Evaluation of methane and carbon dioxide flux from Upper Carboniferous coal-bearing strata to near-surface zone in the Wa³brzych Coal District

Transkrypt

1 Chapter 12 Evaluation of methane and carbon dioxide flux from Upper Carboniferous coal-bearing strata to near-surface zone in the Wa³brzych Coal District Ocena szybkoœci dop³ywu metanu i dwutlenku wêgla z górnokarboñskich formacji wêglonoœnych do strefy przypowierzchniowej w niecce wa³brzyskiej Adam KORUS *, aciej J. KOTARBA **, arek DZIENIEWICZ ** & enryk SECAN ** * Faculty of Physics and Nuclear Techniques, University of ining and etallurgy, Al. ickiewicza 30, Kraków ** Faculty of Geology, Geophysics and Environmental Protection, University of ining and etallurgy, Al. ickiewicza 30, Kraków Abstract Results of a quantitative determination of the forced influx of C 4 and CO 2 from the Upper Carboniferous coal-bearing formations to near-surface zone of the Wa³brzych Coal Basin are presented and discussed. easurements were made in 56 areas using a modified static chamber method. Selection of the areas was based upon: (1) anomalous concentration of methane and carbon dioxide in soil gases, (2) fissures, faults, pit eyes and other surface-connected workings, and (3) genetic compatibility of soil gases and coalbed gases at the measurement site. The forced influx of methane into the near-surface zone varies from 0.1 to 620 dm3/m2h, and that of carbon dioxide ranges from 0.8 to 330 dm 3 /m 2 h. Considering the proposed model and scale of gas migration processes, the potential hazard to the residents has been evaluated by forced influx of coalbed gases into the surrounding houses. Overall probability of C 4 influx at the "very high" and "high" hazard categories is about 28%. The hazard caused by CO 2 at the level of "very high" and "high" categories is about 80%. The increased concentrations of C 4 and CO 2 in the soil atmosphere do not commonly overlap which suggests that: (1) the increased concentrations of ether C 4 or CO 2 in the soil atmosphere cause the permanent hazard at the level of "high" category in the study area, and (2) the possibility of uncontrolled influx of gas (gases) into the buildings is categorised as "high" category. Key words: Wa³brzych Coal Sub-basin, methane, carbon dioxide, forced influx, potential hazard, 1. Introduction The closure of hard-coal mines in the Wa³brzych District of lower the Silesian Coal Basin brought not only the end of exploitation but also the termination of ventilation and drainage of mine workings. This, in turn, resulted in some processes, which affected the near-surface zone in Wa³brzych town and vicinity. The principal identified problems are: (1) recovery of groundwater table in the Carboniferous water horizon, (2) surface deformations (Kowalski, red., 2000) and (3) migration of coalbed gases into the near-surface zone. The processes contributing to the secondary migration of coalbed gases into the surface zone have not been explained adequately. In 1995, a working group from the Department of Fossil Fuels, University of ining & etallurgy in Kraków commenced the field geochemical surveys in 14 areas of Wa³brzych and Boguszów-Gorce towns (Dzieniewicz et al., 2002a). The studies were aimed at measuring the concentration of C 4 and CO 2 in.j. Kotarba (Eds.) Gas hazard in the near-surface zone of the Wa³brzych Coal Distric caused by coal mine closure: geological and geochemical controls. ISBN , Copyrights 2002 by Society of Research on Environmental Changes GEOSFERA, Kraków, str

2 Adam KORUS, aciej J. KOTARBA, arek DZIENIEWICZ & enryk SECAN the near surface zone (Kotarba, 2002; Kotarba et al., 2002; Dzieniewicz et al., 2002a). From 1999 to 2000, the group ran measurement to estimate of coalbed gas flux into the near-surface zone. The volume of migrating gases influences both the safety of residents in homes and the concentration of greenhouse gases in the atmosphere. The concentration of C 4 in air in Wa³brzych town during climatic inversion reaches about 5 ppm and that of CO 2 is about 500 ppm (Korus et al., 1999). Such anomalous concentration may indicate a permanent long-term supply of both gases into the atmosphere. This paper discusses methodology of coalbed gases influx summarises the results of the study and evaluates the hazard caused by the gases for residents in the areas affected by uncontrolled gas flux. l = 200 cm sampling valve static chamber d = 1 cm control hole mud seal soil 2. Principles of measurement of coalbed gases influx into the near-surface zone Coalbed gas (C 4 or CO 2 ) influx into the near-surface zone by soil disruption has been measured by a modified static chamber method. This method is used in near-surface geochemical surveying to determine the flow of gas released from the Earth's surface to the atmosphere (eventhal, 1992). Evaluation of the influx includes the three steps: (1) measurement of natural gas emission, (2) simulation of soil disruption, (3) simultaneous measurement of natural gas emission and gas influx into the near-surface zone. The influx of coalbed gases (C 4 or CO 2 ) into the near-surface zone is calculated from the formula (1): D(s) = E n+d - E n (1) D(s) = influx of forced gas migrating into the nearsurface zone, E n = natural gas (C 4 or CO 2 ) emission, E n+d = sum of natural gas (C 4 or CO 2 ) emission and forced influx of gas (C 4 or CO 2 ) into the nearsurface zone. The concept of measuring both the natural gas emission from the soil and the influx of gas into the near-surface zone by soil disruption is illustrated in Fig. 1. First, the chamber of given volume Vp covers area A is positioned at a location where the possible emission of coalbed gas from soil is occurring. 184 Fig. 1. easurement scheme for determining methane and carbon dioxide flux from Upper Carboniferous coal-bearing strata to the near-surface zone Rys. 1. Schemat pomiaru dop³ywu metanu i dwutlenku wêgla do strefy przypowierzchniowej z górnokarboñskich utworów wêglonoœnych After sealing the chamber, air samples are collected in pre-defined time intervals. The samples are then analysed for C 4 and CO 2. The results are displayed as curves, which reflect the changes in gas concentrations in the chamber with collection time. Second, a well 1-centimetre in diameter and 2 meters deep is made at a measurement site where gas emissions simulate soil disruption are anticipated. Finally, on-site measurements are made of natural gas emission and forced influx into the nearsurface zone. Results are displayed in plots of C 4 and CO 2 concentration changes in the chamber as a function of time. Gas emission from the soil is describe by the formula (2), dm 1 E = (2) dt A E = gas emission, dm/dt = increment of gas mass in chamber in time unit, and A = surface area of emission into the chamber. Given dm = dcv p r ( dc = changes of gas concentration in chamber, r = gas density, V p = chamber volume), its substitution into the formula (2) gives the formula (3) dc 1 E = Vp ρ (3) dt A

3 Applying the total gas equation, gas density may be defined by formula (4): W To p ρ = (4) V po T W = molecular weight of gas, V = volume of 1 mole of gas under standard conditions (p o = 1013 hpa, T o = 273 K), T = soil temperature during measurement (K), T = 273 +t gl ( o C), t gl = soil temperature during measurement ( o C), p = atmospheric pressure during measurement (hpa). Substituting (4) into (3) gives formula (5), which describes gas emission from soil. Formula (5) takes into account all equipment (V p, A) and meteorological (p, t gl ) factors at the moment of measurement: dc W To p 1 E = Vp dt V p T A (5) Substituting relevant values for C 4, transforms formula (5) transforms into (6a), = "natural" C 4 emission (gdm -2 min -1 ), o (6a) = ( = dc/dt for t 0), increment of C 4 concentration in the chamber (ppm/min), V p = chamber parameters: volume (dm3 ), and area A (dm 2 ), p = atmospheric pressure during measurement (hpa), t gl = soil temperature during measurement ( o C), W = 16 g, V = 22.4 dm 3, T o = 273 K, p o = 1013 hpa. For CO 2 formula (5) transforms into (6b), E CO Vp p = n n A (273+ t ) (6b) CO2 2 n = "natural" CO 2 emission (gdm -2 min -1 ), = ( = dc/dt for t 0), increment of CO 2 concentration in the chamber (%/min), W = 44 g, remaining values as in formula (6a). gl Simulation of soil disruption was obtained by drilling of a standard well (diameter = 1cm, depth about 200 cm) at the site of natural gas emission, under the procedure similar to that of near-surface geochemical surveying (Dzieniewicz et al., 2002b). Thus, the additional area (s) was obtained (about 630 cm 2 ) which was the source of additional gas inflow into the chamber forced by the disturbance of soil structure. Applying the steps identical with those used for natural gas emission, the sum of natural and forced gas/gases inflow were measured. The relevant formulas transform into (7a) and (7b): (7a) = ( = dc/dt for t 0), increment of C 4 concentration in chamber during simultaneous measurement of natural emission and forced influx of methane (ppm/min). The remaining symbols are describes in formula (6a). (7b) = ( = dc/dt for t 0), increment of CO 2 concentration in chamber during simultaneous measurement of natural emission and forced influx of carbon dioxide (%/min). The remaining symbols are describes in formula (6b). To determine the variability of emission and forced influx into the near-surface zone in a sampling site 3 5, measuring points were selected in the vicinity. For each such point the values of emission En and sum of emission and forced influx D(s) were determined. The value of forced influx to the sampling area is assumed to be the difference between maximum value derived from formula (7a) or (7b) and the minimum value derived from the formula (6a) or (6b). The parameters describing the determination of gas influx into the near-surface zone are defined by formula (8a) for C 4 and (8b) for CO 2 : C4 C C [En+D(s)C ] max. [E n ] min. 4 D(s) = (8a) 4 4 CO2 CO CO [En+D(s)CO ] max. [E n ] min. 2 D(s) = (8b) 2 2 Taking into account the geometry of the simulation well, formulas (8a) and (8b) give the values 185

4 Adam KORUS, aciej J. KOTARBA, arek DZIENIEWICZ & enryk SECAN of gas flux into the near-surface zone from the unit area D(S). 3. Procedure of chamber measurements Chambers (3 dm, diameter; 10.8 dm 3, volume) were positioned at selected measurement sites. After thorough sealing of the contact between chamber edge and soil, air samples were collected at selected time intervals (e.g. 5, 10, 15 and 30 minutes). Samples were then analysed for C 4 and CO 2 concentrations. These results allowed the determination of the dependence between the increment of C 4 and CO 2 concentrations, and the time measured since the beginning of sampling. easurements of natural (chamber) and disruptionforced influx (chamber + probe) expressed as the increment of gas concentration in the chamber are given in Tab. 1. Values of atmospheric pressure and soil temperatures are listed in Tab Analytical procedure The C 4 and CO 2 samples collected in the chamber were extracted with gas-tight syringes and analysed with a gas chromatograph equipped with flame ionisation detector, thermal conductivity detector, and Carboxen 1000 analytical column. Precision of the measurement of forced C 4 or CO 2 influx from Upper Carboniferous coal-bearing strata to the near-surface zone was estimated to be 10%. 5. Selection of measurement places The end of coal exploitation in 1998 along with the cessation of ventilation and drainage of mines in the Wa³brzych District resulted in the recovery of the Carboniferous ground water table. This rise in the groundwater table caused the migration of significant amounts of coalbed gas to the surface (Kotarba et al., 2002). This migration is facilitated by the system of fractures related to tectonic zones Table 1. Variation of carbon dioxide and methane concentration in static chamber in A-6 site (P-5 area, No. VII line, No. 171 point) Tabela 1. Zmiany stê enia dwutlenku wêgla i metanu w komorze statycznej w rejonie A-6 (obszar P-5, profil VII, punkt nr 171) Chamber Chamber + Probe Time ocation of chamber ocation of chamber (min.) CO 2 vol.% vol.% C 4 vol.% ppm ppm vol.% ppm vol.% ppm ppm vol.% ppm ,000 ppm = 1% Table 2. Atmospheric pressure and soil temperature during measurements in A-6 site Tabela 1. Ciœnienie atmosferyczne i temperatura gleby w czasie pomiaru w rejonie A-6 Atmospheric pressure ocation of chamber Soil temperature ocation of chamber hpa hpa hpa hpa hpa o C o C o C o C o C

5 and magmatic intrusions, as well as numerous shafts, fore-shafts and other surface-connected workings. Fissures and fractures resulting from the subsidence of caprocks over the worked-out seams intensified this migration. During detailed surficial geochemical surveying was completed in the urban areas of Wa³brzych and Boguszów-Gorce over an area of about 9 km 2 (Dzieniewicz et al., 2002a). The measurements were made along 24 lines with cumulative length of about 30 kilometres (Dzieniewicz et al., 2002b). Concentration of carbon dioxide and methane were measured at about 4,500 sites. The results indicated CO 2 concentrations greater than 2% in soil atmosphere at 151 sites (maximum value 17.4%) and more than 1% of methane at 18 sites (maximum value 49.6%). During years and the Department of Fossil Fuels, Faculty of Geology, Geophysics & Environment Protection of the University of ining & etallurgy in Kraków ran detailed geochemical surveys for the purpose of determining the origin of coalbed gases in the Wa³brzych District of the ower Silesian Coal Basin (Kotarba, 1988; 1990a; 1990b; 1990c; Kotarba and Rice 2002). The studies included detailed analyses of molecular and isotopic composition of 62 gas samples collected from hard-coal seams. During , about 200 samples taken at the surficial sites of maximum gas concentrations (Dzieniewicz et al., 2002b) were analysed for stable carbon isotopes in C 4 and CO 2 (Kotarba & Korus, 2002). Comparison of isotopic and molecular compositions of coalbed and soil gases allowed for the genetic identification of methane and carbon dioxide in the soil atmosphere (Kotarba & Korus, 2002). The selection of measurement sites for natural emission and forced influx of coalbed gases was determined by at least two of the following criteria: 1. Occurrence of anomalous methane and carbon dioxide concentrations in subsoil atmosphere; 2. Presence of fissures, faults, shafts, fore-shafts and other surface-connected workings in the vicinity; 3. Genetic link between soil and B coalbed gases in the vicinity. 6. Evaluation of the coalbed gases influx into the near-surface zone in the selected areas in Wa³brzych and Boguszów- Gorce towns Based upon the above criteria, 56 measurement sites were selected in the area of Wa³brzych and Boguszów-Gorce. At these sites, coalbed gas emission and influx were measured in the years ocalities of the sites are given in Appendix 1 of Kotarba (2002). The measurement procedure is presented for the A-6 area (No. 171 site, Profile VII, Area P-5, Fig. 2). At this site, cyclic measurement sessions were undertaken (Dzieniewicz et al., 2002c). During all the seasonal sessions, increased C 4 and CO 2 concentrations were detected. oreover, increased concentrations of both coalbed gases were found A-6 441/ (m) A A Fig. 2. Sketch map of location of A-6 site. 1 - boundary of P-5 area; 2 - subcrops of coal seams on top of Upper Carboniferous strata and number of seam; 3 - faults; 4 - geological cross-sections (cf. Fig. 3); 5 - points of chamber location Rys. 2. apa lokalizacji rejonu A granica obszaru P-5; 2 - wychodnie pok³adów wêgla na stropie utworów górnokarboñskich i numer pok³ad; 3 - uskoki; 4 - przekroje geologiczne (patrz Fig. 3); 5 - punkty lokalizacji komory 441/ /437 B B 436 P-5 AREA 1 0 A (m) A 2 428/ /429 B 2 187

6 Adam KORUS, aciej J. KOTARBA, arek DZIENIEWICZ & enryk SECAN A A-6 A NW SE Žacle ø formation (m n.p.m.) B B SW A-6 SE Žacle ø formation 4 02 Žacle ø formation (m n.p.m.) Fig. 3. Geological cross-sections of A-6 site. 1 - boundary between Upper Carboniferous formations and Quaternary sediments; 2 - boundaries between various Upper Carboniferous formations; 3 - observed coal seam; 4 - inferred coal seam; 5 - faults; 6 - points of chamber location Rys. 3. Przekroje geologiczne rejonu A granica pomiêdzy utworami górnokarboñskimi i osadami czwartorzêdowymi; 2 - granice pomiêdzy poszczególnymi formacjami górnego karbonu; 3 - stwierdzony pok³ad wêgla; 4 - przypuszczalny pok³ad wêgla; 5 - uskoki; 6 - punkty lokalizacji komory in the Carboniferous rocks at a depth of about 4 meters. In the vicinity of site, the surface is covered by Quaternary sediments, about 3 meters thick, composed of weathered crust of claystones in the lower part of the sequence and made land in the upper part. These sediments are underlain by the Carboniferous ower Žacleø Formation, which dip 20 to the east. Beneath the site, hard coal has been mined from No., and seams. A fault system exists close to the site with throws from 5 to 10 meters (Fig. 3). Results of molecular and stable carbon isotope analyses of near-surface gases sampled at No. 171 site indicate that both gases are genetically linked to the migrating coalbed gases (Kotarba & Korus, 2002). In the A-6 area 5 measuring sites were established: central (0) precisely at the selected site and four others (1-4) at the distance of about 5 meters, from the central site along the two mutually perpendicular lines. This positioning was aimed at measuring both emission and influx at the selected site and in the immediate area, to control the local variability of measured values at the sampling site. ocation of sites in the A-6 area is shown in Figs. 2 and 3. Changes in concentrations of C 4 and CO 2 for "chamber" (i.e. natural) and "chamber + probe" (i.e. natural + disturbed) systems, as well as equations describing their changes with time and locality within the A-6 area are presented in Figs. 4 and 5. Relevant functions are second and third degree polynomials in which the numeric coefficients were determined by regression analysis. It was assumed that the fitting coefficient R 2 of a function should be higher than Equations describing the minimum and maximum rates of change in concentrations of respective gases are shown in Figs. 4 and 5. These equations were applied to determination of the following parameters: = dc/dt for t 0 (minimum increment rate of C 4 concentration), = dc/dt for t 0 (maximum increment rate of C 4 concentration), = dc/dt for t 0 (minimum increment rate of 188

7 CABER Point 0 Point 1, 2 and 4 Point C(t) = t t t (R = 1.00) C(t) = 4t t t (R = 0.96) A CABER Point 0 Point 1, 2 and 4 Point C(t) = 3E-0.5t-t t (R = 1.00) 2 2 C(t) = -3E-0.6t + t (R = 0.96) A 6000 CABER & PROBE Point 0 Point 1, 2 i 4 Point CABER & PROBE Point 0 Point 1, 2 i 4 Point Fig. 4. Concentration of methane in static chamber in A-6 site: (A) measurement of natural emission, and (B) measurement of natural emission and forced influx of methane Rys. 4. Stê enie metanu w komorze w rejonie A-6. (A) - pomiar naturalnej emisji; (B) - pomiar naturalnej emisji i wymuszonego dop³ywu metanu CO 2 concentration), = dc/dt for t 0 (maximum increment rate of CO 2 concentration). The measurements for A-1 to A-20 areas were run in 1999, and A-21 to A-56 areas in Increments of C 4 and CO 2 concentrations are listed in Tab. 3. Values of natural emission and forced influx of C 4 and CO 2 are presented in Tabs 4 and 5. In the A-6 area, maximum values of both the emission and the forced influx of methane and carbon dioxide were detected. Estimated methane influx was 508 dm 3 /m 2 h (Tab. 4) and that of carbon dioxide was 330 dm 3 /m 2 h (Tab. 5). Both values indicate a very high hazard potential caused by migration of both gases into adjacent buildings. Intensive migration of C 4 and CO 2 into the nearsurface zone resulted from a fault-related fissure system and shallow exploitation of coal seams. In the other area (A-26), the estimated influx values were: 620 dm 3 /m 2 h for methane (Tab. 4) and 181 dm 3 /m 2 h for carbon dioxide (Tab. 5). These values also indicated a significant hazard caused by migration of both gases into the adjacent buildings. Examples of low hazards influx values should also be presented. Fig. 5. Concentration of carbon dioxide in static chamber in A-6 site: (A) measurement of natural emission, and (B) measurement of natural emission and forced influx of carbon dioxide Rys. 5. Stê enie dwutlenku wêgla w komorze w rejonie A-6. (A) - pomiar naturalnej emisji; (B) - pomiar naturalnej emisji i wymuszonego dop³ywu ditlenku wêgla 7. Evaluation of potential hazard for buildings caused by forced influx of coalbed gases The values of uncontrolled gas migration vary from 0.1 to 620 dm 3 /m 2 h for C 4 and from 0.8 to 330 dm 3 /m 2 h for CO 2. Such high influx into the basements of buildings may be lethal to their residents. Based on theoretical considerations and calculations of the influx of migrating coalbed gases (D(S) parameter see Tabs. 4 and 5), the potential hazard caused by migration of C 4 and CO 2 into the basements may be divided into 4 categories: potential hazard very high (V) D(S) ³ 5 dm 3 /m 2 h potential hazard high () 5 dm 3 /m 2 h ³ D(S) ³ 1 dm 3 /m 2 h potential hazard medium () 1 dm 3 /m 2 h ³ D(S) ³ 0.1 dm 3 /m 2 h potential hazard low () D(S) < 0.1 dm 3 /m 2 h The calculations are based on standard room conditions, 100 m 2 areas and 2 m height. For a high potential hazard category, at D(S) = 1 dm 3 /m 2 h, the gas inflow into such room should reach about 4,800 dm3 (4.8 m 3 ) per 24 hours. Therefore, assuming homogenic gas-mixing conditions and lack of ventilation in a basement, gas concentrations (C 4 or 189

8 Adam KORUS, aciej J. KOTARBA, arek DZIENIEWICZ & enryk SECAN Table 3. inimal and maximal values of increases of methane and carbon dioxide concentration in static chamber Tabela 3. inimalne i maksymalne wartoœci przyrostu stê enia metanu i dwutlenku wêgla w komorze statycznej Place A-1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 A-10 A-11 A-12 A-13 A-14 A-15 A-16 A-17 A-18 A-19 A-20 A-21 ethane Carbon dioxide chamber chamber and probe chamber chamber and probe n min n max n min n max n min n max n min n max ppm/min. ppm/min. ppm/min. ppm/min. %/min. %/min. %/min. %/min , , A A A A A , A A A A A A A-33 A A A A A A A A A A A A A A A A A A A A A A A For abbreviations see the text Skróty w tekœcie 190

9 Table 4. Values of natural emission and forced influx for methane Tabela 4. Wartoœci naturalnej emisji i wymuszonego dop³ywu metanu Site A-1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 A-10 A-11 A-12 A-13 A-14 A-15 A-16 A-17 A-18 A-19 A-20 A-21 E (ch+p)max. E ch(min) E (ch+p)max. - E p(min) D(S) dm 3 /m 2 h dm 3 /m 2 h dm 3 /m 2 h dm 3 /m 2 h Potential hazard symbol A A V A A A V A V A A A A A A A A A A A A A A A V A V A A A A A * A A A V A A A A A V V For abbreviations see the text Skróty w tekœcie 191

10 Adam KORUS, aciej J. KOTARBA, arek DZIENIEWICZ & enryk SECAN Table 5. Values of natural emission and forced influx for carbon dioxide Tabela 5. Wartoœci naturalnej emisji i wymuszonego dop³ywu dwutlenku wêgla Site A-1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 A-10 A-11 A-12 A-13 A-14 A-15 A-16 A-17 A-18 A-19 A-20 A-21 E (ch+p)max. E ch(min) E (ch+p)max. - E ch(min) D(S) dm 3 /m 2 h dm 3 /m 2 h dm 3 /m 2 h dm 3 /m 2 h Potential hazard symbol V V V V V V V A V A V A V A V A V A V A A A A A A V A A A A A A A A A V A A V A A V A V A A A A V A A A V A A For abbreviations see the text Skróty w tekœcie CO 2 ) may reach about 2.4%. For a very high potential hazard conditions at D(S) = 10 dm 3 /m 2 h, gas flux into such a room should be about 24,000 dm 3 /m 2 h (24 m 3 ). Assuming homogenic gas-mixing conditions and lack of ventilation in the basement, gas concentrations (C 4 or CO 2 ) may reach about 12% (!!!). Such a value for methane falls into the explosive concentration range (5 to 15%), and for carbon dioxide it can caused suffocation of humans. 192

11 Number of potential hazard C 4 CO very high high medium low Potential hazard Fig. 6. Number of potential hazard by forced influx of methane and carbon dioxide to near-surface zone Rys. 6. iczba miejsc potencjalnie zagro onych wymuszonym dop³ywem metanu i dwutlenku wêgla do strefy przypowierzchniowej C 4 CO Probability of hazard (%) very high high medium low Potential hazard Fig. 7. Probability of potential hazard by forced influx of methane and carbon dioxide to near-surface zone Rys. 7. Prawdopodobieñstwo wyst¹pienia potencjalnego zagro enia wymuszonego dop³ywu metanu i dwutlenku wêgla do strefy przypowierzchniowej 1.8 Generally, in Wa³brzych and Boguszów-Gorce, the flux of coalbed gases into the near-surface zone was estimated for 56 areas. In the years , 8 areas have a very high (V) C 4 hazard level and 8 areas have a high () C 4 hazard level. For CO 2 20 areas have a very high (V) hazard level and 25 areas with a high () hazard level. Frequency of hazard is illustrated in Fig Conclusions easurements completed at 56 sites support the relationship between increased concentrations of C 4 and CO 2 in the soil atmosphere and the gas hazard caused by uncontrolled migration of both gases into the basements of buildings. The influx might have been caused by soil disruption or by fracturing of foundations in the buildings. The probability of the occurrence of gas hazard caused by gas migration into the near-surface zone in Wa³brzych and Boguszów-Gorce is displayed in Fig. 7. Cumulative probability of C 4 influx at V and levels is about 28% whereas that of CO 2 at the same V and levels is about 80%. Because the increased concentrations of C 4 and CO 2 in the soil atmosphere do not always occur simultaneously, it increased concentration of C 4 or CO 2 cause permanent gas hazard affects the buildings in the studied areas. Based on surficial geochemical surveys (Dzieniewicz et al., 2002a) and the measurements of gas influx into the near-surface zone (Kotarba et al., 2002), principles of hazard control in build- 193

12 Adam KORUS, aciej J. KOTARBA, arek DZIENIEWICZ & enryk SECAN ings located in the areas of increased concentrations of C 4 and CO 2 in soil atmosphere may be initiated. References DZIENIEWICZ., SECAN., KOTARBA.J. & KORUS A. 2002a - Surface geochemical surveying of methane and carbon dioxide in the selected areas of the Wa³brzych Coal District. In: Kotarba.J. (Ed.) Gas hazard in the near-surface zone of the Wa³brzych Coal District caused by coal mine closure: geological and geochemical controls (this volume). DZIENIEWICZ., SECAN., KOTARBA.J. & KORUS A. 2002b - Distribution of methane and carbon dioxide contents in the near-surface zone along the selected geological cross-sections of the Wa³brzych Coal District. In: Kotarba.J. (Ed.) Gas hazard in the nearsurface zone of the Wa³brzych Coal District caused by coal mine closure: geological and geochemical controls (this volume). DZIENIEWICZ., SECAN., KOTARBA.J. & KORUS A. 2002c - Periodical changes of methane and carbon dioxide contents in the near-surface zone along the selected four geological cross-sections of the Wa³brzych Coal District. In: Kotarba.J. (Ed.) Gas hazard in the near-surface zone of the Wa³brzych Coal District caused by coal mine closure: geological and geochemical controls (this volume). KORUS A., NÊCKI J. & ASA J Rozk³ad przestrzenno-czasowy stê enia i sk³adu izotopowego metanu atmosferycznego w rejonie Polski Po³udniowej. W: Buszewski B. (Red.) Chromatografia i inne techniki separacyjne u progu XXI wieku. Toruñ 1999, ISBN : 430. KOTARBA.J Post-mining gas hazards in the nearsurface zones of hard-coal basins: purposes of geochemical study in Wa³brzych basin. In: Kotarba.J. (Ed.) Gas hazard in the near-surface zone of the Wa³brzych Coal District caused by coal mine closure: geological and geochemical controls (this volume). KOTARBA., 1990a - Geneza gazów akumulowanych w górnokarboñskiej serii wêglonoœnej Dolnoœl¹skiego Zag³êbia Wêglowego i po³udniowej czêœci Rybnickiego Okrêgu Wêglowego. W: itwiniszyn J. (Red.) Górotwór jako oœrodek wielofazowy. PAN, Kraków: KOTARBA. 1990b - Geneza gazów akumulowanych w formacji wa³brzyskiej górnego karbonu w polu centralnym KWK "Thorez" w œwietle badañ izotopów trwa³ych. W: itwiniszyn J. (Red.) Górotwór jako oœrodek wielofazowy. PAN, Kraków: KOTARBA. 1990c - Isotopic geochemistry and habitat of the natural gases from the Upper Carboniferous Žacleø coal-bearing formation in the Nowa Ruda coal district (ower Silesia). Org. Geochem., 16, KOTARBA Geochemiczne kryteria genezy gazów akumulowanych w serii wêglonoœnej górnego karbonu niecki Wa³brzyskiej. Zeszyty Naukowe AG, Geologia nr 42, 119 stron. KOTARBA.J. & KORUS A Origin of coal-bed and near-surface gases in Wa³brzych region. In: Kotarba.J. (Ed.) Gas hazard in the near-surface zone of the Wa³brzych Coal District caused by coal mine closure: geological and geochemical controls (this volume). KOTARBA.J. & RICE D.D Composition and origin of coalbed gases in the ower Silesian basin, southwest Poland. Applied Geochemistry, v. 16: KOTARBA.J., DZIENIEWICZ., SECAN., KORUS A., KOINOWSKI K., GOGOEWSKA A., P ONKA A. & WINNICKI A. 2002a - echanism of coalbed gas flux and gas hazard in the near-surfaces zone of the Wa³brzych Coal district - prognoses and monitoring. In: Kotarba.J. (Ed.) Gas hazard in the near-surface zone of the Wa³brzych Coal District caused by coal mine closure: geological and geochemical controls (this volume.) KOTARBA.J., DZIENIEWICZ., KORUS A., SECAN., KOINOWSKI K, GOGOEWSKA A. & GRZYBEK I echanism of coalbed gas flux into the near-surface zone of the Wa³brzych Coal district. In: Kotarba.J. (Ed.) Gas hazard in the nearsurface zone of the Wa³brzych Coal District caused by coal mine closure: geological and geochemical controls (this volume). KOWASKI A., red Eksploatacja górnicza a ochrona powierzchni. Doœwiadczenia z wa³brzyskich kopalñ. Wydawnictwo GIG, Katowice. ISBN EVENTA J odern mobile methane measurement in marshes. United States Geological Survey, Denver, Open-File Report 92-, Streszczenie ikwidacja kopalñ wêgla kamiennego w Okrêgu Wa³brzyskim Dolnoœl¹skiego Zag³êbia Wêglowego spowodowa³a szereg zjawisk, które maj¹ niekorzystny wp³yw na œrodowisko naturalne na powierzchni w obrêbie miasta Wa³brzycha i okolic. Do tych niekorzystnych zjawisk nale y zaliczyæ: rekonstrukcje zwierciad³a wód karboñskiego piêtra wodonoœnego, deformacje powierzchni terenów górniczych, migracje wêglowych gazów z³o owych do strefy przypowierzchniowej. W 1995 r. w rejonie Wa³brzycha i Boguszowa-Gorc rozpoczêto powierzchniowe badania geochemiczne, których celem by³o okreœlenie stê enia metanu i dwutlenku wêgla w warstwach przypowierzchniowych. Kontynuuj¹c te prace w latach przeprowadzono ocenê wielkoœci strumienia gazów z³o owych dop³ywaj¹cych i przemieszczaj¹cych siê w strefie przypowierzchniowej w wyniku przerwania ci¹g³oœci struktury gleby. Iloœæ gazów z³o owych dop³ywaj¹ca do powierzchni mo e wp³ywaæ na bezpieczeñstwo ludzi przebywaj¹cych w budynkach usytuowanych w rejonach zagro onych dop³ywem gazów z³o owych oraz na stê enie C 4 i CO 2 w atmosferze. Ocenê wielkoœci strumienia gazów z³o owych dop³ywaj¹cych do 194

13 strefy przypowierzchniowej wykonano za pomoc¹ zmodyfikowanej metody komór statycznych, której idea jest przedstawiona na Rys. 1. Szczegó³owe pomiary wykonano w 56 rejonach (A-1 do A-56). O wyborze rejonu do pomiaru emisji gazów i wymuszonego dop³ywu strumienia gazów z³o owych decydowa³y nastêpuj¹ce warunki: wystêpowanie anomalnych stê eñ metanu lub dwutlenku wêgla w gazach podglebowych, wystêpowanie szczelin, uskoków, wylotów szybów, szybików i wyrobisk maj¹cych po³¹czenia z powierzchni¹ w rejonie pomiaru, genetyczna zgodnoœæ gazów podglebowych i gazów z³o owych w rejonie pomiaru. Dla ka dego rejonu spe³nione by³y co najmniej dwa z tych warunków. Szczegó³owy sposób wykonania pomiaru strumienia gazów z³o owych dop³ywaj¹cych do strefy przypowierzchniowej zosta³ przedstawiony na przyk³adzie rejonu A-6. Rozmieszczenie komór w trakcie eksperymentu przedstawia Rys. 1, a warunki geologiczne w tym rejonie Rys. 1. Z komór w okreœlonych odstêpach czasu pobierano próbki atmosfery, w których metod¹ chromatografii gazowej oznaczano stê enia C 4 i CO 2. Po przeprowadzeniu symulacji przerwania ci¹g³oœci gleby ponownie pobierano próbki atmosfery i wykonywano oznaczanie stê eñ C 4 i CO 2. Zmiany stê eñ C 4 i CO 2 w komorach w trakcie realizacji eksperymentu zestawiono w Tab. 1. Wartoœci ciœnienia atmosferycznego i temperatury gleby podano w Tab. 2. Na podstawie wyników analiz wykonywane s¹ krzywe opisuj¹ce zmiany stê eñ C 4 (Rys. 4) i CO 2 (Rys. 5) w komorze w zale noœci od czasu trwania pomiaru. Pos³u y³y one do wyznaczenia minimalnych i maksymalnych wartoœci narostu stê eñ C 4 i CO 2. Otrzymane w trakcie badañ wartoœci zestawiono w Tab. 3.. Obliczone wartoœci naturalnej emisji C 4 i CO 2 oraz ich wymuszonego dop³ywu do strefy przypowierzchniowej podane s¹ odpowiednio w Tab. 4 i Tab. 5. Wartoœci te wynosz¹ odpowiednio dla C do 620 dm 3 /m 2 h i dla CO do 330 dm 3 /m 2 h. Na podstawie badañ oceniono równie potencjalne zagro enia dla ludzi wymuszonym dop³ywem gazów z³o owych do budynków zlokalizowanych w rejonach, gdzie dop³yw jest mo liwy (Tab. 4 i Tab. 5). Ocenê wykonano wed³ug zaproponowanej skali zagro eñ, której podstawê stanowi wielkoœæ strumienia dop³ywaj¹cych do budynku gazów z³o owych D(S) i skutki dla ludzi w nich przebywaj¹cych. Przyjêto: potencjalne zagro enie - bardzo du e (V) D(S) ³ 5 dm 3 /m 2 h potencjalne zagro enie - du e () 5 dm 3 /m 2 h ³ D(S) ³ 1 dm 3 /m 2 h potencjalne zagro enie - œrednie () 1 dm 3 /m 2 h ³ D(S) ³ 0.1 dm 3 /m 2 h potencjalne zagro enie - niewielkie () D(S) < 0.1 dm 3 /m 2 h Czêstoœæ wystêpowania zagro eñ wymuszonym dop³ywem gazu/gazów przedstawiono na Rys. 6. Prawdopodobieñstwo wyst¹pienia zagro enia niekontrolowanym dop³ywem jednego gazu lub obydwu gazów do strefy przypowierzchniowej przedstawiono na Rys. 7. ¹czne prawdopodobieñstwo zagro enia dop³ywem C 4 na poziomie "bardzo du e" (V) i "du e" () wynosi ok. 28%, natomiast zagro enie dop³ywem CO 2 na poziomie "bardzo du e" (V) i "du e" () ok. 80%. 195