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 nontypical 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
Alkaline particles
SO2
NOx
1990
(Kallaste et al., 1992)
270
146
14
2001
(Kohv et al., 2002)
48
81
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 m (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).
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