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The changes of flow and chemical composition of groundwater in the vicinity of the thermal experiment


Lenka Rukavičková, Jan Holeček, Karel Sosna

Geoscience Research Reports 48, 2015 (GRR for 2014), pages 159–164

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Published online: 12 October 2015

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A thermal heating experiment was carried out in hard rock massif approx. 100 m below the surface in the Underground Research Center Josef, Mokrsko - Central Bohemia. The rock around the large-diameter borehole (? 80 cm) was heated in cycles up to 90 °C and changes in stress, temperature, chemical composition and in outflow of groundwater were observed in adjacent network of monitoring boreholes. The rock occurring in place of the heating experiment is represented by tonalite of the Central Bohemian Pluton. During the long-term experiment lasting 2 years, samples of groundwater were collected and the intensity of seepage was measured in adjacent monitoring boreholes as well as in broader undisturbed area away of the heating experiment (Fig. 1).
The chemical type of seepage ground waters unaffected by the thermal experiment corresponds mostly to Ca-SO4-HCO3 or Ca-HCO3-SO4 (Fig. 2). The major cation in these ground waters is calcium, but sodium prevailed in waters collected shortly after the beginning of heating.
A man-made silica-based geopolymer was used as thermal-conductive sealing between the heater in central borehole and the surrounding rock. It was found by the leaching tests that the geopolymer sealing influences the chemical composition of ground waters. Water in contact with the geopolymer loses calcium, and partly also magnesium, while sodium is dissolved and released from the geopolymer into groundwater. The rise of temperature during the thermal experiment accelerated the ion exchange on the geopolymer surface (Fig. 3).
It has also been proved that heating of the rock affected its hydraulic conductivity. The fractures parallel to the gallery face widen because of thermal relaxation of the rock mass into the space of the drift. The extension of fractures is accompanied by the increase of seepage into the adjacent boreholes (Fig. 4). Fractures perpendicular to the gallery face exhibit a decrease of hydraulic conductivity due to increased tension in the rock massif during the heating cycle.


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