An assessment of groundwater contaminant source … · surface gasifiers, rate of ... Acid Rock Drainage • In “normal” groundwater, ... Application of environmental isotopes

An assessment of groundwater contaminant source and evolution from underground coal gasification

at the Majuba pilot plant

Lehlohonolo Mokhahlane

Ph D research at the Institute for Groundwater Studies

CONTENT

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1. Introduction

2. Background

3. Objectives

4. Methodologies

5. Results

4. Summary

1. INTRODUCTION

Introduction• Underground coal gasification (UCG) is a chemical process that converts

coal in-situ into a gaseous product at elevated pressures and temperatures

• This is achieved by injecting steam and air or oxygen into coal seams, which is then ignited to initiate the gasification reactions

Why UCG

• Conventional coal mining is eliminated with UCG, reducing operating costs, surfacedamage and eliminating mine safety issues such as mine collapse and asphyxiation;

• Coals that are unmineable (too deep, low grade, thin seams) are exploitable by UCG,thereby greatly increasing domestic resource availability;

• No surface gasification systems are needed, hence, capital costs are substantiallyreduced;

• No coal is transported at the surface, reducing cost, emissions, and local footprintassociated with coal shipping and stockpiling

• Most of the ash in the coal stays underground, thereby avoiding the need forexcessive gas clean-up, and the environmental issues associated with fly ash wastestored at the surface;

• There is no production of some criteria pollutants (e.g., SOx, NOx) and many otherpollutants (mercury, particulates, sulfur species) are greatly reduced in volume andeasier to handle.

• UCG eliminates much of the energy waste associated with moving waste as well asusable product from the ground to the surface;

• UCG, compared to conventional mining combined with surface combustion,produces less greenhouse gas and has advantages for geologic carbon storage.

2. BACKGROUND

Conceptual model of UCG

Potential limitations for UCG

• Significant environmental consequences:

aquifer contamination and

ground subsidence

• Coal deposits may not be suitable because some may have geologic and hydrologic features that increase environmental risks to unacceptable levels

• UCG operations cannot be controlled to the same extent as surface gasifiers, rate of water influx, distribution of reactants in the gasification zone, and the growth rate of the cavity

UCG impact on water pressure

Organic compounds are contained and continuously destroyed

Monitoring Wells Positioning

Groundwater contamination

Process gas containing contaminants can escape when there are outward pressure gradients and

a permeable flow path

Risk Based Decision Making

Natural and man made pathways

3. THE PROBLEM STATEMENT & OBJECTIVES

3. Problem statement

• What is the impact of UCG on local groundwater(across immediate aquifers) post gasification?

• How big is the cavity?

• What is the chemical evolution of thegroundwater?

• What are the flow paths for the pollution plume?

• Can any groundwater receptors be impactedupon by UCG?

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3. ObjectivesThe aim of this project is to assess the impact UCG hason the regional groundwater at the Majuba UCG site.The specific objectives are:

• Collect and review available research and work fromsite

• Conduct geophysical survey

• Drilling of verification borehole and analysis of wasteand surrounding rocks

• Chemical evolution of organic and inorganic chemistrypollutants in the groundwater resources postgasification

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Objectives cont…

• Determine of groundwater flow rate

• Investigate the interaction between deepgasified formation and upper potentiallyproductive aquifers

• To model the pollution plume from the cavity

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Study limits

• Post gasification

• Site specific

• Two seasons on monitoring

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Source Pathway Receptor

4. METHODOLOGY

Verification drilling• Allows for core samples with residue products to be

obtained

• Samples will be used to define contaminant source term (ABA, Geochemical analysis)

• Quenching might flush out residue products

• Might not be effective if cavity size is unknown

Geophysics

• Density logging, a radioactive source and detector are lowered down the borehole and the source emits medium-energy gamma rays into the formation

• Can be useful if residue samples cannot be retrieved and also to determine the size of the cavity

Hydraulic conductivity

Packer test

Acid Rock Drainage

• In “normal” groundwater, access to oxidising species is poor and acid-base reactions tend to dominate over oxidation reactions

• Acid-base reactions such as carbonate dissolution and silicate hydrolysis consume protons and carbon dioxide, and release alkalinity and base cations

• In mines, the atmospheric environment is rapidly introduced to the deep reducing geosphere. This carries the possibility of intense and rapid oxidation of sulphide minerals such as pyrite, to such an extent that these acid-generating redox reactions may dominate over acid-base “neutralisation” reactions and result in the phenomenon of “acid rock drainage” (ARD)

Stratification

Johnstone et al, 2013

Summary of MethodologyMethod / Scope of work Description Timeframe

Literature review Desktop review of available papers 6 months

Drilling of boreholes (6) Verification boreholes to obtain UCG

residue and lithology

18 months

Aquifer testing Packer tests to establish flow rate 3 months

Geophysics Reflection seismology and down hole 2 months

Desktop geochemical

study

GW monitoring - Chemical evolution 6 months

Geochemical modeling Numeric transport flow 6 months

Conceptual model Post gasification 12 months

Scenario modeling Quenching, subsidence , borehole

failure

4 months

Groundwater Monitoring On going groundwater monitoring 2 years

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5. RESULTS

Isotope study• In natural waters the isotopic ratio (2 H, 3 H, 18O) of oxygen

and hydrogen varies due to chemical, biological and physicaldevelopments

• During evaporation water that is taken up into the gaseousphase gets enriched in the lighter isotopic fractions of δ16Oand δ1H leaving behind water that is concentrated in δ18O andδ2H

• The interpretation of evaporation can lead to quantitativelyidentifying the admixture between various surface waterbodies and subsurface waters

• This provides a tool to determine the conditions duringgroundwater recharge by determining the 2H and 18Ocompositions in borehole samples

Isotope study

Tritium analysis

6. SUMMARY

Summary

• Isotope study found the shallow and deep aquifers had adistinctive isotopic signature for both stable isotopes

• The ABA study for the first verification borehole iscomplete

• The depth and presence of thick dolerite presents newchallenges for geophysical study

• On-going groundwater monitoring

• South Africa does not have a lot of experience in deepaquifer system, hence sampling equipment is limited

Thank you

References• Burton E, Friedmann J, Upadhye R. Best practices in underground coal gasification. Lawrence

Livermore National Laboratory; 2006.• Craig, H and Gordon, L, I.,Deuterium and oxygen 18 variations in the ocean and the marine

atmosphere. In Stable Isotopes in Oceanographic Studies and Paleotemperatures, pages 9–130, Pisa, Italy, 1965. Laboratorio di Geologia Nucleate

• Gordon, J.J, Edward, T.W.D, Bursey, G.G and Prowse, T.D., Estimating evaporation using stable isotopes: Quantitative results and sensitivity analysiss for two catchments in northern Canada, 1993, Nordic Hydrology, 24, 79-94

• Love, D.; Beeslar, M.J.; Blinderman, M.; Pershad, S.; van der Linde, G.; Van der Riet, M. Ground water monitoring and management in Underground Coal Gasification. Paper presented at Unconventional gas – just the facts, Groundwater Division of the Geological Society of South Africa & Mine Water Division of the Water Institute of South Africa, Pretoria, South Africa, 18-19 August 2014.

• IAEA, 1981. Stable Isotope Hydrology – Deuterium and Oxygen-18 in the Water Cycle. J.R. Gatand R.Gonfiantini (eds.), Technical Report Series No. 210, International Atomic EnergyAgency, Vienna.

• Krzysztof, K and Krzysztof, S. (2014) Chemical and toxicological evaluation of undergroundcoal gasification (UCG) effluents: The coal rank effect. Ecotoxicology and environmentalsafety, 112, pp. 105-113.

• Mook, W.G. 2000. Environmental Isotopes in the Hydrological Cycle, Volume 1: Introduction. IHP-V, UNESCO, Paris, 280p. URL: http://www.iaea.or.at/programmes/ripc/ih/volumes/volumes.htm[Last accessed on 14 July2015]

• Liu, S., Li, J., Mei, M and Dong, D. (2007) Groundwater pollution from Underground Coal Gasification, Journal of China University of Mining and Technology, 17(4), pp. 0467-0472.

• Yurtserver, Y and Payne, B.R., Application of environmental isotopes to groundwater investigation in Qatar, proceeding of a symposium, IAEA, Vol.2, pp465-490, Viena, 1978

• Malaria in Mpumalanga Province, South Africa, withspecial reference to the period 1987–1999 J.M. Govere,D.N. Durrheima, M. Coetzee and R.H. Hunt., 2001, South African journal of science