Geochemical and Hydrologic Controls on Abandoned Coal Mine Discharge
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Jill Burrows Ph.D. Candidate, Lehigh University, “Geochemical and Hydrologic Controls on Abandoned Coal Mine Discharge” Water samples were collected from 23 Coal Mine Discharges (CMDs) in the summer and fall of 2012 in the anthracite coal region of Pennsylvania to evaluate the changes in geochemistry and hydrology over time by comparing the results to studies conducted on the same discharges in 1975, 1991, and 1999 by the U.S. Geological Survey. Geochemical modeling was used to establish a timeline for inorganic pyrite dissolution.
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Geochemical and Hydrologic Controls on Abandoned Coal Mine Discharge
- 1. Geochemical and Hydrologic Controls on Mine Drainage: Anthracite Coal Fields, PA, 1975-2012 J.E. Burrows1, S.C. Peters1, and C.A. Cravotta, III2 1 Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 2 USGS, U.S. Geological Survey, Pennsylvania Water Science Center, New Cumberland, PA 17070
- 2. Exposure to oxygen and moisture produces Fe2+, SO4, and acid: FeS2 + 14Fe3+ + 8H2O 15Fe2+ +2SO4 2- +16H+ (1)
- 3. Fe+3 Ferric Iron Fe+2 Ferrous Iron Aerobic High pH Anaerobic Low pH
- 4. Exposure to oxygen and moisture produces Fe2+, SO4, and acid: FeS2 + 14Fe3+ + 8H2O 15Fe2+ +2SO4 2- +16H+ (1) Fe2+ is transformed through the following reactions: Fe2+ + 0.25O2 + H+ Fe3+ + 0.5H2O (2) Fe3+ + 3H2O Fe(OH)3 + 3H+ (3) Fe2+ + 0.25O2 + 2.5 H2O Fe(OH)3 + 2H+ (4)
- 5. pH Site 2 Site 3 Higher pH results in the precipitation of Fe Lee et al, 2002, Appl. Geochem. Site 1
- 6. Wood et al., 1999, Quat. Jour. Eng. Geo.
- 7. Wood et al., 1999, Quat. Jour. Eng. Geo.
- 8. Scranton Wilkes-Barre Bethlehem Sampling Sites Coal Mining Operations Cities
- 9. Scranton Wilkes-Barre Bethlehem Sampling Sites Coal Mining Operations Cities
- 11. 1975 1991 1999 2012 Growitz et al., 1985, USGS Report Wood, 1991, USGS Report Cravotta, 2008, Appl. Geochem. This Study
- 12. 2.5 3.5 4.5 5.5 6.5 7.5 2/9/99 7/9/99 12/6/99 5/4/00 10/1/00 pH Sampling Date (M/D/Y) pH Askam shaft 2.5 3.5 4.5 5.5 6.5 7.5 pH pH Honey pot Outfall 0 20 40 60 80 100 120 140 160 180 Fe(mg/L) Fe Honeypot outfall 2.5 3.5 4.5 5.5 6.5 7.5 pH pH Valley View 0 50 100 150 Fe(mg/L) Fe Valley View 0 50 100 150 2/9/99 7/9/99 12/6/99 5/4/00 10/1/00 Fe(mg/L) Sampling Date (M/D/Y) Fe Askam shaft
- 13. 0 10 20 30 40 50 60 1975 1985 1995 2005 Fe(mg/L) Sampling Year 0 100 200 300 400 500 600 700 1975 1985 1995 2005 SO4(mg/L) Sampling Year pH SO4Fe Median Std Deviation Std Deviation Median Std Deviation Median pH SO4Fe A) B) C) D) E) F) MeanMean Mean 0 500 1000 1500 2000 2500 3000 SO4(mg/L) 0 1 2 3 4 5 6 7 1975 1985 1995 2005 pH Sampling Year 0 20 40 60 80 100 120 140 160 180 200 Fe(mg/L) 2.5 3.5 4.5 5.5 6.5 7.5 pH
- 14. 0 10 20 30 40 50 60 1975 1985 1995 2005 Fe(mg/L) Sampling Year 0 100 200 300 400 500 600 700 1975 1985 1995 2005 SO4(mg/L) Sampling Year pH SO4Fe Median Std Deviation Std Deviation Median Std Deviation Median pH SO4Fe A) B) C) D) E) F) MeanMean Mean 0 500 1000 1500 2000 2500 3000 SO4(mg/L) 0 1 2 3 4 5 6 7 1975 1985 1995 2005 pH Sampling Year 0 20 40 60 80 100 120 140 160 180 200 Fe(mg/L) 2.5 3.5 4.5 5.5 6.5 7.5 pH
- 15. 0 10 20 30 40 50 60 1975 1985 1995 2005 Fe(mg/L) Sampling Year 0 100 200 300 400 500 600 700 1975 1985 1995 2005 SO4(mg/L) Sampling Year pH SO4Fe Median Std Deviation Std Deviation Median Std Deviation Median pH SO4Fe A) B) C) D) E) F) MeanMean Mean 0 500 1000 1500 2000 2500 3000 SO4(mg/L) 0 1 2 3 4 5 6 7 1975 1985 1995 2005 pH Sampling Year 0 20 40 60 80 100 120 140 160 180 200 Fe(mg/L) 2.5 3.5 4.5 5.5 6.5 7.5 pH
- 16. 0 10 20 30 40 50 60 1975 1985 1995 2005 Fe(mg/L) Sampling Year 0 100 200 300 400 500 600 700 1975 1985 1995 2005 SO4(mg/L) Sampling Year pH SO4Fe Median Std Deviation Std Deviation Median Std Deviation Median pH SO4Fe A) B) C) D) E) F) MeanMean Mean 0 500 1000 1500 2000 2500 3000 SO4(mg/L) 0 1 2 3 4 5 6 7 1975 1985 1995 2005 pH Sampling Year 0 20 40 60 80 100 120 140 160 180 200 Fe(mg/L) 2.5 3.5 4.5 5.5 6.5 7.5 pH
- 17. Time Interval Parameter ’75-‘91 ’91-‘99 ’99-‘12 ’75-‘12 Fe (mg/L) 0.080 0.005 0.081 0.165 SO4 (mg/L) 0.600 <0.001 0.091 0.002 pH <0.001 <0.001 0.046 0.008 Non-parametric Matched Pairs Significance Level p<0.05
- 18. 0 10 20 30 40 50 60 1975 1985 1995 2005 Fe(mg/L) Sampling Year 0 100 200 300 400 500 600 700 1975 1985 1995 2005 SO4(mg/L) Sampling Year pH SO4Fe Median Std Deviation Std Deviation Median Std Deviation Median pH SO4Fe A) B) C) D) E) F) MeanMean Mean 0 500 1000 1500 2000 2500 3000 SO4(mg/L) 0 1 2 3 4 5 6 7 1975 1985 1995 2005 pH Sampling Year 0 20 40 60 80 100 120 140 160 180 200 Fe(mg/L) 2.5 3.5 4.5 5.5 6.5 7.5 pH
- 19. EPCAMR John Welsh Flux = Concentration x Discharge
- 20. 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 1975 1985 1995 2005 Feflux(mg/s) Sampling Year 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 1975 1985 1995 2005 Discharge(m3/s) Sampling Year 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 1975 1985 1995 2005 SO4Flux(mg/s) Sampling Year Median Std Deviation Median Median Std Deviation Std Deviation Fe Flux Fe Flux SO4 Flux SO4 FluxDischarge Discharge A) B) C) D) E) F) Mean Mean Mean 0 0.5 1 1.5 2 2.5 3 Discharge(m3/s) 0 0.5 1 1.5 2 2.5 SO4Flux(mg/s) 0 0.05 0.1 0.15 0.2 0.25 FeFlux(mg/s)
- 21. Demchak et al., 2004, Jour. Env. Qual.
- 22. Above: exposed rock surfaces facilitate O2 transport and continual pyrite dissolution, alkalinity consumption Demchak et al., 2004, Jour. Env. Qual.
- 23. Demchak et al., 2004, Jour. Env. Qual. Above: exposed rock surfaces facilitate O2 transport and continual pyrite dissolution, alkalinity consumption Below: groundwater inputs with low dissolved O2, resulting in a decrease of pyrite oxidation
- 29. 0 20 40 60 80 100 120 140 160 180 200 Fe(mg/) pH SO4Fe A) B) C) 2.5 3.5 4.5 5.5 6.5 7.5 pH 0.5 1.5 2.5 3.5 4.5 5.5 6.5 1975 1985 1995 2005 pH Sampling Year Mean Std Dev 7 12 17 22 27 32 37 42 47 52 1975 1985 1995 2005 Fe(mg/L) Sampling Year Mean Std Dev Mean 0 100 200 300 400 500 600 700 800 1975 1985 1995 2005 SO4(mg/L) Sampling Year 0 500 1000 1500 2000 2500 3000 SO4(mg/L) Mean Mean Std Dev Above: black Below: gray
- 30. Sampling Year Drainage Type ‘75-’91 ‘91-’99 ‘99-’12 ‘75-’12 Above pH 0.075 0.106 0.204 0.108 Fe 0.867 0.089 0.402 0.799 SO4 0.611 0.050 0.866 0.402 Below pH 0.003 0.004 0.099 0.043 Fe 0.045 0.007 0.091 0.028 SO4 0.289 0.009 0.084 0.004 Non-parametric Matched Pairs Significance Level p<0.05
- 31. Sampling Year Drainage Type ‘75-’91 ‘91-’99 ‘99-’12 ‘75-’12 Above pH 0.075 0.106 0.204 0.108 Fe 0.867 0.089 0.402 0.799 SO4 0.611 0.050 0.866 0.402 Below pH 0.003 0.004 0.099 0.043 Fe 0.045 0.007 0.091 0.028 SO4 0.289 0.009 0.084 0.004 Non-parametric Matched Pairs Significance Level p<0.05
- 32. 0 20 40 60 80 100 120 140 160 180 200 Fe(mg/) pH SO4Fe A) B) C) 2.5 3.5 4.5 5.5 6.5 7.5 pH 0.5 1.5 2.5 3.5 4.5 5.5 6.5 1975 1985 1995 2005 pH Sampling Year Mean Std Dev 7 12 17 22 27 32 37 42 47 52 1975 1985 1995 2005 Fe(mg/L) Sampling Year Mean Std Dev Mean 0 100 200 300 400 500 600 700 800 1975 1985 1995 2005 SO4(mg/L) Sampling Year 0 500 1000 1500 2000 2500 3000 SO4(mg/L) Mean Mean Std Dev Above: black Below: gray
- 33. Exposure to oxygen and moisture produces Fe2+, SO4, and acid: FeS2 + 14Fe3+ + 8H2O 15Fe2+ +2SO4 2- +16H+ (1) Fe2+ is transformed through the following reactions: Fe2+ + 0.25O2 + H+ Fe3+ + 0.5H2O (2) Fe3+ + 3H2O Fe(OH)3 + 3H+ (3) Fe2+ + 0.25O2 + 2.5 H2O Fe(OH)3 + 2H+ (4)
- 34. 0 0.05 0.1 0.15 0.2 0.25 0 20 40 60 80 100 120 Relativefrequency Dissolved Oxygen (% Saturation) 0 0.05 0.1 0.15 0.2 0.25 0 20 40 60 80 100 120 Relativefrequency Dissolved Oxygen (% Saturation) Below-Drainage Above-Drainage
- 35. Below-Drainage Above-Drainage 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 20 40 60 80 100 120 140 Relativefrequency Alkalinity (mg/L CaCO3) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 20 40 60 80 100 120 140 Relativefrequency Alkalinity (mg/L CaCO3)
- 36. 0 20 40 60 80 100 120 140 160 180 200 Fe(mg/) pH SO4Fe A) B) C) Above: black Below: gray 2.5 3.5 4.5 5.5 6.5 7.5 pH 0.5 1.5 2.5 3.5 4.5 5.5 6.5 1975 1985 1995 2005 pH Sampling Year Mean Std Dev 7 12 17 22 27 32 37 42 47 52 1975 1985 1995 2005 Fe(mg/L) Sampling Year Mean Std Dev Mean 0 100 200 300 400 500 600 700 800 1975 1985 1995 2005 SO4(mg/L) Sampling Year 0 500 1000 1500 2000 2500 3000 SO4(mg/L) Mean Mean Std Dev
- 37. 0 0.5 1 1.5 2 2.5 3 3.5 4 0 1 2 3 4 5 6 7 8 Fe(moles) S (moles) Pyrite (2:1) Samples (2.3:1) Molar Ratio S:Fe
- 38. 0 2 4 6 8 10 12 14 16 2 3 4 5 6 7 8 9 10 -LogActivity pH
- 39. 0 2 4 6 8 10 12 14 16 2 3 4 5 6 7 8 9 10 -LogActivity pH Fe tot 1999 Fe(III)1999 Fe tot 2012 Fe(III)2012
- 40. 0 2 4 6 8 10 12 14 16 2 3 4 5 6 7 8 9 10 -LogActivity pH Fe tot 1999 Fe(III)1999 Fe tot 2012 Fe(III)2012 Goethite Jarosite Schwertmannite Ferrihydrite
- 41. 0 1 2 3 4 5 6 7 8 2 3 4 5 6 7 8 9 10 -LogActivityFe(III) pH Jarosite Schwertmannite Ferrihydrite
- 42. 0 1 2 3 4 5 6 7 8 2 3 4 5 6 7 8 9 10 -LogActivity pH Fe tot 1999 Fe(III)1999 Fe tot 2012 Fe(III)2012 Jarosite Schwertmannite Ferrihydrite
- 43. 1 - .5 - 0 - -.5 - Troilite Ferrihydrite Jarosite Schwertmannite FeSO4 (aq) Pyrite Fe++ Fe3+ Fe(OH)++ Fe(OH)2 + FeOH+ FeOFe++ 2 3 4 5 6 7 8 9 10 pH I I I I I I I Eh(V)
- 44. Conclusions • Differences in pH, Fe, and SO4 were significant (p<0.05) for below-drainage mines • Above-drainage discharges did not see any significant changes • Fe(II) is the dominant Fe species, and transformation to Fe(III) may be limited by O2 transport. • Saturation of Fe(III) precipitates varies with pH and Fe and SO4 concentrations: increasing pH and decreasing concentrations of Fe and SO4 limit the precipitation of K-jarosite and schwertmannite and favor precipitation of Fe(III) oxides.
- 45. Thank you! • EPCAMR • Earth Conservancy • PA DEP • PA GIS and Tax Assessors Offices • LU Environmental Initiative • LU EES Department • Kayla Virgone • Joe Solly • Kate Semmens • Paul Henry • George Yasko
- 46. References • Pine Knot Tunnel Discharge image http://www.undergroundminers.com/oakhill.html • Cravotta, C.A., III, 2008a, Dissolved metals and associated constituents in abandoned coal-mine discharges, Pennsylvania, USA. Part 1: Constituent quantities and correlations, Appl. Geochem., 23, 166-202. • Cravotta, C.A., III, 2008b, Dissolved metals and associated constituents in abandoned coal-mine discharges, Pennsylvania, USA. Part 2: Geochemical controls on constituent concentration, Appl. Geochem, 23, 203-226. • Lee, G., Bigham, J.M., Faure, G., 2002, Removal of trace metals by coprecipitation with Fe, Al, and Mn from natural waters contaminated with acid mine drainage in the Ducktown Mining District, Tennessee: Appl. Geochem., 17, 569-581. • Growitz, D.J., Reed, L.A., Bear, M.M., 1985, Reconnaissance of mine drainage in the coal fields of Eastern Pennsylvania, U.S. Geological Society Water-Resources Investigations Report, 83-4274. • Wood, C.R., 1991, Water quality of the large discharges from mines in the anthracite region of Eastern Pennsylvania, U.S. Geological Society Water-Resources Investigations Report, 95-4243. • Wood, S.C., Younger, P.L., Robins, N.S., 1999, Long-term changes in the quality of polluted minewater discharges from abandoned underground coal workings in Scotland, Quat. J. of Eng. Geo., 32, 69-79.
- 47. Discharges Sampled • Coalbrook Mine (lower Wilson Creek Shaft) • Gravity Slope (Peckville Shaft) • Old Forge Borehole • Duryea Breech Seep • Butler Mine tunnel (Pittston Water Level Tunnel) • South Wilkes-Barre Boreholes • Buttonwood Outfall • Beaver Meadow Outfall • Oneida Tunnel • Scott Ridge Mine Tunnel • Cameron Mine Airshaft • Cameron Mine Drift • Silverbrook Mine • Colket Mine • Tracy Airhole • Rowe Tunnel Discharge • Valley View Tunnel • Jermyn Mine • Honeypot Outfall • Maysville Mine Borehole at Ranshaw • Henry Clay Stirling Mine Pump • Big Mtn Mine no. 1 Slope • Markson Columnway • Porter Tunnel near Tower City
Notas do Editor
- Today I’m going to be talking about the controls on geochemistry and hydrology in the Anthracite coal field of Pennsylvania and how they vary over time. It is important to know what these discharge characteristics are going to be in the future for prioritizing AMD treatment systems This study will show you the importance of using current samples in consideration of treatment system design for the long-term, as well as planning for the functioning of that system as the mine discharge evolves with timeAbandoned mines, with their drainage systems no longer intact or functioning, create the aqueous and oxidizing environment required for acid, iron, and other metals to leach from the remaining rock and drain into nearby waterways as abandoned mine drainage, or AMD.
- The following equations detail the dissolution of pyrite.
- This diagram shows the iron cycle. Fe+3 is ferric iron and forms in aerobic high pH conditions, almost all iron solid phases are in the form of ferric iron
- The oxygenation of ferrous iron has been identified as the rate limiting step in pyrite dissolution, and that’s going to become important a little later.The mine drainage produced contains high concentrations of metals such as Fe, Mn, Pb, Cu, As, Ni, Zn, Cr, and Al (Benner et al., 1997). Toxic metals in CMD cause environmental impairment by killing plant and animal species, reducing biodiversity, disrupting the food chain, destroying the buffering capacity of water bodies thereby making them more susceptible to changes, and altering vital habitats (Gray 1997). Field studies and modeling experiments have been conducted to study the transport and precipitation of Fe in streams and have generally identified pH, dilution, and instream-biota as the controlling factors in Fe transport and precipitation in streams impaired by CMDs. Studies have also been conducted to quantify the relative contributions of biotic and abiotic processes on the attenuation of AMD; however, above pH 5, biological processes are insignificant compared to abiotic processes.
- Increased pH generally favors solid phases of metal sulfates and oxyhydroxides, influencing their precipitation. These graphs show mine drainage from 3 sites.Describe axes on graph. With an increase in pH, there is a decrease in Fe in solution.Sequential extraction experiments indicate metals are associated primarily with the Fe/Mn oxides fraction, indicating increased oxygenation aids in the remediation of metals in CMDs through co-precipitation with Fe and Mnoxides. Lab and field experiments have shown that schwertmannite and jarosite are the dominant precipitates in CMD samples, and schwertmannite, jarosite, and ferrihydrite are precursors to the more stable goethite and hematiteHowever, most of these studies were interested in the down-stream transformations of Fe, and not the factors controlling the geochemistry at the point of discharge. It has been suggested that CMD water quality improves over time following the cessation of mining and the closure of a mine.
- Wood et al. 1999 looked at the effect of the number of year a mine has been abandoned, and the water quality of the discharge. On this graph, pH is on the y axis, and discharge age, or years since mine abandoned is on the x-axis. Mines that have been abandoned more recently tend to have lower pH, while mines abandoned decades to over one hundred years ago tend to be circumneutralSimilar trends in water quality were seen with iron
- Total iron in mg/L is on the y axis, and discharge age, or years since mine abandoned is on the x-axis. In general, you find higher iron concentrations in younger abandoned mines, and lower iron concentrations in older abandoned mines. The same trend was seen for sulfate. It is hypothesized that decreasing rates of acid production will be observed over time because of diminishing quantities of unweathered pyrite, decreased access of O2 to the subsurface after mine closure, and relatively constant influx of alkalinity from groundwater. however, when evaluating how individual mine geochemistry changed over time, only six sites were analyzed with a maximum of 20 years of record at one site, approximately 10 years available at the other 5, and discharge data and hydrologic regimes were not evaluated. But what we wanted to know how this hypothesis works for the anthracite coalfield in pennsylvania, especially hen you take hydrology in to account, and whether this knowledge could help better inform treatment options
- In 2012, 24 mine discharges in the anthracite field of pa were sampled at base flow for a wide range of geochemical and hydrologic parameters. Here are the 4 coal fields, sample sites are the black dots
- Samples were collected at baseflow, which is defined as flow below the 25 percentile on nearby streams (Lackawanna and Little Schuylkill Rivers) as representative of hydrologic conditions (USGS). Here are the locations of the comparison rivers in reference to the cmd locations
- Samples were retrieved as close as possible to the point of discharge to avoid possible variations in chemistry due to photo effects that have been reported for several metals species including Fe. Samples were filtered on-site using a 0.45 um filter for anion and dissolved cation analysis. Water quality data was collected onsite with Manta II multi-probes (Eureka Environmental) and included pH, Eh, dissolved oxygen (DO), and discharge. Probes were calibrated prior to use in the field, and calibrations verified upon return. All samples were stored on ice while in the field, and refrigerated at 4°C upon return to Lehigh University. Samples for cation analysis were acidified to a pH of 2 in the field using HNO3. These 24 discharges were identified based on GPS location and mine name, as being sampled in
- 1975, 1991, 1999, 2012We’re going to look at how the concentrations of Fe and sulfate, and the pH changed across this time period, but first I want to show you the variation these discharges seen in one year
- These graphs show the pH and Fe at 3 discharges in a year and a half time period. This data doesn’t consider dischargeMine discharge varies with precipitation in complex ways; however, inter-site differences are expected to be greater than intra-site differences (Cravotta et al., In review).The low point in iron at honeypot and askam shaft corresponds to a day that had higher discharge, about 30-40% higher, so dilution may explain the lower concentration.So now lets look at what these discharges do over the course of 37 years.
- graphs show variations in pH, Fe, and SO4 with the respective descriptive statisticsThe samples obtained from CMDs in 1975, 1991, 1999, and 2012 evolved with similar patterns that are reviewed by analyte.
- The mean pH increased significantly (Wilcoxon matched pairs test, p=0.008, Table 1) from 5.0 in 1975, to 5.7 in 2012 (Figure 2A, 2D) with 19 of the 24 sites increasing. There were two pH outliers measured in 2012, with one of these values being 1.8 pH units lower than previous observations in 1975, 1991, and 1999. Overall, the range of pH values increased due to the majority of sites having higher pH values, though three sites stayed the same or decreased.
- The mean Fe concentrations decreased from 42 mg/L to 18 mg/L between 1975 and 2012 (Figure 2E). The site with highest concentration of Fe in 1975 saw a decrease from 190 mg/L in 1975 to 21 mg/L in 2012. Overall, the standard deviation of Fe concentrations decreased by 69% over the study period with the largest decrease observed between 1975 and 1991.
- Concentrations of SO4 exhibited a decrease in concentration at most sites between 1975 and 2012. The mean SO4 concentration decreased from 540 mg/L in 1975 to 250 mg/L in 2012 (Figure 2F) with 21 out of 24 sites decreasing. The highest concentration of SO4 in 1975 was 2,800 mg/L; this same site still had the highest concentration in 2012, though the concentration had decreased almost 80% to 630 mg/L. Similar to Fe, the standard deviation of SO4 concentrations decreased by 71% between 1975 and 2012 with the largest decrease between 1975 and 1991. The decrease in range of the Fe and SO4 concentrations indicates that over time these discharges are becoming more homogenous. This is consistent with the hydrology of the anthracite coalfield in that most discharges are sourced from large underground mine pools that receive water from multiple coal seams.
- Over the study period (1975-2012), SO4 concentrations decreased while the pH increased and Fe showed no significant change at the CMDs sampled in the anthracite coal region of PA between all years sampled (Figure 2). These results indicating improvement of water quality over time at abandoned coal mine discharges that are consistent with Wood et al., 1999, and results seen in similar time-series studies
- Demchak et al., 2004, saw a 79% decrease in mean Fe concentration in CMDs in West Virginia over a 32 year time period, while this study saw a 57% decrease in the mean over a 37 year period. Stewart and Skousen, 2003, saw significant increases in pH and decreases in Fe at CMDs in West Virginia over a 36 year period. In a study of the mine drainage impact to receiving streams using historical water quality datasets dating as back as 1906, pH increased and sulfate decreased in receiving streams over time (Raymond, 2009). However, this study focused on mine drainage receiving streams, not the discharges themselves, and may include changes in water quality due to the installation of treatment systems, and not natural attenuation.One thing to point out as well is how much a discharge can change in a decade. Using older samples for consideration in treatment system design may not be appropriate.
- discharge makes a difference in the amount of toxic metals produced, so I used discharge to calculate fluxThe discharge and flux data presented here is for context purposes, and is representative of just one point in time and does not necessarily reflect what the occurred during the full year. So that said, let’s take a look at that data
- There was a decrease in mean discharge between 1975 and 1991, followed by increases between both 1991 and 1999, and 1999 and 2012. Fe and SO4 fluxes both exhibited a slight decrease in mean, range, and maximum value between 1975 and 1991. However, between 1991 and 1999, and 1999 and 2012, there were increases in the mean flux for Fe and SO4, as well as discharge. The fluxes of these constituents are variable and tend to track with increases in discharge between 1991 and 2012, suggesting that dilution has an influence on CMD geochemistry over time. I had hypothesized that decreasing rates of acid production will be observed over time because of diminishing quantities of unweathered pyrite, decreased access of O2 to the subsurface after mine closure, and relatively constant influx of alkalinity from groundwater. so far, we’ve looked at how the pH, fe and sulfate concentrations of these discharges have changed over time without really look to far in to any differences in geology or hydrologic regime of any of these discharges. I’m going to do that by characterizing these discharges as above or below drainage
- Below drainage refers to a coal seem being situated below the water table. I hypothesize that below-drainage systems receive groundwater inputs with low dissolved O2, resulting in a decrease of pyrite oxidation and the gradual improvement of CMD water quality. Above-drainage systems with exposed rock surfaces facilitate O2 transport and continual pyrite dissolution , alkalinity consumption, and less change in water quality over time. The rain does not necessarily have to be acidic. Also, both above and below drainage mines will receive recharge from the surface. The point with above drainage mines is that large volumes of mined rock continue to be exposed to oxygen and infiltrating water that promote the oxidation and transport of oxidation products.
- Below drainage refers to a coal seem being situated below the water table. I hypothesize that below-drainage systems receive groundwater inputs with low dissolved O2, resulting in a decrease of pyrite oxidation and the gradual improvement of CMD water quality. Above-drainage systems with exposed rock surfaces facilitate O2 transport and continual pyrite dissolution , alkalinity consumption, and less change in water quality over time. The rain does not necessarily have to be acidic. Also, both above and below drainage mines will receive recharge from the surface. The point with above drainage mines is that large volumes of mined rock continue to be exposed to oxygen and infiltrating water that promote the oxidation and transport of oxidation products.
- Below drainage refers to a coal seem being situated below the water table. I hypothesize that below-drainage systems receive groundwater inputs with low dissolved O2, resulting in a decrease of pyrite oxidation and the gradual improvement of CMD water quality. Above-drainage systems with exposed rock surfaces facilitate O2 transport and continual pyrite dissolution , alkalinity consumption, and less change in water quality over time. The rain does not necessarily have to be acidic. Also, both above and below drainage mines will receive recharge from the surface. The point with above drainage mines is that large volumes of mined rock continue to be exposed to oxygen and infiltrating water that promote the oxidation and transport of oxidation products.
- METHODS OF ENTRY TO UNDERGROUND MINES:a. Slope--follows dip of coal bed; worked from bottom upward.b. Shaft--vertical access across multiple coal beds.c. Tunnel--horizontal access across multiple coal beds.d. Drift--horizontal access follows strike of coal bed.From chuck’s slide
- boreholes
- Flooded airshaft, silver creek
- Gravity drainage: mining took place up-dip, and then water is able to freely drain out
- TunnelSo taking a look at the water quality if we sepearate above/below
- graphs show variations in pH, Fe, and SO4 with the respective descriptive statistics
- When we look at the above drainages, there weren’t many significant changes seen between any of the time periods, and overall, the changes were not significant. The only one that comes close to even being marginally significant is pH
- However, when we look at the below-drainage sites, there is much more significance in the changes we are seeing thereWhy would this happen? Once the mine is flooded, access to oxygen is decreased, reducing pyrite dissultion, and the mine is getting alkalinity from groundwater flowing in, which adds buffering capacity to the water. These mines may be showing some natural attenuation or self-remediation. Whereas the above drainage mines are receiving in-puts of oxygen rich water, which allow pyrite to continue dissolution, and any alkalinity inputs are fully or partially consumed by the acid generationConversely, above drainage mines have exposed rock and continual pyrite dissolution, continuing the addition of acid
- Increases in pH, decreases in iron more significant for below dischargesTo view this in terms of pyrite dissolution, we can go back to the equations I showed at the beginning of the presentation
- Remember at the beginning of the talk how I said The oxygenation of ferrous iron has been identified as the rate limiting step in pyrite dissolution, and that’s going to become important a little later.Well if we look at the distribution of dissolved oxygen between the above and below discharges
- Below mine discharges tend to be completely saturated and have lower dissolved oxygen, while above-drainage mines tend to have more variable levels of dissolved oxygen
- Now if we look at alkalinity, above-drainage mines are on low end here. A couple reason for this could be less alkaline inputs, they might not be receiving as much groundwater as below drainage mines, however, the alkalinity in above drainage mines may be being consumed by the higher sulfide oxidation and acidity generation
- Observations across the sampling period 1975-2012 are consistent with the controls on the rates of pyrite dissolution. Initial pyrite dissolution occurs in oxygen-rich systems immediately after mine closure resulting in both secondary sulfate mineral precipitation and export of Fe and SO4, followed by a decrease in Fe and SO4 concentrations and an increase in pH until steady-state concentrations are reached as the landscape returns to the natural long-term weathering rates. Below-drainage systems receive alkaline groundwater inputs with low dissolved oxygen, resulting in a decrease of pyrite oxidation and the gradual improvement of the discharge water quality. Above –drainage systems maintain exposed rock and receive inputs of acidic rain resulting in continual pyrite dissolution, alkalinity consumption, and less significant changes in water quality over time. But if we’re basing this off a pyrite system, you might notice that there’s something up with these values. It looks like there’s a lot more sulfate than iron.
- There is much less Fe than SO4 considering a pyrite source for these dischargesWe’d expect there to be more sulfate than iron for a couple reason. SO4 tells you more about dilution, it’s not limited by mineral precipitation the way Fe is.
- What I will be showing on this plot is activity versus pHActivity is generally the concentration of a chemical species available to react when taking in to account that you do not have an ideal system, there are other ions present, and complexes can form. This axis is the neg log activity, meaning that higher activities where there’s more of that species available to react in the water are towards the top of the y-axis.I used the geochemical modeling program phreeqc to model the activity of Fe(III). Only data from 1999 and 2012 was complete enough to use in modeling
- Fe(III) activity decreases with increasing pH.the samples in this study were retrieved as close as possible to the point of discharge soi’m not expecting any photo effects, and the lack of correlation of Fetot to pH indicates they have a large Fe(II) component which is not controlled by the solubility of hydroxide mineral species or SO4 complexation.The lack of correction between Fetot and pH may indicate that the transformation to Fe(III) is kinetically limited by lack of oxygen, which is likely the rate limiting step in mine drainage treatment. This shows that changes in pH alone do not indicate that a mine discharge is improving in water quality, which is why even though we saw significant improvements in pH, the change in iron was smaller and less significantNow I’m going to add the saturation indices which will show which common CMD solid phases are controlling Fe(III) in solution. Saturation indices were modeled using Act2 in Geochemist’s workbench
- Almost all samples appear to be oversaturated in goethite, while the meta-stable phases, K-jarosite, schwertmannite, and ferrihydrite, are undersaturated over the pH range. At lower pH values (approximately pH 2 to 5.25), K-jarosite formation is predominant over ferrihydrite and schwertmannite. Between approximately pH 5.25 and 5.75, schwertmannite becomes the dominant meta-stable phase, followed by ferrihydrite predominance over pH 5.75. Now let’s look at what happens when as iron concentrations decrease over time, and we’ll do that by looking more closely at ferrihydrite, schwertmannite, and jarosite
- Note I changed the scale to zoom in on the region of interest
- The gray lines in Figure 5 denote solid phase saturation at for two end-member SO4 concentrations (pK = 4.06 and 2.06) to reflect changing solubility due to decreases in SO4 concentrations in later samples. Predominance ranges are shifted in lower SO4 conditions: in lower SO4 environments, K-jarosite predominance is restricted to lower pH values (pH 2 to 3.75 compared to 2 to 5.25 at higher SO4 concentrations). The window for schwertmannite predominance is shifted from pH 5.25 to 6 at higher SO4 concentrations, to 4 to 4.75 at lower SO4 concentrations. When iron concentrations decrease, the saturation index for jarosite and schwertmannite move up higher making it less likely those species are going to be precipitating.
- Eh values for samples taken in 1999 and 2012 are above 0 V for all samples. Different solid phase stability fields are observed for elevated and low Fe(III) and SO4 end-member concentrations in water (Figure 6). At low concentrations of Fe and SO4 (pK = 6.06 and 4.05, respectively) aqueous complexes dominate below approximately pH 5.25, and ferrihydrite dominates above approximately pH 5.25. Fe(II) is present at lower oxidation levels. At elevated concentrations of Fe and SO4 (pK = 3.01 and 2.06, respectively), K-jarosite formation occurs below pH 5.25, schwertmannite forms between pH 5.25 and 6, and ferrihydrite formation forms above pH 6. At lower oxidation levels, FeSO4 is present.The statistical insignificance of the change in Fe concentration between 1975 and 2012, and the marginal significance in variation of SO4 concentration may indicate that these constituents have plateaued, while pH has continued to increase to the extent that the resulting mineralogy of precipitates at the point of discharge has changed. The increase in pH seen in samples may contribute to an increase in saturation of ferrihydrite, K-jarosite, and schwertmannite; however, decreases in SO4 concentration over time may limit the potential for saturation of K-jarosite and to a lesser extent schwertmannite, in the future (Figure 5). Lower Fe and SO4 concentrations over time may lead to a decrease in K-jarosite formation and instead favor the formation of aqueous Fe complexes (Figure 6).
- In conclusion, we have seen a decrease in Fe and sulfate concentration, and an increase in pHThe system appears to be controlled by Fe(II) and limited by oxygenation,And variations in discharge have resulted in little variation in actual iron fluxChanges in concentrations can results in shifts in the solid phases expected to form