Origin of groundwaters in the Upper Silesian Coal Basin according to stable-isotope composition

The total area of the Upper Silesian Coal Basin is 7500 km2. The Carboniferous coal-bearing deposits (8200 m thick) are covered by permeable Mesozoic and Quaternary rocks in the northern and eastern parts. In the southern part, these deposits are covered by Tertiary clayey impermeable formations (up to 1000 m thick). Two hydrological regions can thus be distinguished (I and II). The contemporary recharge areas for the Carboniferous aquifers are mainly located in region I. The drainage of these aquifers is mainly by coal mines, as a result of water pumping (down to 600 m). For reasons of safety and economy, it is essential to estimate the potential inflow of water into the mines and to establish its condition (static or dynamic). Therefore, the origin of groundwaters within the whole basin must be known. The traditional hydrochemical and hydrodynamical methods do not solve the problem, but the environmental isotope techniques represent a new approach. 133 water samples were collected from different mines and boreholes, from the Tertiary, Triassic and Carboniferous aquifers. The δ18O and δD values of all samples were measured in the laboratories of the IAEA and the Institute of Physics and Nuclear Techniques of the Academy of Mining and Metallurgy. The water chemistry was determined in the laboratories of the Geological Institute, Upper Silesian Branch, at Sosnowiec, and the Geological Enterprise at Katowice. The tritium content of some of these samples was determined at depths of down to 300 m (region I) and down to 400 m (region II); 14C was determined at depths of down to 400 m (region I) and down to 260 m (region II). The δ18O values vary from -11.6 per mille to +0.3 per mille and the δD values vary from -72.9 per mille to -1.2 per mille. The TDS values vary from 0.14 g/L to 215 g/L, with a simultaneous differentiation in chemical composition. Conclusions: (a) Both chemistry and isotopic composition permit three groups of waters to be distinguished: infiltration waters (1), mixed infiltration and relict waters (2), and relict (synsedimentary) waters (3). Their vertical range is shown. (b) First group: HCO3-SO4-Ca-Mg waters, TDS = 10 g/L, δ18O = -12.1 per mille to -8.8 per mille, δD = -77.1 per mille to -62.0 per mille, tritium (5 ± 2) TU to (18 ± 2) TU, observed only down to 300 m. Carbon-14 was detected in this group down to 400 m (11.1 ± 1.3%). It can be concluded that the aquifer recharge took place in Holocene and Pleistocene. (c) Second group: Mainly Cl-SO4-Na or Cl-Na-Ca waters. TDS = 14-230 g/L, δ18O -8.8 per mille to -1.0 per mille, δD = -62.0 per mille to -15.0 per mille. Mixed waters of groups (1) and (3) with older relict waters. There is no correlation between the stable-isotope content and total mineralization, probably because relict waters were squeezed out from Tertiary sediments which had different initial mineralization. (d) Third group: CI-Na waters, TDS = 32-52 g/L, 14C free; the isotope composition corresponds to SMOW. These waters have to be considered as synsedimentary/Tertiary waters. (e) Waters from region II show a depletion of heavy isotopes down to 600 m. This indicates the presence of mixing processes forced by the mining activity