1. GEOCHEMICAL INVESTIGATION OF AN UNDERGROUND COAL MINE, MPUMALANGA, SOUTH AFRICA
Albie Steyn, Robert Hansen and Eelco Lukas
Institute for Groundwater Studies

2. OUTLINE Introduction Research approach Study objectives
Site description
Data collection
Data analysis and hydrogeochemical description
Geochemical modelling
Conceptual model
Geochemical model
Geochemical model results
Mitigation implications
Conclusion and recommendations
Markedly with respect to…

3. INTRODUCTION
Serious environmental consequences have resulted from mining activities in South Africa:
Poor environmental and water management and,
Acid mine drainage (AMD).
Large scale closure of mining operations (cessation of abstraction of groundwater from the mine voids) → significant national concerns.
Markedly with respect to…

4. Albie Steyn Albie Steyn The legacy of mining cannot be ignored
The generation of large volumes of mine wastewater by South African mines have the potential to adversely affect an already scarce water resource
Post-mining prediction of mine water quality and proper management therefore becomes a necessity

5. TRANSFORMATION OF WATER MANAGEMENT AND PROTECTION ASPECTS IN SOUTH AFRICA
EQUALITY
SUSTAINABILITY
LIABILITY
NWA
(Act No.36, 1998) +
MPRDA
(Act No.28, 2002)
The promulgation of new regulatory frameworks in the NWA and MPRDA, which focuses more on sustainability, equality and liability have led to the transformation of water management and protection aspects in SA.
On completion of mining activities, the owning company is required to undertake a process that should culminate in the granting of a mine closure certificate.
The process undertaken should satisfy various requirements that mainly relates to the identification of long-term risks and liabilities.
Quantification of risks by means of forward reaction path models becomes imperative.
Identification of long-term risks

6. RESEARCH APPROACH
Complexity of natural systems → necessitates multidisciplinary, integrated research approaches.
Geochemical modelling = invaluable tool
Interpretation of complex systems.
Prediction and anticipation of how geochemical systems evolve over time.

7. RESEARCH APPROACH Hydrogeological assessment
Mineralogical and geochemical assessment
Geochemical modelling
An integrated assessment approach was implemented that included hydrogeological, mineralogical and geochemical assessment and geochemical modelling techniques to predict the quality of water emanating from the mine waste.

8. STUDY OBJECTIVES The primary objectives of this study:
To quantify long-term water quality of mine waste drainage for the purpose of determining environmental risk to sensitive receptors, and
to recommend appropriate water management strategies.

9. The decommissioned underground mine is located in the Ermelo coal field within the Mpumalanga Province, South Africa (Figure 1).
Figure 1: Coalfields in the Mpumalanga Province, South Africa (After Huisamen & Wolkersdorfer, 2016).

10. Mining activities: 1900s to 1980s
Underground workings started filling with water
517 ha
Mined depth: mbgl (< 30 mbgl)
Bord-and-pillar mining with some stooping
The mine consists of many separately mined compartment
Mining activities commenced in the early 1900s and ceased in the mid-1980s, where after the workings started filling with water.
Most mining occurred at depths shallower than 30mbgl.
The main mining methods employed was bord-and-pillar mining with some stooping.
Mining activities ceased in the 1980s – underground workings started filling with water.
Mined depth: mbgl (most mining < 30 mbgl)
Main mining method: bord-and-pillar mining with some stooping.

11. PRE-MINING ENVIRONMENT
Significant elevation differences exist across the Mpumalanga coalfields.
Regional highs → coal is present in the
hills.
Flow
Due to the regional highs, coal is only present in the hills

12. POST-MINING ENVIRONMENT
Excavation activities has created a new water table aquifer, with new flow vectors, allowing oxygen and water
to mix with sulfides → AMD.
Mining creates pathways for water ingress and oxygen diffusion (subsidence areas);
Due to regional highs and shallow mining conditions water that accumulates in the mine workings has ample opportunity to discharge towards the local streams.
Environmental impacts result from the interaction of the deposit type, host rock and characteristics, the mining methods and the receiving environment.

13. POST-MINING ENVIRONMENT
Flooding of the underground mine workings, representing more than 80 years of mining → decant of AMD.
Two decants in well-defined flow channels.
Albie Steyn
Albie Steyn

14. DATA COLLECTION
Solid-and water sample collection → aimed at providing useful information for mineralogical and hydrogeochemical assessments.
Solid sample collection
Sample collection were aimed at providing useful information for mineralogical and hydrogeochemical assessment which is vital for the geochemical characterisation of the mine waste and the overall conceptualisation of the system
Solid samples were collected with hand augers and during drilling, where after it was submitted to several accredited laboratories for geochemical tests and analyses.

15. DATA COLLECTION
Additional field campaigns → to acquire water chemistry data that augments existing water quality databases.
Water sample collection
During water sample collection, groundwater levels were measured and water columns were profiled to ensure that the boreholes are sampled on a representative basis.
Through-flow bailing equipment was used to obtain depth specific samples which was submitted to an accredited laboratory and analysed for metals, metalloids, major and minor ions.

16. DATA ANALYSIS HYDROGEOCHEMICAL DESCRIPTION
Underground void mine water:
Dominant Ca-SO4 water type (65% of void samples)
Minor hydrochemical water types
Decant mine water:
Dominant Ca-SO4 water type (97% of decant samples)
Devoid of alkalinity
Compiled datasets were evaluated by means of descriptive plots (Piper and Durov)
The underground mine water as well as the decant mine water displays dominant CaSO4 water type.
Figure 2: Piper Diagram showing the hydrochemical water types.

17. DATA ANALYSIS HYDROGEOCHEMICAL DESCRIPTION
Underground void mine water:
pH range:
TDS range: 48–3620 mg/L
Decant mine water:
pH range: 2.35–4.47
TDS range: 644–3772 mg/L
The underground mine water is characterised by acidic to alkaline water, whereas acidic pH prevails in the decant mine water.
Figure 3: Durov Diagram showing the hydrochemical water types with pH and TDS concentrations.

18. GEOCHEMICAL MODELLING
Modelling of geochemical processes → tool to quantify the medium to long-term geochemical risks associated with the waste material.
Quantification is built on Conceptual understanding which is built on the foundations of the data collected, and scientific principles.
Figure 4: Evolution and fate of contaminants.

19. CONCEPTUAL MODEL XRD analyses: Primary minerals: Secondary minerals:
quartz
muscovite
plagioclase
K-feldspar
kaolinite
calcite and
pyrite [FeS2]
Secondary minerals:
gypsum
hematite, goethite
ankerite
smectite-type clay minerals.
XRD analyses revealed that pyrite is the primary sulphide bearing mineral, and calcite is the primary carbonate mineral.
It is expected that when FeS2 are exposed to oxygen and moisture, the FeS2 will be oxidised producing AMD if not successfully neutralised.
It can be inferred from the acidic pH of the decant mine water that under the current hydrogeochemical conditions all alkalinity is consumed along the flow path and it is therefore assumed that the system is not effectively buffered by calcite.
Mine water quality is strongly related to oxidation of FeS2 minerals in terms of acidity, sulphate and metal(loids).
Major concerns are Fe, Mn, Al, Ni, Pb, As and Se that exceed target quality guideline concentrations.
This process includes the incongruent dissolution of silicate minerals within the mine workings and buffering by soluble secondary hydroxides formed such as Al-hydroxide and Fe-hydroxides.
Figure 5: Conceptual model for the geochemical evolution of mine water (After Azzie, 2002).

20. Mineral abundances (XRD) + Reaction rates
GEOCHEMICAL MODEL
Adsorption (HFO)
The Geochemists Workbench modelling code was used for all modleling purposes.
This software code is able to define the dynamic geochemical system by using mineral abundances, water qualities, mineral reaction rates and mineral reactive surface areas.
The mineral phases with acquired reaction rates were reacted with most recent mine void water to simulate concentrations and metals and metalloid mobility in the mine decant.
Assumptions made during data processing, data input and the model setups and simulations were made to follow a conservative approach to simulate realistic processes The system was calibrated against recent mine water analyses of mine decant water quality.
Recent water chemistry ↓
Calibration data
Mineral abundances (XRD) + Reaction rates
Assumptions

21. Mineral abundances (XRD) + Reaction rates
GEOCHEMICAL MODEL
Adsorption (HFO)
Iterative process with constant sensitivity analysis and changes to parameters and rates to achieve outputs that most closely match the measured parameters in the monitoring data set.
Recent water chemistry ↓
Calibration data
Mineral abundances (XRD) + Reaction rates
Sensitivity analysis

22. GEOCHEMICAL MODEL
Oxygen availability → play an integral role in which geochemical processes are dominant:
Sensitivity analyses: alterations of the oxygen content available to react within the system
Scenario 1: Fixed oxygen fugacity
Scenario 2: Sliding fugacity
Scenario 3: Unfixed oxygen fugacity
Sensitivity analysis was focused on oxygen availability.
Different scenarios under different oxygen fugacities were simulated

23. GEOCHEMICAL MODEL RESULTS
Constituents of concern:
Acidic pH
> SO4
> metals and metalloids
Table 1: Mineral proportions and rates for forward reaction path models (constituent concentrations in mg/L).
Following the model outcomes after a 100-year simulation period the main constituents of concern is the acidic pH, elevated SO4 and mobile metals and metalloids.
Due to the acidic nature of the mine water, Mn, Pb, Ni, Se and to a lesser extent As, remain mobile.
Due to the adsorption capacity provided by the presence of iron oxides and hydroxides As poses a negligible risk to the local environment.

24. GEOCHEMICAL MODEL RESULTS
Changes to the FeS2 weathering rate as well as the oxygen fugacity is most sensitive. a) b)
…and controls the outcome of both the resultant water chemistry and the pH of the system.
Figure 6: Transient evolution of pH (a) and SO4 (b) during weathering reaction sequence over 100 years at different oxygen fugacities.

25. MITIGATION IMPLICATIONS
The models suggest that water quality is not expected to improve → necessitates mitigation and water management decisions.
Acidic pH
SO4
Metals and metalloids
Need to mitigate/remediate
Standard management practices like mine void flooding and the closure of drainage pipes are not options due to the shallow mining conditions and elevation differences.

26. MITIGATION IMPLICATIONS
Diverting the contaminated water to more accessible and manageable areas → water can easily be treated by means of conventional and unconventional active and passive methods.
Conventional methods:
Lime dosing
Addition of fly ash
Semi-passive treatment (BaCO3)
Desalination plants
Passive treatment (wetlands)
EXPENSIVE
Water treatment by means of conventional methods is an option, however these options are usually prohibitively expensive at decommissioned mine sites that can continue to discharge contaminated water for centuries post closure.

27. MITIGATION IMPLICATIONS
Unconventional methods:
Bioremediation using microbes to do in-situ remediation of specific contaminants.
Inorganic or biotic reduction.
Allows contaminants to be treated within the mine voids, thus reducing exposure risks on surface.
Very few examples exist where classic efficient bioremediation principles have been applied at a site in South Africa.
Combination of conventional and unconventional methods.
Unconventional methods such as bioremediation where the mine water is treated in-situ, thus reducing exposure risks on surface, may be more viable options.
Unfortunately very few examples exist where this has been effectively applied.

28. CONCLUSIONS AND RECOMMENDATIONS
The simulated results obtained from the described methodology are in good accordance with the most recent monitoring data for the investigated site.
The kinetic numerical method can successfully be used to make improved estimates of long-term mine water quality evolution within order of magnitude accuracies.
Risks owning to the decant of acidic water to the surface (Al, Mn, Ni and Pb are highly mobile):
Detrimental ecological impacts, and
regional impact on river systems.
Main risks are associated with acidity and high metal and metalloid loadings that may have detrimental ecological impacts and regional impact on river systems.

29. CONCLUSIONS AND RECOMMENDATIONS
Further refinement and additional studies:
Incorporate and evaluate the role of microbiology in AMD generation within models.
Evaluation of the effectiveness of in-situ bioremediation and its effect on the discharging mine water.
Although enlightening results have been achieved with this approach…

30. REFERENCES
Annandale, J.G., Beletse, Y.G., De Jager, P.C., Jovanovic, N.Z., Steyn, J.M., Benade, N., Lorentz, S.A., Hodgson, F.D.I., Usher, B. & Vermeulen, D Predicting the environmental impact and sustainability of irrigation with coal mine water. Report to the Water Research Commission. Report No. 1149/01/07.
Azzie, B.A.M Coal mine water in South Africa: Their geochemistry, quality and classification. University of Cape Town, Cape Town. (DPhil Thesis).
Bethke, C.M Geochemical and Biogeochemical Reaction Modeling, 2nd Ed. Cambridge University Press, Cambridge.
Coetzee, H., Hobbs, P.J., Burgess, J.E., Thomas, A., Keet, M., Yibas, B., Van Tonder, D., Netili, F., Rust, U., Wade, P. & Maree, J Mine water management in the Witwatersrand gold fields with special emphasis on acid mine drainage. Report to the inter-ministerial committee on acid mine drainage.
Huisamen, A. & Wolkersdorfer, C Modelling the hydrogeochemical evolution of mine water in a decommissioned opencast coal mine. International Journal of Coal Geology, 164(1)3-12.
Usher, B.H The evaluation and development of hydrogeochemical prediction techniques for long-term water chemistry in South African Coalmines. Institute for Groundwater Studies, University of the Free State, Bloemfontein. (DPhil Thesis).

31. Thank You