Field Research on Thermal Anomalies Indicating Sulflde-Oxidation

Chapter 39

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Field Research on Thermal Anomalies Indicating Sulflde-Oxidation Reactions in Mine Spoil Weixing Guo and Richard R. Parizek Department of Geosciences, Pennsylvania State University, University Park, PA 16802 Field investigations were conducted at two reclaimed surface coal mining sites in western Pennsylvania. A number of deep holes that penetrated to the mine floor and shallow holes 0.3 to 0.6 m deep were installed on transects containing the deep holes. Temperature within the mine spoil and at ground surface was measured during the last three years. Spoil temperature was 1.5 to 2.2°C higher than the background temperature, which was measured in a deep hole outside mine spoil, over the field investigation period. Thermal anomalies were detected near buried coal refuse in mine spoil, which was confirmed by using geophysical methods and test drilling. Application of sewage sludge showed an effect in suppressing spoil temperature. The intensity of thermal anomalies was found to decrease with time at an average rate -0.1C/year. This may be the result of gradual depletion of pyrite in mine spoil. Acid mine drainage (AMD), a well known water pollution problem in some coal and metal mining districts, results from the oxidation of sulfide minerals. Through a series of reactions in which pyrite is oxidized, hydrogen ions and sulfate are produced. Highly acidic mine waters can leach heavy metals from spoil into surface and subsurface waters. Soil erosion also can be excessive where acidic waters have come into contact with and killed vegetation. Field study of spoil temperature is of importance in the practice of mined-land reclamation. Temperature can be measured comparatively easily and accurately in the field using simple equipment. Minor heat differences resulting from sulfide-oxidation reactions can be detected because mine spoil is a poor medium for heat transfer. The perturbation of mine spoil temperature due to the exothermic pyrite oxidation reactions has been known for a long time (1,2). The ability to determine the location of chemically active pyrite-concentrated materials is very desirable in the practice of mined-land reclamation. Financial limitations often do not allow one to treat all disturbed areas in an attempt to abate 0097-6156/94/0550-0645$06.00A)

© 1994 American Chemical Society Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


646

ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION

acid mine drainage or to slow its production. Geophysical methods have been applied to locate the sites of coal refuse, a highly concentrated pyritic material (3,4). However, it is more important to locate the chemically active coal refuse, because the presence of pyritic material does not necessarily generate acid mine drainage unless other conditions are favorable. Thermal anomaly investigation has the advantage in providing the information on the locations of active pyrite-concentrated zones in mine spoil, which is a primary concern in land reclamation and acid mine drainage control. Long-term spoil-temperature monitoring may reveal the intensity changes of pyrite-oxidation reactions and AMD generation from mine spoil. The effect of abatement measures can be evaluated by monitoring the intensity changes of the thermal anomalies in mine spoil. Spoil temperature should decrease with time as the result of pyrite depletion, even if no abatement measures are taken. Spoil temperature should be further suppressed if AMD generation is under control. Long-term temperature observation is required to test this hypothesis, but no such published efforts are known. Most field temperature observations reportedly lasted for only one year. Non-uniform temperature distribution in spoil may also induce air circulation which supplies oxygen for pyrite oxidation. The amount of oxygen supplied through convection may be much greater than that supplied by diffusion together with that dissolved in infiltrating water. Field investigation of the distribution of thermal anomalies in spoil will help us understand the mechanisms of oxygen transport in mine spoil. The primary objectives of this research were to locate hot spots in mine spoil and to characterize the sulfide-oxidation intensity changes with time using thermalsurveying methods. Hot spots are referred to as the locations where the oxidation reactions are taking place at a significant rate and where temperature is higher than the normal or background temperature due to the heat released from pyrite-oxidation reactions. Both near-surface and deep temperature surveys were conducted to determine if air, surface or near-surface surveys could be used for this purpose and if so, during what season of the year deeper temperature anomalies might be detected at or near land surface. Methods and Materials Two sites were selected for this study (Figure 1). Site 1 is located near Clearfield, Pennsylvania. It was mined by surface methods from 1968 to 1982. The lower Kittanning coals were mined to a depth of nearly 33 m. An older portion of the mine, mainly in the west of the site, was abandoned 20 years ago. This part of spoil was not treated with sewage sludge and was covered by only sparse evergreen trees (no sludge area). The central part of disturbed land was treated with sludge in 1986 (old sludge area). In the summers of 1988 and 1989, sewage sludge, mixed with wood chips and lime, was spread over the east part of the mine site (new sludge area). These sludgetreated areas were covered by dense grass. Several seeps were identified as the drainage from the site. The water quality of the seepage remains poor long after all mining activity has ceased (Table I).

Alpers and Blowes; Environmental Geochemistry of Sulfide Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

39. GUO & PARIZEK

Sulfide-Oxidation Reactions in Mine Spoil

647


Table L Seep-water Chemistry at Site 1 (February, 1990) [Data from C. Cravotta, U.S. Geological Survey, written commun.; all data in mg/L except pH (std. units) ] 2

2+

pH (lab)

S0 "

Ca

3.4

673

99.3

4

Mg

2+

Cu

93.5

2+

0.093

Fe

2+

0.47

Fe (total)

Alkalinity (total)

0.58

0.0

During the summer of 1989,15 deep holes with a 15.24 cm open diameter, were drilled (Figure 2). One piezometer (1.27 cm internal diameter (ID), polyvinyl chloride (PVC) pipe) was installed in each deep hole screened at the bottom 0.33 m. Piezometers provided access for temperature probes and were used to determine the presence of saturated spoil and the elevation of the water table. The size of the piezometers was chosen to limit the heat convection through the pipes. One additional deep hole, the Control Hole, was drilled about 600 m outside of the mining-disturbed area. It was drilled through the undisturbed coal-bearing formations to a depth of 26.4 m, below the lower Kittanning coal seams that were mined. The Control Hole contained a PVC piezometer 10.16 cm in diameter with a 3.05 m long screen at its bottom. The background temperature was measured in this Control Hole. The site contains 130 shallow holes (Figure 2). In each shallow hole, two short PVC pipes (1.27 cm ID) were installed, 0.3 and 0.6 m in length respectively. The bottom ends of these short pipes were left open. Shallow holes were aligned along five transect lines distributed over the site. The spacing between two shallow holes was approximately 15 m. The effects of thermal anomalies on the temperature near the ground surface were investigated by measuring the temperature in these shallow holes. Fine sand, bentonite and cuttings were used to backfill the boreholes. All of the deep and shallow holes were capped to prevent heat convection through the pipes and the entry of rain or snow. Site 2 is located in Clarion, Pennsylvania. Prior to 1968, the lower and upper Clarion and the lower Kittanning coals were mined from a portion of the site. During 1972 and 1973, the site was mined more extensively for the upper and lower Clarion coals. In this site, 20 deep holes and 140 shallow holes were established in the same manner as described above (5). Digital thermometers, together with PSI-400 temperature probes were used for quick temperature measurement. The precision of these thermometers is ± 0.15°C and the minimum reading unit is 0.05°C. Digital thermometers also helped to reduce the delay time between inserting the sensors and taking the readings. Spoil temperature at shallow depths is strongly affected by diurnal changes of ambient temperature. Shallow-hole temperatures were measured each day in the very early morning. Three groups of students worked at the same time to minimize the time difference, which may cause serious errors in shallow temperature measurements. Τ Γ Π Γ ' Τ Ϊ

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Chemin! litre??

^ it N. *v v . B r. rush



648


Figure 1. Location of the field sites.

John A. Thompson's Strip Coal Mine (Clearfield, Pennsylvania)

Control Hole

NORTH West Line

Central line North Line ,'Τ-14

ÎT-12

' • T-13 Τ - 9 . - · South Line T-15

5

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·•

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,

I? Seeps

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• Shallow Holes

et .

-ο

•

Τ-4

Τ-5

East Line

0

Deep Holes

0

T-l

100

(m)

AWeir

Figure 2. Location of monitoring wells at Site 1.


39. GUO & PARIZEK


649


Results and Discussion The temperature of spoil reflects its energy balance. The input energy includes solar radiation, geothermal conduction, chemical reactions and decomposition of organic materials, as well as the energy transferred by groundwater and spoil gas. Solar radiation and chemical reactions are the principal energy sources while the contributions of the other heat sources are relatively small under the conditions investigated. Heat radiation from spoil surface to the atmosphere is the major process dissipating energy. Some losses also are associated with mine-water seeps that drain the spoil. Seasonal Variation and Amplitude Attenuation. The temperature data measured in spoil revealed that spoil temperature is chiefly controlled by the solar radiation at the ground surface. The monthly average temperature at the ground surface, observed at Philipsburg Weather Station, 15 km from Site 1, from September, 1989 to May, 1992 is shown in Figure 3. The temperature data observed in selected deep holes show strong seasonal variations (Figure 4), which have a similar pattern to that shown in Figure 3. These data plots look like sine or cosine curves similar to those observed in a natural soil profile. Data presented in Figure 4 imply that solar radiation was the most important energy source in the spoil thermal regime. Heat from other sources was not strong enough to distort these sine-like curves. The amplitude of seasonal temperature variation tended to decrease with increasing depth, as observed in natural soil profiles not influenced by in-situ heat production (6,7). Thermal Anomalies Induced from Oxidation Reactions. Thermal-surveying data clearly show that spoil temperature was higher than the background temperature, which was measured in the Control Hole. Figure 5a shows the temperature values at various depths measured in both mine spoil and in the Control Hole. A similar phenomenon was also observed at Site 2 (Figure 5b). These diagrams are only selected examples that show this general tendency. The average temperature difference between the spoil holes and the Control Hole was about 1.5 to 2.2°C, and varied with time, depth and location within the mine spoil. Based upon field measurements over the period of observation, thermal anomalies were identified near T-3 at Site 1, TO-2, TO-7, TO-9 and TO-19 at Site 2. Spoil temperature at these locations was substantially higher than the background temperature. All of the thermal anomalies are not generated by pyrite oxidation reactions. Thermal anomalies observed in mine spoil may be induced by a number of causes. To locate active pyrite oxidation reaction zones, the thermal anomalies caused by processes other than pyrite oxidation reactions must be excluded. (i) Heterogeneity of spoil physical properties: Various spoil and bedrock materials have different physical properties, such as thermal diffusivity, water content, etc. Spoil with lower thermal-conductivity values may cause temporary local thermal anomalies due to a time lag effect. If there is no heat source in the spoil, the temperature difference between this part of spoil and its surrounding areas should change with the seasons. When the entire spoil begins to warm up, the temperature within this part will be lower than its surrounding temperature and vice versa. An appearance of



650


Time

(Months)

Figure 3. Monthly average temperature (September, 1989 to May, 1992) recorded at Philipsburg Weather Station, PA.


39.

GUO & PARIZEK


1989

1990

1991

651

1992

25 C-3.05 (m) C-6.10(m) C-9.15(m)


20 Η

151

ίο Η ε

—τ—·—ι—

5

1 — — ι —r~— —ι

1

1

10

15

Time

1

r

1

20

25

30

35

(Months)

1992

T3-3.05 (m) T3-6.10 (m) T3-9.15 (m) 1

10

1

1

I

15

Time

1

1

20

I

25

•

I

30

r-

35

(Months)

Figure 4. Temperature measured in (a) the Control Hole and (b) T-3 at depths 3.05 m, 6.10 m and 9.15 m. The first month is September, 1989.




652



39.

GUO & PARIZEK

Sulfide-Oxidation Reactions in Mine Spoil 653

thermal anomalies all year long, as observed at both sites, indicates that these thermal anomalies were not caused by the heterogeneity of spoil physical properties. (ii) Groundwater flow: Many studies have proven that groundwater may transfer heat, causing temperature differences within aquifers (8). However, this is not the principal cause of temperature variations noted in this study. The groundwater temperature was observed to be almost uniform in the region. The water table was detected at T-10 only, which is located in a topographic low area, while all the other deep holes drilled in the spoil at Site 1 were dry during the entire investigation period. Groundwater temperatures, measured in T-10 and the Control Hole, were relatively stable and lower than the spoil temperature all the time. (hi) Heat production from oxidation reactions: hi spite of the fact that reactive pyrite is contained in the undisturbed rock sequence penetrated by the Control Hole, the oxidation reactions would not start or would proceed only very slowly because of the lack of oxygen. In the spoil, oxidation reactions release a significant amount of heat, 374.74 kJ for each mole of pyrite oxidized. This heat production can cause the spoil temperature to be higher than that of the unmined material. In addition, the intensity of heat production varies with location in spoil, in part because of the uneven distribution of reactive pyrite within the spoil. To ensure that thermal anomalies were consistent with coal refuse sites, geophysical methods were applied at Site 2. Results of geophysical surveys are consistent with the existence of highly reactive pyrite-concentrated coal refuse. Apparent conductivity surveying data showed high conductivity values appearing nearby TO-2, TO-7, TO-9 and TO-19 at Site 2 (4). These same areas were determined to be hot spots, with temperature differences of 2.8 to 8.0°C. A magnetometer survey was conducted in 1986 to locate more precisely the buried coal refuse (9). Areas near these hot spots were confirmed as sites of buried coal refuse by test drilling. Six samples of the buried coal refuse ranged in total sulfur content from 1.5% to 5.9% by weight (5). Based on the evidence collected from our field investigation, it is clear that pyriteoxidation reactions generate detectable thermal anomalies and these anomalies can be used to locate active pyrite-concentrated regions in mine spoil. In general, the intensities of thermal anomalies observed in this study were relatively low, compared to the temperatures reported in the literature. In most cases, spoil temperatures were in the range of 8.5°C to 13.0°C at both sites. Temperatures in excess of 80°C have been observed in low grade copper dumps (10). The best time to detect hot spots is in the spring, when the surface temperature disturbance reaches a minimum. During the rest of the year, it is not easy to distinguish these thermal anomalies from solar heating unless much stronger anomalies are present. Shallow Temperature Interpretation. Thermal anomalies at ground surface should reflect variations in the intensity of pyrite oxidation in the spoil as well as the seasonal changes in air temperature, solar radiation, infiltration and evaporation of spoil water, surface slope and orientation, shade and other factors. Because these shallow holes were buried only 0.3 and 0.6 m deep, they were more likely to be influenced by short-term weather and environmental changes. Without the disturbance of the heat from pyrite oxidation, temperatures at 0.3 m should be higher than those



654


at 0.6 m during warm seasons. When the temperature disturbances induced by pyrite oxidation reactions are great enough, reversed thermal gradients result. Lower temperatures at 0.3 m during warm seasons, will be very useful in locating reactive hot spots. Shallow temperatures near a hot spot are also expected to be higher than those of the surrounding area. Thermal anomalies were detected by measuring the near-ground surface temperature. Figure 6 shows the temperature taken along South Line at Site 1 in May, 1990. Two "temperature humps" were found. One thermal anomaly, between hole numbers 5 and 15 along South Line, is located neaor T-3, in which strong thermal anomalies were observed. Another temperature hump was detected between hole numbers 30 and 40 in the areas treated with sludge in 1988 and 1989. This hump was probably due to the dark color of sewage sludge material, which absorbed more heat from solar radiation. It is interesting to note the effect of sludge treatment on the ground-surface temperature. Recently, sewage sludge has been widely used in mine-spoil abatement (11,12). Sewage sludge may increase the pH, moisture and organic-matter contents, and cause a reduction in heavy-metal concentrations. The general effects of such treatment have been to reduce temperature, to help in revegetation, and to slow down the sulfide-oxidation reactions by depleting part of the oxygen available in spoil. From the field data obtained from treated and non-treated portions of the mine, it was found that the temperatures of shallow holes in untreated areas generally were higher than those in recently sludge-treated areas. The lowest shallow-hole temperatures were found in the older sludge treatment area (Figure 7). The reversed temperature gradients, that may indicate in situ heating effects, were not detected. This observation implies that the intensity of the thermal anomalies produced by sulfide-oxidation reactions was not strong enough to reverse the normal gradient directions between 0.3 m and 0.6 m depths during warm seasons. Intensity Changes of Thermal Anomaly with Time. Thermal-anomaly surveys are useful not only in locating acid-producing sites within mine spoil, but also in studying the intensity changes of pyrite-oxidation reactions. Thermal anomalies are induced by the heat generated from pyrite-oxidation reactions. The intensity of these anomalies should decay with time, because the amount of pyrite in spoil decreases with time. The difference between the spoil temperature and the background temperature should decline. Because the spoil temperature is strongly influenced by seasonal changes and air temperature fluctuations, the decay of spoil temperature due to the intensity change of pyrite-oxidation reactions is easily smeared. Under some circumstances, the spoil temperature may show an increase instead of a decline. To distinguish the spoil temperature changes caused by changes in pyrite-oxidation reactions from those caused by seasonal variations, the magnitude of the difference between spoil and the background temperature versus time should be considered. In order to reduce the effect of air-temperature fluctuations and the heterogeneity of spoil thermal properties, the average values of temperature data were used in interpretations. Positive temperature departure from normal values were recorded between June, 1990 and February, 1992 (Figure 8). This observation indicates that the weather in this area became warmer. The background temperature increased at a rate of 0.44°C


39. GUO & PARIZEK


655

ΙΟ-

υ ο

14-

u

13-

tu


15-

tm

α

12-

ε Γ

111030 Hole

Number

Figure 6. Temperature observed along South Line at Site 1 in May, 1990.

0.3m 0.6m

ε

0

5

10

15

20

25

30

35

40

45

Hole Number

Figure 7. Shallow temperature observed in different spoil areas in March, 1990. "No sludge" refers to the area not being treated with sewage sludge; "old sludge" refers to the area treated with sewage sludge in 1986; "new sludge" refers to the area treated with sewage sludge in 1988 and 1989.


656

ENVIRONMENTAL GEOCHEMISTRY OF SULFIDE OXIDATION 10 ι • " • •


4>

Time (Months)

Figure 8. Monthly temperature departure (September, 1990 to May, 1992) based on the average values from 1931 to 1991 recorded at Philipsburg Weather Station, PA. per year from 1989 to 1992, which is consistent with the air temperature changes. The spoil temperature at T-3, where hotspots were located, decreased at a rate of 0.08°C per year during the same period. Conclusions Acid mine drainage is the result of sulfide-oxidation reactions in which pyritic materials are oxidized and acid is released. Spoil temperature perturbed by the exothermic pyrite-oxidation reactions can be used to locate hot spots within a mine spoil, and to study the intensity changes of sulfide-oxidation reactions under field conditions. Field research was conducted at two surface mining sites in western Pennsylvania. Thermal surveying methods were successfully used to locate the "hot spots" in mine spoil which varied from less than 10 m to nearly 30 m in thickness. Spoil temperature was found to be 1.5°C to 2.2°C higher than the background temperature over the period of field investigation. This temperature difference varied with time and space. Our field temperature data showed that the spoil was cooling off slowly many years after mining ceased, at a rate of approximately -0.1°C/year. This temperature die-off trend most likely reflects the slowing down of the sulfide-oxidation reactions in mine spoil due to the depletion of pyrite. This temperature-monitoring method is economical, efficient and accurate, and a sensitive indicator of sulfide-oxidation reaction rates and sites of these reactions. Only active, concentrated zones of sulfide oxidation can be identified by measuring the thermal anomalies both at and near ground surface and along boreholes drilled in mine spoil. This technique can be used to obtain information on the spatial distribution of hot spots within the spoil to identify where special, additional abatement steps should be taken. This application may be very important when budgets are limited and AMD generation persists following initial attempts of mine reclamation. Real-time temperature monitoring of reclaimed mines is likely to provide more sensitive indications of the early benefits, or lack thereof, of reclamation and acid-abatement measures than soilwater- and groundwater-quality monitoring methods.


39.

GUO & PARIZEK


657

Acknowledgments Financial support for the early stage of this study was provided from the U.S. Bureau of Mines through grant No. G l 125132-4261. Continuation of temperature monitoring at Site 1 was supported by Federal funds from the U.S. Bureau of Mines and the National Mine Land Reclamation Center under cooperative agreement CO 388026.


Literature Cited 1. Harries, J.; Ritchie, A. Water, Air and Soil Pollution 1981, 15, 405-423. 2. Jaynes, D.B.; Rogowski, A.S.; Pionke, H.B. Water Res. Res. 1984, 20, 233-250. 3. Williams, J.H.; Henke, J.R.; Pattison, J.R.; Parizek, R.R.; Homberger, R.J. Hydrogeology and Water Quality at a Surface Coal Mine in Clarion County. Pennsylvania; Penn. State Univ., Coal Research Section, 1987. 4. Schueck, J.H. Mine drainage and surface minereclamation.U.S. Bureau of Mines Information Circular 9183, 1988, pp. 117-130. 5. Fielder, D. Unpublished M.S.Thesis, Penn State Univ., 1989. 6. Hillel, D. Introduction to Soil Physics; Academic Press: New York, 1982; pp. 155-175. 7. Ciolkosz, E.R.; Cronce, R.; Cunningham, R.; Petersen, G. Soil Science 1985, 193, 232-238. 8. Wang, J.; Xiong, L. InHydrogeologicalRegimes and Their Subsurface Thermal Effects; Geophy. Monograph 47; American Geophysical Union: Washington, DC, 1989; pp. 87-99. 9. Ladwig, K.J. Ground Water Monitoring Rev. 1983, 3, 46-51. 10. Beck, J.V. Biotech. Bioeng. 1967, 9, 487-490. 11. Plass, W.T. J. of Soil and Water Conservation 1978, 29, 119-121. 12. Wittwer, R.; Carpenter, S.; Graves, D. In Symposium on Surface Mining Hydrology and Reclamation: Univ. of Kentucky: KY, 1980; pp. 193-197. RECEIVED March 26,

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Field Research on Thermal Anomalies Indicating Sulflde-Oxidation

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