O wpływie tektoniki uskokowej na zawartość jonu chlorkowego w wodach triasu olkuskiego

Abstract

Influence of disjunctive tectonics on chloride-ion content in the Triassic waterbearing horizon of the Olkusz region (near Cracow) In the present paper the authors have examined the concentration of chloride ion in the Triassic waterbearing horizon basing their examination on 190 analyses of ground water taken at the time of pumping tests from the bore-holes distributed on the territory of about 130 km2. The tectonics of the examined area is well known due to the dense net of ore-prospecting bore-holes. The content of chlorides in the Triassic waters is between 2 ppm to 344 ppm. It has been reported that the distribution of the concentration of chlorides in the examined waters is uni-modal in character, asymetric with the positive skewness (Fig. 1). Upon the basis of the shape of the distribution curve it has been stated that the concentration value of chloride background is about 12 ppm, whereas the concentration values exceeding 30 ppm have been recognized to be anomally high. The relation between the concentration of chloride ion and the depth measured in relation to the terrain surface has not been recorded, nevertheless the increase of the chloride content in the base part of the examined aquiferous horizon i.e. in the Roethian has been recorded. The map of the distribution of the chloride ion content in the Triassic waters {Fig. 2) shows its considerable differentiation in the horizontal plane. There are areas of apparently higher concentration in relation to the background and also of relatively low concentration of Cl. The picture shows certain regularities which can be explained by the conditions of circulation and exchange between waters occuring in the Triassic aquiferous horizon and the waters present in underlying formations. The Triassic waterbearing horizon of the fissure-karstic type about 110 m thick including Roethian and the Lower and Middle Muschelkalk is underlain by the Permian sediments of continental origin. In the Permian deposits there are present mineralized waters of the total ion concentration -reaching up to 22 000 ppm and the chloride ion content to 12 000 ppm. Because of the low permaeability of the Permian deposits the circulation of waters there is very constrained. In the earlier papers of the authors (Motyka et al., 1972, Witczak et al., 1975, Adamczyk et al., 1978) it has been demonstrated that the waters occuring in the Permian deposits originally salty in the entire profile undergo gradual freshening mainly by way of diffusion. The ascending in this way salts are removed by the fresh waters circulating intensively in very permeable Triassic limestones and dolomites. The vertical range of the diffusive elevation of chloride ion within the Triassic horizon is small. The consequence of this process is probably only a little increase in the content of this ion in the base portion of the Triassic profile (Fig. 7). However, in the zones where fissures accompany faults and the piezometric head in the Permian formations is higher than in Triassic, the salty waters can be elevated by way of convection. In this way large quantities of ion Cl— can be convected not only from Permian but also from the older formations occuring in the base. Depending on the permeability of the migration ways of salt waters and the difference of piezometric pressures as well as on the velocity of the horizontal ground-water flow in Triassic the influence of the convectional elevation of chloride-ion can reach different height. The mentioned process can also influence accordingly the increase of the concentration of chlorides in Triassic waters. Bearing all this in mind the graph of horizontal distribution of ion Cl— presented in Fig. 2 and 6 becomes clear. It is evident there that areas including places with higher concentration of this ion than in the chloride background are mainly situated in the vicinity of fault zones. The elongated shape of the places with the higher concentration of chloride in the Triassic waters corresponds with the main direction of tectonic disjunctive dislocations. Apart from this the shape of hydrochemical anomalies can be influenced by the regional direction of the underground waters flow which was dominant at the time when the samples were being taken. At present on the described territory the movement of the ground-waters in Triassic is different. This is caused by the intensive drainage of the zinc-lead ore mines.