Micro and Macro Scale Design and Performance of Dry Covers - Waihi, New Zealand

P.J.B. Fransen, J Ruddock, T. Matuschka, S.D. Miller, W.J. Russell

Prevention & Mitigation of ARD -

Summary of methods for the prevention of mine drainage

(Ref. Global ARD Guide)

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Waihi Climate

Air temperature -5° to 28° CRainfall: 2100 mm a-1

ET: 700 mm a-1

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Cover and Climate Types

General guidance on appropriate cover types

(Ref. Global ARD Guide)

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Waihi

Waihi – New Zealand

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Waihi Tailing Storage Facility

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Storage 2 rehabilitation

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Storage 2 rehab cover and land use

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Objectives:

1. Restriction on generation of acid drainage in the short and long term

2. Rehabilitation of the downstream shoulder to pasture and native plantings

3. Surface water control to prevent ponding, infiltration and erosion

Achieved by:Multi-layered cover system

1. Sealing layer - oxidation control and geochemical security

2. Soil layer – insulation & vegetation growth medium

Slope geometry – surface runoff and erosion control

Embankment cover design

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Geochemical objectives & targets

Primary method to achieve these targets is limestone addition to the waste rock

Sealing layer will reduce leachate flow rates

Objectives - minimise risk of adverse water quality effects- minimum after-care and long term geochemical security

TargetsMn – receiving water quality standard in consents is 2 g/m3

Reduction in underdrainage flows rates < 1 L/s

pH = 6 for minimising Mn release and control of Al and Cu

SO4 – 250 g/m3 recommended at closure (c. 10 kg/ha/d = double worst case estimate)

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Cover design

Dec 2009 Waihi Dry Covers 12

Storage 2 cover assessment

Diffusion is the dominant O2 transport mechanism in WRDNo advection or convection

Oxygen concentration < 1% at 2m depth from surfaceNo O2 below 5m depth even in areas not covered

No oxidation at depth

Pyrite oxidation is not significant below 1-2m depth in uncovered areas

Some heating during construction of PAF waste areas

Sealing layer diffusion coefficients < 2.1 x 10-8 m2/s2-orders of magnitude lower than bulk waste rock

Important oxygen barrier

Sulphate generation rate - 93-99.5% reduction (to 5.5 kg/ha/d) with use of cover

ANSTO study in 1994 – oxygen, temperature, and diffusion on waste rock and covers

ANSTO = Australian Nuclear Science & Technology Organisation

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Zone G – sealing cover specifications

Diffusion coefficient is the critical factor determining the performance of zone G

De - function of moisture content or degree of saturation

90% degree of saturation (81-100% range) @ De = 2.1 x 10-8 m2/s

zone G permeability < 1 x 10-8 m/s

Embankment design and material specifications

Construction control - compact waste rock in 250mm thick layers – zone G

Field saturation levels – 85% minimum, 90% median

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Construction Monitoring

Geotechnical testing of foundations and placed embankment fill

Dry Density

Air Voids

Degree of saturation

Scala Penetrometer

Shear vane

Particle Size Distribution

Permeability – Triaxial Cell – zone G sealing layer

Solid Density of Soil Particles

Failed tests result in re-compaction of finished areas and retesting

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Sealing layer reduces leachate flows

Prevention of water ingress

Zone B – tails upstream barrier

Zone G – embankment cover

Graph shows performance for Storage 2 – L9 had the highest flows 0.00

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ar-97

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Expected flows less than 0.25 L/s

Storage 2 zone G – performance

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Sulphate trends

250 g/m3

recommended at closure

Equiv. 10 kg/ha/d

= double worst case estimate

Zone G – performance

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Storage 1A – slope geometry

Slopes• 4:1 to 3.2:1 (horiz:vert)• 10 m berms - reverse & longitudinal fall• collector sumps and subsoil drainage

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Pasture Dry Weight Monitoring Locations

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TSF2-A

TSF2-B

TSF2-C

TSF1A-D

Con

trol

Pasture Productivity

Storage 2dry matter (grass) production

18 embankment sites

6 control sites-flat alluvials

Average = 15 kg/ha/d (range 9-23 kg/ha/d)

Performance target 80% - met in 2008

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Monitoring & inspection

Peer review

Receiving water quality standards for discharge & no adverse effects

Successful Closure

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Contributors

Pieter Fransen - Environmental Manager, Newmont Waihi Gold

Jeff Ruddock - Surface Mining Manager, Newmont Waihi Gold

Trevor Matuschka - Geotechnical Engineer, Engineering Geology Ltd, NZ

Stuart Miller - Environmental Geochemistry International Pty Ltd, NSW

Wayne Russell – Groundwater Services Ltd, NZ