BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004
Investigation Concerning Influence Of Seismic Activity On The Regime Of Hydrothermal Deposit Pchelinski Bani, Bulgaria
Boryana Yordanova Deneva PhD student, University of Mining and Geology "St. Ivan Rilski" Sofia, Bulgaria
Abstract Pchelinski Bani hydrothermal deposit located in the central western part of Bulgaria. It is a hydrogeological structure of a fissure-vein type. The terrain in the investigated area is built up mainly by granodiorites. The investigations on the thermal water regime have been carried out on the basis of an analysis of data obtained from regime observations on the thermal spring discharge for a 28-year period and from observations on the water level in a monitoring borehole for a 20-year period. The analysis of the data has shown that the spring discharge decreases sharply, even in some cases the outflow has totally stopped for few hours, after earthquakes with a magnitude over 3.8 according to the Richter scale, occurring on the territory of Bulgaria and its neighboring countries. Key words: thermal water; earthquakes; Balkan Peninsula.
Introduction Pchelinski Bani hydrothermal deposit is located in the central western part of Bulgaria. The main water source is the captured thermal spring also known as "Pchelin" captation. The spring has an expressed postseismic reaction – there is a sharp decrease in its discharge immediately after strong earthquakes, occurring both on the territory of Bulgaria and its neighboring countries. The water from the hydrothermal deposit is determined as fresh - with mineralization 0.92 – 0.98 g/l, hyperthermal (72.8ºC), sulfate sodium by composition with slightly alkaline reaction (pH up to 8.0). The contents of metasilic acid, fluoride and radon respectively are: up to 112 mg/l; up to 10 mg/l and 110 ÷ 120 Em (407 ÷ 444 Bq/l. Nitrogen dominates among the gases diluted in the water - 96.7 volume %, followed by argon (1.7 vol.%) and helium (0.25 vol.%). Pchelinski Bani hydrothermal deposit was included in many regional geological investigations. A. Boué in 1884 gives the first information about the geology of this region. Later on, the famous Bulgarian scientists G. Zlatarski in 1893 and G. Bonchev in 1968 have investigated the region. The latest results from the geological mapping of the region are summarized by Iliev & Katskov (1990, 1993) on the Geological map of Bulgaria in scale 1: 100 000 – map sheet Ihtiman. A hydrogeological investigation of the region has been carried out in the period 1965-1967. Its main purpose was to increase the mineral water yield. K. Shterev (1964) and P. S. Petrov et al. (1970) investigated the hydrothermal deposit. Full hydrogeological characteristic of the hydrothermal deposits from the Dolna Banya thermal water basin, including also that of Pchelinski Bani, was made in the paper of Pentchev et al. (2003). The investigation on the regime of the hydrothermal deposit and the assessment of its exploitation resources was made in the paper of Zahariev Deneva (2003). P. Petrov (1983) who has studied the consequences of the earthquake in Vrancea, Romania in 1977 on groundwater regime in different hydrogeological structures of Bulgaria, has provided the first purposeful investigation on the post-seismic reaction of the Pchelinski Bani hydrothermal deposit. As a result, one of the constructed boreholes (Borehole N2) has been equipped as a seismo- hydrogeological observation station with a continuing water level recorder. This report presents an attempt in the investigation of the impact of seismic events, occurring on the territory of Bulgaria and its neighboring states, on the regime of hydrothermal deposit Pchelinski Bani. The investigation is based on the analysis of data obtained from regime observations on the thermal spring discharge and the fluctuations of the water level in the monitoring borehole, standing near to the spring.
Geological and hydrogeological conditions Pchelinski Bani hydrothermal deposit is located in the northern part of a larger hydrogeological structure, named as Dolna Banya thermal water basin. The basin encompasses Dolna Banya valley
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BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004
and slopes of surrounding mountains - northern slopes of Rila mountain and southern Sredna Gora mountain (Pentchev et al., 2003). On the territory of the investigated region Paleozoic, Mesozoic and Neozoic formations are found. Basic collectors of thermal water are granodiorites of the Gucal pluton (guγδK2), which outcrop within the region of Gutzal and Pchelin villages. The pluton is built of large-grain granodiorite, leuco – to mesocratic with massive texture whose mineral composition includes plagioclase, potassium feldspath, quartz, amphibole, biotite, apatite, titanite, sericite, epidot. The pluton age is determined to 72.5 million years (Katskov et al., 1993). The hydrothermal deposit belongs to a regional tectonic structure, known as the Maritsa fault, which divides the Rhodopes massif from the Sredna Gora Mountain. Its width is about 10 - 20 km. It represents a deep fault, composed of a series of sub-parallel, mainly vertical south-verging faults, where the northern slope of the Rhodopes massif is slipped. The Maritsa deep fault is one of the zones with the most active seismic activity for the territory of Bulgaria. In the region of the hydrothermal deposit, thermal water outflows in the intersection zone of two fault systems. Initially, mineral water has been drained by several small springs at an elevation of 632.6 m a.s.l., coming out from a 3 to 10 m wide fault of east-west direction and steep incline to the south. The springs have appeared at the place where the fault is intersected by transverse tectonic fissures of northeast orientation. The open part of the fault is filled with cavernous tectonic breccia from granodiorite fragments that are intensively changed, covered and fused by ferrous hydroxides, zeolites and other materials, deposited by thermal waters (Zahariev Deneva, 2003). The springs were captured in a general captation shaft in 1937. Their total discharge before the capturing amounted to 11.17 l/s and then to 11.7 l/s at temperature of the water 73ºC. Three hydrogeological boreholes - NN 1, 2 and 3 were drilled close to the thermal spring during 1965-1967. Their depths are respectively: 497 m, 348.9 m and 350.3 m passing entirely into granodiorites. Only two of the boreholes had crossed the thermal zone but no artesian flow was obtained as a result. In the course of the hydrogeological studies, experimental tests of these boreholes have been made. During the experimental water pumping, the discharge of the thermal spring has decreased and at its end the outflow has completely stopped. The discharge reached its initial values following a period of a month and a half. Borehole N1 has been liquidated after the end of the surveying works. Borehole N3 was adapted for pumping exploitation. In 1982, Borehole N2, which was situated at a distance of 44 m north-east from the thermal spring, has been equipped with a continuous level recorder by the National Institute of Hydrology and Meteorology at the Bulgarian Academy of Sciences. Nowadays the main water source in the hydrothermal deposit is the captured thermal spring known as "Pchelin" captation. Mineral water flows out at an elevation of 632.9 m. During the last 10 years, the spring discharge was varying around 8.8 l/s.
Information base The investigation of the seismic activity impact on the thermal water regime is carried out empirically - on the basis of an analysis of data obtained from regime observations on the spring discharge and the observations of water level fluctuations in Borehole №2. The regime observations on the spring discharge cover a period of 28 years, 1975 - 2003. The data obtained from water level observations are collected for a 20-year period, 1983 - 2003. Regular regime discharge observations have been made by the Specialized Hospital for Rehabilitations in Momin prohod during 1977-2000. They have been carried out with changing frequency. Up-to-date measurements of the discharge of the thermal spring were made by the author in August 2001, March and December 2003. Data obtained from the continuing level recorder in Borehole №2 encompass the period 01.01.1983 - 31.09. 1990. Since September 1990, when the recorder has been damaged, the observations have been carried out once a week. National Institute of Meteorology and Hydrology gave up the data available for this investigation. The values are average per month. Due to the lack of comparable data for individual periods of time, the correlation between the spring discharge and water level fluctuations is difficult to be determined at this first stage of investigation.
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BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004
Results and discussion Graphical interpretation of the data obtained from regime observations on the thermal spring discharge for a period of 28 years (1975 – 2003) is represented on Figure 1. There are several extremely low discharge values. The reference shows that these values were measured immediately after earthquakes occurring on the territory of Balkan Peninsula. Data for discharge variations and provoking earthquakes are summarized in Table 1. Locations of the epicenters of the earthquakes which had affected the thermal spring discharge are visualized on Figure 2.
Table 1 Discharge variations and corresponding seismic activity data Discharge Earthquakes Magnitude Geographical Discharge, according coordinates Depth № Date Date Region l/s Richter km scale, M Lat. Long. Velingrad, 1 06.11.1977 7.000 03.11.1977 5.4 42.08 24.09 13 Bulgaria Vrancea 2 11.03.1977 4.000 04.03.1977 7.2 45.34 26.30 110 Romania Aegean 3 09.08.1983 4.600 06.08.1983 6.9 40.14 24.74 10 Sea 4 20.08.1999 0.555 Marmara 17.08.1999 7.4 40.76 29.97 15 Sea 5 23.08.1999 1.005 Marmara 6 30.09.1999 7.610 29.09.1999 5.1 40.70 29.35 10 Sea Yambol, 7 10.08.2001 7.585 09.08.2001 3.8 42.55 26.48 15 Bulgaria 8 25.04.2002 0.500 Gniljane, 24.04.2002 5.7 42.43 21.51 10 9 26.04.2002 2.580 Kosovo 10 29.04.2002 0.000 Kumanovo, 29.04.2002 4.5 42.06 21.84 10 11 30.04.2002 6.500 Macedonia
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Discharge data Discharge data affected by earthquakes
Figure 1 Visualization of thermal spring discharge data during 1975-2003.
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BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004
Figure 2 shows that the thermal spring discharge is affected by seismic activity occurring in the regions of Aegean Sea, Marmara Sea, Macedonia, Romania and Bulgaria. The most distant earthquakes which have significantly affected the thermal water regime, are those from Vrancea (1977), Romania, and Marmara Sea (1999).
EARTHQUAKE MAGNITUDE VrVrancanceaea Moldova 7 to 8 5 to 6 4 to 5 3 to 4 Romania
Yugoslavia Bulgaria
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Macedonia
Turkey MarMarmaramara sseaea Ellada Albania 070140 AegeaAegeann sseaea kilometers
Figure 2 Locations of the earthquake epicenters affected the thermal spring discharge
Figure 3 gives a graphical presentation of data obtained from water level observations in monitoring borehole during the period 1983-1990, when the measurements have been carried out with continuing level recorder. Several smaller and two great sharp falls of water level are registered. These two falls have occurred during the periods October 1986 - December 1986 and March 1988 - June 1988. For the whole period, October 1986 - June 1988, water level had decreased totally with 60 cm. It has been established that strong seismic activity on the territory of Bulgaria (Strazica) and Northern Greece (Aegean Sea) was registered during this period. The data of seismic activity that possibly had affected the water level variations are presented in Table 2.
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Water level data during the period 01.01.1983 - 30.09.1990
Figure 3 Visualization of water level data obtained during the period 01.01.1983 - 30.09.1990.
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BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004
Table 2 Seismic activity that possibly had affected the water level in Borehole N2. Geographical Magnitude, Depth № Date coordinates Region M km Lat. Long. 1 06.08.1983 6.90 40.14 24.74 10 Aegean Sea 2 12.06.1984 4.30 40.32 22.78 10 Aegean Sea 3 25.10.1984 5.30 40.10 21.61 40 Northern Greece 4 21.02.1986 5.00 43.27 26.02 15 Bulgaria, Strazica 5 12.09.1986 4.10 40.24 27.36 10 Marmara Sea 6 26.10.1986 4.10 40.76 29.05 10 Marmara Sea 7 07.12.1986 5.44 43.23 26.01 13 Bulgaria, Strazica 8 08.08.1987 4.50 40.13 24.97 10 Aegean Sea 9 30.05.1988 4.30 40.29 25.93 10 Aegean Sea
During the period 30.09.1990 - 15.12.2003 the water level in the borehole was not automatically recorded because the continuing recorder was seriously damaged. The measurements were taken manually once a week. That is why the specific water level variations caused by earthquakes were not recorded. Parallel measurements of the spring discharge and water level in monitoring borehole were conducted during 28.01.1983 - 01.10.1990. Only one change in thermal spring discharge caused by earthquake has been registered. As a consequence of earthquake with a magnitude of 6.9 according to the Richter scale, occurring on 06.08.1983 in the territory of Northern Greece, thermal spring discharge had decreased with 7.6 l/s (from 12.3 to 4.6 l/s) and water level had dropped with 7 cm. After this earthquake, the discharge had almost immediately increased to its normal values, but the water level had retained its value for three months and then had increased to the value before the earthquake. These processes are visualized on Figure 4.
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Water level data Thermal spring discharge data
Figure 4 Parallel observations of thermal spring discharge and water level in Borehole N2
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BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004
Conclusions After an analysis of the presented data from the regime observations of the spring discharge and the water level in the Borehole N2 the following conclusions could be made: (a) The discharge of the mineral water spring decreases or even in some cases fully disappears after earthquakes with magnitude over 3.8 (Richter), which epicenters are registered in Bulgaria, Romania, Macedonia, Kosovo, Turkey and Greece. (b) In the period 01.01.1983 - 30.09.1990, when the level in the borehole N2 was continuously recorded with a measuring device, two points of sharp level lowering were observed, namely at the end of year 1986 and in the middle of year 1988. In those cases the total value of the level lowering was about 0.60 m. Because of the lack of other impact sources of the water level in the borehole (for example technogenous) it could be stated that its changes have appeared as a result of strong seismic activity in the region of Strazica (Bulgaria, 1986) and Aegean Sea (Greece, 1988). These results give a reason for future more detailed investigation of the hydrothermal deposit Pchelinski Bani. The investigation work has to include the following: (a) Implementation of continuous water level monitoring - installation of a continuing device in Borehole N2; (b) Implementation of parallel measurements of thermal spring discharge and water level in Borehole N2; (c) Investigations for possible variations in water chemical content and temperature as a result from seismic activity; (d) Including the hydrothermal deposit in extensive monitoring set for investigation of seismic impacts on similar hydrogelogical structures of fissure-vein type on the territory of Bulgaria and its neighboring countries.
Reference Asmanov, A. (1940) The Bulgarian mineral sources. Sofia, Bulgaria (in Bulgarian). Bulgaria: Catalogue of earthquakes 1981-1990. (1993) Geophysical Institute at Bulgarian Academy of Sciences, Seismological department. Ed.: D.E.Solakov, S.D.Simeonova. Sofia. Dimitrova, R. Katzkov, N. (1990) Geological map of Bulgaria in scale 1:100 000, sheet Velingrad.. Sofia, Bulgaria (in Bulgarian). Iliev, K. Katskov, N. (1990) Geological map of Bulgaria in scale 1:100 000, Ihtiman sheet. Sofia, Bulgaria (in Bulgarian). Katskov, N. Iliev, K. (1993) Explanatory note to the geological map of Bulgaria in scale 1:100 000, Ihtiman sheet. Sofia, Bulgaria (in Bulgarian). Pentchev, P., Zahariev,V. Deneva, B. (2003) Hydrogeology of the Dolna Banya thermal water basin. Annual of the University of Mining and Geology “St. Ivan Rilski” – Sofia, Vol. 46, p. 281-286. Petrov, P. Hydrogeological occurrences caused by the earthquake (seismo-hydrogeological phenomena) (1983) In: Vrancea Earthquake in 1977. Its after effects in the people’s Republic of Bulgaria. Publishing House of the Bulgarian Academy of Sciences. Sofia, p.96 -112 (in Bulgarian). Petrov, P. S., Martinov, Sv., Limonadov & K., Straka, Y. (1970) Hydrogeological researches on the Bulgarian mineral waters. Technika, Sofia, Bulgaria (in Bulgarian). Shterev, K. (1964) Mineral waters in Bulgaria. Nauka i Izkustvo. Sofia, Bulgaria (in Bulgarian). Zahariev, V., Deneva, B. 2003. Investigation on the regime of Pchelinski Bani mineral water occurrence and current assessment of its exploitation resources. Annual of the University of Mining and Geology “St. Ivan Rilski” – Sofia, Vol. 46, p. 315-320.
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