technical Centre of the Ministry of Environment, Tallinn, 91 (in Estonian). Available at http://www.envir.ee/itk/sisetiitel_01.pdf .

Mandre, M. (1995) Air pollution and growth conditions of forest trees In: Mandre, M. Dust Pollution and Forest Ecosystems. Publications of the Institute of Ecology, Estonian Acad. Sci., 1995, 18-22.

Pets, L.I., Vaganov, P.A., Knoth, I., Haldna, Ü., Schwenke, H., Schnier C., Juga, R. (1985) Micro-elements in the ashes of the Baltic Thermoelectric Power Plant, Oil Shale, 2, 4 379-390.

Sofiev, M., Kaasik, M., Hongisto, M. (2003) Model simulations of the alkaline dust distribution from Estonian sources over the Baltic Sea basin, Water, Air and Soil Pollution (in press).

Voll, M., Trapido, M., Luiga, P., Haldna, Ü., Palvadre, R., Johannes, I. (1989) Spread of atmospheric wastes from power equipments and oil shale processing enterprises. In: M. Ilomets (Ed.), Natural State and Development of the Kurtna Lakeland. Tallinn, Valgus, 29–42 (in Estonian).