SOME HYDROGEOCHEMICAL FEATURES OF THE NAZAROVO BROWN COAL DEPOSIT

A. D. NAZAROV, S. V. SHISHMAREVA

(Presented by the scientific seminar of the Department and Research Institute of Hydrogeology and Engineering Geology)

The Nazarovo brown coal deposit is located in the southwestern part of the Kansk-Achinsk basin and is confined, in structural-tectonic terms, to the Nazarovo trough, in the geological structure of which deposits of the Lower and Middle Paleozoic, Mesozoic, and Cenozoic take part [1]. The beds of brown coal are confined to Jurassic deposits, in which, according to the scheme adopted at the conference on the development of unified stratigraphic schemes for the West Siberian Lowland, held in 1960 in Novosibirsk, two formations are distinguished [2].

The lower Makarovo Formation of Lower Jurassic age lies on eroded Paleozoic deposits and is represented by sandstones, siltstones, argillites, and beds of brown coal. The Middle Jurassic deposits rest concordantly on the Lower Jurassic deposits and are distinguished under the name of the Itat Formation, which, in turn, according to its lithological composition, is divided into a coal-barren and a coal-bearing subformation.

The coal-barren subformation, occupying the lower part of the formation, is represented by a sequence of medium-grained sandstones with a thickness of 140 m.

The coal-bearing subformation is composed of sandstones, argillites, siltstones, carbonaceous rocks, and beds of brown coal, and has a thickness of about 200 m. The Jurassic deposits over the area of the deposit are everywhere overlain by Quaternary deposits, represented by loams, clays, sands, and pebbles. Their thickness ranges from 0.5 to 45 m.

At present, at this deposit, the upper coal seam, the “Moshchny” seam of the Itat Formation, which has a rather persistent areal distribution, is being mined by the open-pit method.

In the area of the open pit, in June 1968 the authors carried out a hydrogeochemical survey, during which groundwater of the Quaternary deposits, the coal and subcoal horizons, and surface waters were studied within the zone of active water exchange.

Surface waters are characterized by an alkaline medium, a hydrocarbonate-magnesium-calcium composition, and total mineralization ranging from 243.6 to 391.4 mg/l. Under the influence of drainage waters from the open pit, the amount of sulfates in the Adadymka River increases significantly. Among the microcomponents, copper, zinc, titanium, manganese, barium, and silver were found in the waters.

The waters of the Quaternary deposits, studied from wells and test pits, are characterized by a varied component composition. Thus, the waters of the wells—

The villages of Adadym and the town of Nazarovo are bicarbonate waters with a mixed cation composition and are slightly alkaline. Mineralization ranges from 416.01 to 504.41 mg/l; the content of free carbonic acid is from 66.0 to 158.4 mg/l, oxygen from 1.0 to 3.0 mg/l, and sulfate ion from 12.0 to 32.5 mg/l; gas saturation is weak. Among the dissolved gases, carbon dioxide, methane, and nitrogen are noted. The presence of methane indicates contamination of the waters. This is also confirmed by the presence in the waters of nitrite ion up to 0.6 mg/l. The possibility of inflow of bog waters is not excluded, since the well from which the water and gas samples were taken is located 100–130 m from the edge of the bog. The complex of microcomponents occurring in the waters is represented by zinc, titanium, manganese, copper, and barium.

The waters of the deposits of Lake Chertovo have an entirely different composition. The lake is, in all probability, confined to a depression of erosional-tectonic origin. The latter may be indicated by the presence in this area of narrow zones of low-amplitude deformation cutting through interbeds of rocks in the coal seam, and by the strong fragmentation of the coal, which is not observed in other parts of the open pit. By the time of the survey the lake had been drained, and water remained only in small depressions. Its composition is sulfate-bicarbonate-calcium, and the medium is alkaline. The content of free carbonic acid is 44–26.4 mg/l, dissolved oxygen 2 mg/l, and sulfate ion 100 mg/l. Gas saturation is insignificant. The composition of the dissolved gas is carbon dioxide–nitrogen. Among the microcomponents, copper, zinc, titanium, manganese, and barium were detected.

The drainage of the lake led to compaction of the silt, as a result of which a large quantity of water is squeezed out and flows into the open pit in the form of springs with discharge up to 0.5 l/sec. The silt waters are characterized by higher mineralization (1014.17 mg/l), a high content of free carbonic acid (424 mg/l), sulfate ion (100 mg/l), and a nitrogen–carbon dioxide composition of the dissolved gas. In chemical composition the waters are bicarbonate-sodium-calcium. The content of ferrous iron is 0.5 mg/l, ammonia 0.6 mg/l, and bicarbonate ion 1037.0 mg/l. Among the microcomponents, manganese ion 1037.0 mg/l was detected. Among the microcomponents, manganese (72 µg/l) and copper (traces) were detected.

Immediately beneath the silt (at a depth of 30–40 cm) lies a layer strongly enriched in organic matter, of the sapropelic type. Analysis of water taken from a small pit and consisting of a mixture with silt water showed that it is characterized by a sulfate-bicarbonate-calcium composition, elevated gas saturation (90 cm²/l), and a nitrogen–methane–carbon dioxide composition of dissolved gases. Mineralization is 628.76 mg/l. The content of free carbonic acid reaches 528 mg/l, sulfate ion 100 mg/l, ammonia 6.0 mg/l, and ferrous iron 15 mg/l. Among the microcomponents, only barium (6.6 mg/l) and manganese (traces) were detected. In addition, in the waters and rocks of the sapropelic layer, judging by the strong odor and the very rapid appearance of a continuous sulfide film on metallic objects when they are immersed in the water or in the rock, hydrogen sulfide is probably present in considerable quantities.

The waters of the coal horizon, exposed by boreholes, adits, and springs, differ markedly in chemical composition from the waters of other horizons. Despite their connection with the waters of the Quaternary deposits, their chemical composition differs from the latter in the high content of free carbonic acid (79.2–488.4 mg/l), ferrous iron and ammonia, significant gas saturation, and the predominance of carbon dioxide in the composition of the dissolved gases. The waters are bicarbonate in composition, rarely sulfate-bicarbonate, with a mixed cation composition.

Mineralization ranges from 268.35 to 325.56 mg/l. The sulfate ion is found in significant quantities (79.2–488.4 mg/l) in all water samples. Its presence in the waters can be explained by the dissolution of various sulfide minerals, which are found in the coals even with the naked eye. Thus, for example, when examining a small number of coal samples on the eastern wall of the quarry, inclusions of pyrite were noted by us, and along individual cracks there were coatings of covellite and thin films of malachite.

This can also explain the presence in the waters of ferrous and ferric iron and hydrogen sulfide.

The complex of microcomponents present in the waters is represented by zinc (1.19–39.15 μg/l), manganese (trace–9.0 μg/l), barium (0.4–4.52 μg/l), titanium (trace–2.78 μg/l), copper and silver (traces). The coals, according to spectral analysis of the ash, are characterized by a broader complex of microcomponents, of which the most frequently encountered are titanium (0.03–0.6%), strontium (0.01–1.0%), nickel (trace–0.02%), chromium (0.001–0.003%), zirconium (0.002–0.03%), copper (trace–0.006%), cobalt (trace–0.006%); zinc (trace–0.006%), beryllium (trace–0.003%), tin (trace–0.001%), vanadium (trace–0.006%), molybdenum (trace–0.001%), and others are present more rarely.

The waters of the subcoal horizon, studied in several boreholes, are characterized by a hydrocarbonate–sodium–calcium composition with mineralization of 357.12–545.49 mg/l, a nitrogen composition of dissolved gases with weak gas saturation (50 cm³/l at the wellhead, despite the release of free gases, the composition of which could not be studied). The medium is slightly alkaline. The content of free carbon dioxide ranges from 79.2 to 448.8 mg/l. The sulfate ion occurs in smaller quantities than in the waters of the coal horizon. Among the microcomponents, copper, zinc, manganese, titanium, and barium were detected in the waters in insignificant quantities, except for zinc.

The waters of the subcoal horizon are characterized by a constant temperature (8°C), regardless of fluctuations in air temperature.

Thus, the results of the studies show that the surface and underground waters of the Nazarovo brown-coal deposit are characterized by the widespread distribution of the sulfate ion, the greatest amount of which is contained in silt waters and in the waters of the coal horizon and the sapropel layer. Such a broad distribution of sulfates is probably caused by the dissolution, in the zone of active water exchange, of various sulfide minerals of the coal-bearing deposits.

The waters of the coal horizon are characterized by a number of features that distinguish them from the waters of other horizons. These include elevated contents of sulfates, ferrous iron, free carbon dioxide, increased gas saturation, a high content of carbon dioxide gas in the composition of the dissolved gas, and the presence of hydrogen sulfide.

REFERENCES

1. M. P. Burtsev. Kansk-Achinsk Basin, Publishing House of the Academy of Sciences of the USSR, Moscow, 1961.
2. Proceedings of the Interdepartmental Conference on the Development of Unified Stratigraphic Schemes, Novosibirsk, 1961.