Kontakty między poziomami wodonośnymi w olkuskim rejonie kopalnictwa rud

Zbigniew Wilk, Jacek Motyka

Abstract


Contact between water-bearing horizons (Olkusz ore mining region near Cracow)

The region under description is situated north-west of Krakow, not far from the town of Olkusz, on faces of Mesozoic beds, which constitute the Kraków-Silesia Monocline. The Mesozoic cover deeps gently towards NE. Triassic deposits occurring in the base of the Mesozoic cover overlie discordantly, partly directly over folded Devonian and Carboniferous rocks, and partly over Permian deposits of the continental origin. A dense network of faults is a characteristic of the geological structure of the region; it divides it into a system of tectonic troughs and horsts. In the region described above, a vast programme of investigations connected with the development of zinc and lead ore-minding and building of new mines has been carried into practice recently. As a result, a very good surveying of hydrogeological conditions has been achieved. It applies especially to the Triassic water-bearing complex, which is the main starting point of mine affluents, a factor of water hazard to mines, but also the source of water supplies. Table 1 shows a succession of water-bearing horizons and impervious strata in the region of Olkusz, as well as their lithologic character. The area under examination can be divided into two parts, as far as its hydrogeology is concerned. The substantial difference between the two parts lies in the fact that in its eastern part there occurs a Jurassic water-bearing complex, absent in the western part. Apart from this fact, a Quaternary water-bearing complex is of no great importance in the eastern part, while it is of considerable value in the western one (Fig. 1). The water-bearing horizons, mentioned in Table 1, are often hydraulically connected with each other. The intensity of hydraulic connection may be of various types. As far as types of these conctacts and factors indispensable to their occurrence are concerned, they can be systematized in the way shown in Fig. 3. Fig. 14 presents a scheme of connections between individual water-bearing complexes, as well as types of hydraulic contacts participating in these connections. The distribution of more important types of hydraulic connections is shown on a map (Fig. 1). Under the circumstances of a natural hydrogeological régime, the movement of subsurface water took place in the whole area, mainly in the direction of the main draining stream, i.e. the Biała Przemsza river. The situation changed as a result of drainage of the Muschelkalk waterbearing horizon, which was a large scale undertaking carried out by the ore-mining industry. Foundation of an artificial base of drainage on the level of the Muschelkalk bedrock resulted in formation of a vast depression zone. The shape of zone is formed, to some extent, by fault tectonics and by hydraulic contacts with upper water-bearing complexes and surface waters. As a result of the situation described above, the hydraulic gradient has increased in the eastern and south-eastern parts of the Olkusz region, while in the northern and western parts there has occurred an inversion of the direction of water movement from the Biała Przemsza river to the mine drainage system. Moreover, the hydraulic contacts between water-bearing complexes have changed and become more important. In connection with the decrease of piezometrie pressure in the Muschelkalk in the eastern part of the region, the difference between the height of water level in the Jurassic and in the Muschelkalk has increased. Accordingly, the amount of water, which can penetrate from the Jurassic to the Triassic in the contact zones of both horizons, has grown up directly proportionally. The phenomenon concurred to a disastrous disappearance of water, which occurred in the Jurassic horizon, east of Olkusz in 1969 and 1970. One can assume a hypothesis that the increased difference of pressures makes the suffusion process work and creates a direct connection between water-bearing complexes in some fault zones in which such a connection had not existed before because of their being filled with unconsolidated, ill-previous material. The same refers to the connection between the Jurassic and Triassic water-bearing complexes by means of badly liquidated bore-holes and buried karst zones. The erosional contact between the Quaternary and Triassic waterbearing horizons is of the gratest practical importance; it appears in the buried zone of the preglacial valley of the Biała Przemsza river. Previously, under natural conditions, it had been a zone of pressure discharge and ascension of Triassic artesian waters. At present, however, the valley has changed into a zone of recharge and constant supplying of mine water in flow. The significance of contact of the Muschelkalk horizon with the Roethian one has not been surveyed in detail, so far. On the grounds of general data one can suppose, however, that an artificial decrease of hydraulic head in the upper complex (of the two) enables ascensive water penetration from the lower complex, i.e. from Roethian. This also refers to the importance of contacts of the Triassic horizon with the Paleozoic one. The significance of contacts, mentioned above, could be shown in the course of simulation of the Triassic drainage process performed by mines with the help of analogue computer. The authors of the present paper carried out model tests, having assumed the boundary conditions that allow, among others, for some contacts described above. It appeared that the desirable conformity of the course of process performed on the model to the actual course could be achieved in that way.

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