The historical silver-barite mines of the famous Kogel mining area can be found 40 km northeast of Innsbruck in Tyrol/Austria, close to Brixlegg. Since medieval times, silver-bearing fahlore, and in the 19thcentury as well as from 1947 to 1968, barite was mined there. One of these underground mines is the Georgi-Unterbau with the huge Schwerspat stope (pit) and its flooded underground works.
In 1887, the miners started to drive the Georgi-Unterbau adit, which opened up rich fahlore and barite resources. Starting in 1900, a 100 m deep two-compartment blind shaft was sunk 320 m southeast of the mine entrance.
This shaft opened up the 20, 40, 70, and 100 m main levels and the 10, 75, and 80 m sublevels (Pirkl 1961; Mutschlechner 1984; Krischker 1990; Han-neberg and Schuster 1994) and connected the 14-Nothelfer stope and the Bar-bara stope with each other. It was partially to fully flooded in the 1950s and the pre-flooding water make was about 100 L min-1 (Hießleitner 1951;
Schmidegg 1953); it was pumped out again in 1984 and 1988 for ore In April 2002, and in May 2006 the author conducted a tracer test in the
Reiche Zeche Schacht, based on the results of the previous investigations (Fig. 120). Na-fluorescein was chosen as the tracer, which proved to be sta-ble in the mine water as has been shown by an earlier laboratory investiga-tion with the Reiche Zeche mine water and Na-fluorescein. 50 g of Na-fluo-rescein were injected into the shaft at a depth of 75 m during the 2002 tracer test, between the ½ 5 and 6 levels (½ 5. and 6. Sohle). From the velocity measurements (ca. 0.06 m min-1), the shaft’s cross section (4–5 m²) and a piston-flow-model, the tracer was expected to reappear after a minimum of 17 hours plus the opening time of the tracer probe (approx. 6–10 hours). Yet, even after 17 days, no tracer was observed at the outflow of the shaft. Three more tracer tests with Na-fluorescein at depths of 40 m, 65 m, and 324 m were conducted in 2006. Only tracer from the injection point at a depth of 40 m, just above the ½ 5 level, reappeared at the shaft’s outflow. After seven weeks the tracer test was terminated as no tracers from the 65 m and 324 m deep injection points could be fround (Wolkersdorfer et al. 2007).
Those results comply with the velocity measurements of Zittnan et al.
(1990), who observed down welling between the ½ 5 and 6 levels. If that and the tracer tests’ results, as discussed in the previous paragraph, repre-sents the real flow situation in the flooded Reiche Zeche shaft – and from all the measurements conducted so far, this hypothesis cannot be rejected – the often published conceptual hydrogeological model of the mine (e.g. Baacke 2000; Baacke and Degner 2000; Bayer 1999) must be reconsidered. The real
conductivity, mS cm-1
2.1 2.2 2.3 2.4 18.8 19.0 19.2 19.4 19.6 19.8
depth below water surface, m
0
50
100
150
200
250
300
temperature, °C
½ 5. Sohle
6. Sohle
7. Sohle
8. Sohle
11. Sohle
½ 10. Sohle
Fig. 120. Temperature and electrical conductivity measurements in the Reiche Zeche shaft (2 February 2001). Sohle: level.
temperature, °C 20 22 24 26 28 30 32 34 36 38
depth, m below water surface
0 50 100 150 200 250 300 350 400 450
velocity, m min-1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
½ 5. Sohle
6. Sohle 7. Sohle 8. Sohle
11. Sohle
½ 10. Sohle
Fig. 119. Temperature and velocity measurement in the Reiche Zeche shaft (modi-fied after Zittnan et al. 1990). Sohle: level. No error-bars were given by the authors.
324 Flooded Underground Mines: Case Studies 12.12 Reiche Zeche, Erzgebirge, Saxony, Germany 325
differences. While the upper one had a mean temperature of 8.53±0.01 °C and electrical conductivity of 324±26 µS cm-1, the lower one’s mean tem-perature and electrical conductivity was 8.63±0.03 °C and 332±23 µS cm-1, respectively. In contrast to the temperature, no stratification could be deduced from the electrical conductivity profile, which showed a relatively continu-ous increase from ca. 335 µS cm-1to ca. 345 µS cm-1. The chemical compo-sition of the mine water behaved similarly (Table 29). After the injection of 200 kg of sodium chloride tracer as a saturated brine in August 2001, the be-fore described stratification significantly changed, albeit the brine was not injected into the blind shaft itself but into a small drop-hole 4 m west from it. Instead of two mine water bodies, four of them appeared to exist, separated at the 20-, 40-, and 70-m levels. Yet, six months later, the previous stratifi-cation was restored with a lower temperature (ca. 8.1 °C) and a higher elec-trical conductivity (ca. 480 µS cm-1).
Interestingly, the stratification pattern in the Georgi-Unterbau blind shaft was stable for at least 19 months, although the temperature difference of 0.1 K between the two mine water bodies is extremely small. On the other hand, the injection of the sodium chloride brine shows that the stratification can easily be destroyed although it had virtually reestablished itself six months thereafter. Unger (2002) numerically modelled a part of the flooded mine and showed that the water seeping through the Schwerspat pit into the 14-Nothelfer pit and from there to the 40-m level is responsible for the tem-perature drop in the water column above the 40-m level and thus the ob-served stratification. While the physico-chemical measurements and the chemical analyses of the water indicate good mixing within the upper and lower layers, the results of the tracer test clearly showed restricted mixing be-prospecting (Krischker 1990). Finally, in 1990, the blind shaft was flooded
for the last time and stationary hydraulic conditions exist there today. The Georgi-Unterbau mine is typical of mines that have only one shaft that con-nects all the flooded levels.
All parts of the mine are within the Devonian Schwaz Dolostone (Schwazer Dolomit) of the Northern Tyrol Greywacke area (Nordtiroler Grauwackenzone). Typically, the Schwaz Dolostone is a very hard, light white to light grey dolostone, being highly brecciated and fissured in the area investigated. The dolostone hosts silver- and mercury-bearing fahlores (among them schwazite) as well as barite, the mineralisation being bound, but not restricted, to the breccia zones (Pirkl 1961; Wöbking 1982; Arlt and Di-amond 1998). Grundmann and Martinek (1994), and Schnorrer (1994, 1996) described 20 ore minerals as characteristic of the Schwaz Dolostone and over 132 minerals for all the rocks in the Schwaz-Brixlegg area.
The first physico-chemical investigation of the flooded blind shaft was conducted in December 2000. It became clear that the mine water was strat-ified, clearly showing two water bodies separated from each other at the 40 m level. Further investigations, including two tracer tests with 15 µm micros-pheres and Na-fluorescein, were conducted in August 2001 and February 2002, where 22 L min-1(32 m³ d-1) and 31 L min-1(45 m³ d-1) of mine water were flowing out, respectively (Unger 2002; Wackwitz 2002; Wolkersdorfer et al. 2002). About 300 m³ d-1are coming from other inflows, including 150–
200 m³ from a major fault zone, the Grünwies fault, summing up to 300–
400 m³ d-1of total flow (Table 12).
Based on a piston-flow-model (non-dispersive laminar Poiseuille flow), the mean effective velocity of the mine water in the shaft was calculated to be 4 · 10-3m min-1. However, the tracer test resulted in a mean effective ve-locity of 3 · 10-2m min-1above the 40-m level. This difference is typical for all tracer tests conducted by the author so far, whereas the velocity does not fit into the 95% confidence interval of 0.3–1.7 m min-1 (Wolkersdorfer 2002b). Similar results for porous media were described by Ren et al. (1996).
The low velocity in the Georgi-Unterbau blind shaft is due to the mine being a “single shaft mine”, whereas most of the other mines studied are “multiple shaft mines” or include both shafts and galleries. None of the tracers injected below the 40-m level could be retrieved.
12.13.2 Investigations and Appearance of Stratification
Between December 2000 and February 2002, eight temperature and electri-cal conductivity profiles and two time series were measured in the Georgi-Unterbau blind shaft. Before the tracer test in August 2001, two mine water bodies, separated at the 40-m level, were identified, based on temperatures
depth Li HCO3- SO42- NO3- Sb Cu Zn As
0.15 0.003 214 40.6 4.9 1.0 0.5 0.2 0.2
55 0.002 218 41.0 4.9 1.0 0.6 0.2 0.2
Table 29. Exemplified composition of the Georgi-Unterbau mine water (samples BRX-0208-BS0.1 and BRX-0308-BS55 of 2/3 August 2001, filtered through 0.45 µm cellulose acetate filter). Concentrations in mg L-1, depth in m, temp in °C, elec-trical conductivity in µS cm-1; K, Cl, Ag, and Hg below the detection limits.
depth temp pH cond Ca Mg Sr Na Ba
0.15 8.6 7.1 410 60.4 30.5 1.0 0.6 0.08
55 8.3 7.6 411 62.3 30.7 1.0 0.7 0.06
326 Flooded Underground Mines: Case Studies 12.13 Georgi Unterbau, Tyrol, Austria 327
ilar to what was observed in August 2001. During the period of investigation (18 to 24 February 2002), the water temperature of the shaft stayed stable at about 9.2 °C and the electrical conductivity remained around 430 µS cm-1. The outflow of the shaft was also stable at 31.5 L min-1(43–48 m³ d-1); the pumping capacity of the mini-piston pump (sampling pump) was 0.726 L min-1(1 m³ d-1). From the 50 g of Na-fluorescein used, only 3.2 g was recovered, which equals a recovery rate of 6.4%. As the breakthrough curve (Fig. 81) shows, the tracer concentration increased after the first peak at 19 February. This might be caused by a density flow of the tracer. The tracer was injected at a relatively high concentration and due to its higher density it first moved downward; later, after dilution, it was transported out of the shaft again. It can therefore be concluded that after a longer time period, more of the tracer would have been discharged and the recovery rate would have been higher.
The peak concentration of the tracer (excluding the background) was about 300 µg L-1at a background concentration of 70 µg L-1, which resulted from the 2001 tracer test. At the end of the tracer test, the tracer concentration was still as high as 13 µg L-1. The calibration of the fluorometer gave a lower de-tection limit of 7 µg L-1and the maximum concentration design dependent was 7,000 µg L-1(the concentrations in Wolkersdorfer et al. [2002] are wrong as the fluorometer was back calibrated afterwards).
From those results, a mean effective velocity of 0.03 m min-1was calcu-lated, which fits well with the effective velocity calculated from the previous tracer test (0.02 m min-1). This is 7–8 times the velocity calculated ahead of the tracer test (0.004 m min-1) using a piston-flow-model, which is not un-usual for mine water tracer tests. So far, the reasons are unclear but the wall roughness of the shafts and adits might play an important role. When com-paring the results of the two tracer tests, it is clear that the first tracer test had a larger dispersion than the second one, due to the longer flow path of the first test (48 m) than the second one (10 m).
Unfortunately, the reasons for the stratification in the Georgi Unterbau blind shaft could not be determined by the two tracer tests, because the tracer could not be recovered in the deeper parts of the flooded shaft. It is possible that the deeper parts of the shaft are less hydraulically connected and there-fore diffusive flow might dominate. At least from the tracer concentration between the 40- and 70-m levels, diffusive transport can be assumed; other-wise, the tracer distribution around the injection location should have been distributed differently. On the other hand, as the Rabenstein tracer test shows, extremely slow water movements cannot be excluded.
In conclusion, this tracer experiment – like the Rabenstein experiment – showed that no fast convective mixing occurs in the deeper parts of the flooded mine as was observed at the Niederschlema and Straßberg mines.
tween the two layers. Furthermore, the temperature and electrical conduc-tivity patterns confirm that there is a turbulent flow regime in the shaft (Fig.
121).
12.13.3 Results and Interpretation
To investigate the situation, two multiple tracer tests were conducted in the mine. Tracers were injected into different depths of the blind shaft and an underground mining lake connected to the shaft. From the tracers injected, only the tracer from the underground mining lake (“Big Lake”: green mi-crospheres) and sodium fluorescein from the blind shaft could be detected.
A subsequent measurement of the sodium fluorescein concentration in the shaft showed a nearly-normal distribution of the tracer cloud around the tracer’s injection depth at 55 m, with tracer concentrations between 200 and 600 µg L-1between 40 and 80 m depth. This indicates diffusive mass flow was occurring in that part of the shaft. An evaluation of the breakthrough curve of the green microspheres results in a mean flow duration of 52 hours for a distance of 48 m, yielding a mean effective velocity of 0.02 m min-1for the flow between the “Big Lake” and the blind shaft’s outflow.
In February 2002, the negative experiences of the first tracer test were con-sidered and the results therefore were better. The sodium fluorescein showed a well-established breakthrough curve within the expected time frame,
sim-12.13 Georgi Unterbau, Tyrol, Austria 329
conductivity, mS cm-1
0.33 0.34 0.35 8.50 8.60 8.70
depth, m below water surface
0
10
20
30
40
50
60
70
80
90
100
temperature, °C
20 m level
100 m level 70 m level 40 m level
Fig. 121. Temperature and electrical conductivity measurement in August 2001 at the Georgi Unterbau Shaft. The temperature stratification at the 40 m level, even when significantly lower than 1 K, was stable for more than 3 years.
328 Flooded Underground Mines: Case Studies
und Umweltforschung (1986), Herbert (1989), Herbert and Sander (1989), and Czolbe et al. (1992). From the examples in those publications, four salt mines, three in Lower Saxony (Hope and Ronnenberg near Hannover, Beien-rode near Wolfsburg) and another one in Saxony-Anhalt (Peißen near Bern-burg) are described.
Hope is located 30 km north of Hannover/Germany and was flooded with saturated brine from salt cavern dissolution. Flooding started on 12 March 1984, after the mine was abandoned in January 1984. Before flooding, 16 in-situ measuring installations were installed to monitor the flooding process.
Furthermore, an underground dam was constructed to study safety dams for radioactive waste disposal. The whole programme was initiated in a rela-tively short time period, only six month ahead of the planned flooding. Be-sides the geochemical, geomechanical, geophysical, and the dam monitoring installations, the aim was to investigate “mass and heat transport in flooded, not backfilled galleries, mine workings, and shafts” (GSF – Gesellschaft für Strahlen- und Umweltforschung 1985; referred sentence translated from Ger-man).
Mining in the Hope salt dome began in 1907 with the 521.2 m deep Adolfsglück shaft; in 1909, the 628.5 m deep Hope shaft was developed. The salt dome has a depth of about 4,000 m, a maximum diameter of 6 km, and consists of different salt types of Permian (Zechstein) age: rock salt, sylvite, kieserite, and anhydrite. A total volume of about 1.6 · 106m³ was flooded with saturated NaCl brine (density: 1,200 g m-3) from the Empelde cavern field. During the flooding process and as long as the unflooded parts of the mine were accessible, the brine was regularly analysed by taking several samples underground. Also, the flow path of the brine was controlled and monitored (Herbert 1989). The F&E research project participants conducted the last measurements in October 1988.
The 477 m deep Peißen shaft near Bernburg has a diameter of about 5 m, was developed in 1900, and underwent controlled flooding between 1972 and 1974. It is one of 11 shafts in the salt mines in the Bernburg main anti-cline and is located within a small synanti-cline, the Peißen synanti-cline. Mining in the Bernburg area started on 31 January 1852 with the Manteuffel shaft, which was where the world’s first potassium salt mine was developed (Rie-mann 1913; Hiltscher 1999). All the mines are within the Zechstein salt for-mation. Czolbe et al. (1992) investigated the flooded shaft through tempera-ture and density logs as well as chemical analyses and a tracer test with a radioactive γ-tracer. Furthermore, they modelled the hydrodynamic situation of the shaft with the code KASOMO.
The salt mines investigated by Uerpmann (1980) are the Asse I and Asse III (near Wolfenbüttel), the Beienrode shaft near Braunschweig, the Desde-mona shaft near Göttingen, and the Deutschland shaft of the flooded Ron-Therefore, it can be concluded that the missing hydraulic connection between
the workings in the deeper parts prevents convective mixing of the mine water. This fact has aroused little attention, though Uerpmann (1980) made a similar observation. It might therefore be concluded that the flooded mine between the 40- and 70-m levels acts as a hydraulic barrier, preventing mass transport between the upper and lower parts of the shaft.