Natural Analogues for Safety Cases of Repositories in Rock Salt, Salt
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NEA/RWM/R(2013)10 Isotope monitoring of water appearances in salt mines: The Polish experience Marek Dulin ski1, Kazimierz Rozan ski1, Krzysztof Brudnik2, Piotr Kolonko3, Joanna Tadych4 1Faculty of Physics and Applied Computer Science, AGH Univ. of Science and Technology, Krakow, Poland 2W ieliczka Salt Mine S.A., Wieliczka, Poland 3Klodawa Salt Mine S.A., Klodawa, Poland 4Inowroclaw Salt Mines Solino S.A., Inowroclaw, Poland Isotope analyses of water appearances in Polish salt mines started in the early 70s of the last century. Soon after it became clear that oxygen and hydrogen stable isotope composition of water creates a unique possibility of determ ination of the origin of waters in mines and has a crucial role in the risk assessment associated with water inflows. Chem ical composition of mine waters, although useful, cannot be used to distinguish between syngenetic (safe) and meteoric (potentially dangerous) water types occurring in mines. The work provides a brief overview of isotope investigations of water appearances in four Polish salt mines (Wapno, Inowroclaw and Klodawa Salt Mines located in north central Poland and Wieliczka Salt Mine located in southern Poland) performed over the past forty years. Introduction Polish salt deposits are situated in the north-central and southern part of Poland (Figure 1). They are represented either by salt domes or salt beds, usually covered by water bearing Tertiary and Quaternary formations, sometimes surrounded by older deposits. There are three groups of water which make their appearance in salt mines: i) meteoric waters of infiltration origin , recharged during both Quaternary and pre-Quaternary climates; ii) syngenetic waters; iii) technological waters and waters of unknown origin. While the first and third category of waters pose a certain danger to the operation of mines, the second category of water is usually safe for salt beds as they occur in a full saturation state and in limited amount, being remnants of the initial salt-forming brines. Sediments surrounding salt deposits are sources of water of meteoric origin which become dangerous when entering the salt formations. Dissolution of different salt minerals along water flow paths in mines leads to enhanced mineralisation of this water. The type of mineralisation is independent of water origin and is entirely controlled by the chemical composition of the given salt deposit. Therefore, the chemical composition of brines appearing in mines cannot be used as an indicator of the origin of water. Investigations of brines in Polish salt mines carried out in the past forty years clearly demonstrate that only stable isotope composition (2H/1H and 18O/16O isotope ratios) of water provide valuable information about its origin. To some extent, also the tritium content in water and 14C content in dissolved carbonates can be helpful in determination NATURAL ANALOGUES FOR SAFETY CASES OF REPOSITORIES IN ROCK SALT, © OECD 2014 123 NEA/RWM/R(2013)10 Figure 1: Salinary provinces in Poland, and the investigated areas (Zuber, et al., 1997) of water origin although they should be treated with a caution at low measured concentrations due to possible isotope exchange with water vapour or carbon dioxide present in the atmospheres of mines. In the following, the Polish experience in determination of water origin in salt mines is briefly described. We focused on four salt mines localised in Wapno, Klodawa, Inowroclaw and Wieliczka (cf. Figure 1). Discussion The Wapno Salt Mine Exploitation in Wapno village started in 1826. Initially it had the form of an open pit gypsum mine. Over the years the open pit mine was transformed into the deep mine. At this time the uppermost Levels I and II were exploited within the gypsum cap of the underlying salt dome. Exploitation of salt started in 1911. In years 1927-1930 two shafts were build (one of them was almost immediately flooded). In 1964 a new shaft started to operate. Initially, the salt was exploited at Levels IV-IX (406-543 m depth). The problem with dangerous water inflows to the mine appeared when the exploitation started at Level III. The isotopic characteristics of waters appearing at Level III are presented in Figure 2. The modern origin of waters in the well N1 and at Level III was clearly indicated by the presence of tritium and high 14C content. Stable isotope composition of waters modified with respect to the values expected for Holocene infiltration suggested the existence of efficient isotope exchange with water of crystallisation of the hydrated salt minerals. It was clear that water appearing at the third level is of meteoric origin. However, this information was ignored by the mine managers. In the night of 4 August 1977, within five hours about 1.5 million cubic meters of water flooded mine corridors and chambers. The catastrophe was probably unavoidable but its large scale and high costs (also including damages of buildings on the surface) were caused by several main reasons: i) intensive exploitation at the third level; ii) excavation of large chambers (15 m wide, 180 m long, up to 12 m high) leading to weakening of safety pillars, iii) too strong explosives used during the exploitation; iv) poor management of the mine ignoring repeated warnings about the infiltration origin of waters appearing at Level III. 124 NATURAL ANALOGUES FOR SAFETY CASES OF REPOSITORIES IN ROCK SALT, © OECD 2014 NEA/RWM/R(2013)10 Figure 2: Isotopic characteristics of inflows to the Wapno Salt Mine (Zuber, et al., 1997) Left: north-western part of the salt dome. Right: south-eastern part. Arrows indicate presumable directions of water flow. The Klodawa Salt Mine The Klodawa deposit is situated in the central part of a 63 km long salt structure. That part pierces through the Mesozoic strata and forms a diapir 26 km long and 0.5-2 km wide. The gypsum-clayey cap is at a depth of 40-500 m, up to 250 m thick, and is surrounded by Quaternary and Tertiary water-bearing sediments. Below the Tertiary sediments, the diapir is surrounded by Mesozoic formations. Salt is exploited at four main levels: 450, 525, 600 and 750 m and several intermediate levels. Three shafts are operated. Yearly production of salt exceeds 600 ktonnes. The volume of exploitation chambers is approximately 18 million cubic meters, with typical dimensions 120 x 12 x 12 m. The isotopic characteristics of water appearances in the Klodawa Mine are presented in Figure 3. There are three main groups of water identified in the mine on the basis of their isotopic composition. The first group (diamonds in Figure 3) represents syngenetic waters. They are of Zechstein age and represent different stages of evolution of sea water during desiccation of shallow lagoons. They are usually closed within salt pockets and occur in limited amounts; their occurrences usually disappear after some time. Points representing stable isotope composition of these waters form characteristic “hook” shape on the d2H-d18O diagram. In some water appearances of measurable amounts of tritium had been found. Its presence in brines has been interpreted as a result of isotope exchange with atmospheric water vapour inside the mine. Their stable isotope composition is shifted towards the WMWL (Figure 3). The second group of points (squares in Figure 3) represents waters posing a certain risk for the operation of the mine. They are of meteoric origin and of different age. Most of them carry significant evaporation signature and at the present stage of knowledge it is difficult to decide whether the evaporation took place before infiltration or after their appearance in mine passages. Some of these waters were most probably recharged in climatic conditions significantly colder than those prevailing at present in central Poland. Other waters of this type most probably represent Holocene infiltration. In contrast, the appearances no. 7/450 and 14/450 undoubtedly represent pre-Quaternary infiltration, characterised by significantly higher air temperatures than the present ones. NATURAL ANALOGUES FOR SAFETY CASES OF REPOSITORIES IN ROCK SALT, © OECD 2014 125 NEA/RWM/R(2013)10 Figure 3: Stable isotope composition of waters appearing in the Klodawa salt mine (selected data) 10 0 -10 -20 o 2 -30 > -40 XUO -50 -60 -70 -80 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 d18O [%o] V-SMOW The third group of points in Figure 3 (triangles) represents technological waters and waters of unknown origin. Technological waters are relatively easy to recognise. Usually they are formed by condensation of atmospheric water vapour or appear as drops on the connectors of pipelines. They create characteristic erosion holes on the floor of mine corridors. Some water appearances within this group are, however, difficult to explain. Their stable isotope composition may result from complicated, multi-stage isotope exchange processes with atmospheric water vapour and/or crystallisation water in salt minerals. This needs further, in-depth investigations. The Inowroclaw Salt Mine The Inowroclaw salt diapir is about 2.5 km long and 1 km wide. The gypsum cap is at a depth of 6-221 m, while the salt dome is situated at depth of 122-272 m. In the 19th century salt was exploited by two underground mines and solution chambers which were created through wells. In 1907 both mines were catastrophically flooded, while the solution chambers were exploited until 1924 to a depth of approximately 180 m. Between 1924 and 1933 a deeper mine was built below the older one, with the first level at a depth of 470 m. Its exploitation began in 1933. Ten exploitation levels have been created since then, each separated by approximately 18 m of salt deposits. Stable isotope composition of water appearing inside the Inowroclaw Salt Mine is presented in Figure 4.