SUSTAINABLE MINE REHABILITATION: RESOLVING THE TECHNICAL ISSUES

Transcription

1 SUSTAINABLE MINE REHABILITATION: RESOLVING THE TECHNICAL ISSUES GARRY WILLGOOSE*, GREG HANCOCK THE UNIVERSITY OF NEWCASTLE * MEMBER, NSW MINING & PETROLEUM GATEWAY COMMITTEE

2 SUSTAINABLE MINE REHABILITATION Economic landuse Post-mining landuse no less productive than pre-mining? That there is a potential productive landuse after mining may be less productive? Natural landuse That post-mining ecosystems are as sustainable and robust against external impacts as pre-mining. Design lifetimes 100 years, 1000 years, years, (forever?). The time at which rehab stabilises.

3 THREE FOCI OF THIS TALK Erosion, long-term waste containment, and landform stability. Soils and long-term ecosystem sustainability, Constructed soil profiles, nutrients, salts. Final pit voids vs in-filled pits. Long-term term groundwater impacts.

4 LONG TERM LANDFORM STABILITY

5 LANDFORM EVOLUTION MODELLING (SIBERIA, CAESAR, CHILD) Use erosion models to predict the change in a landform over 10 s-1000 s years. Key concepts Traditional erosion processes (water, air). Non-traditional erosion processes (creep, fauna, tree throw, debris flow, landslide). Gully development is a natural outcome of coupling of water erosion and landform over long term (armour failure). Long term stability of protective structures (bunds, benches, covers, rock armours on caps). Post failure behaviour. Monte-Carlo risk assessment of failure probability and postfailure consequences. Geomorphically stable landform design e.g. concave slopes better in the long-term.

6 LANDFORM: RANGER EVOLUTION USING SIBERIA

7 DERELICT LAND APPLICATION The focus of a book chapter in the near future from AUSIMM Hancock and Willgoose Sustainable mine rehabilitation - 25 years of the SIBERIA landform evolution and long-term erosion model, From Start to Finish, AUSIMM. The major missing component is we need to assume/predict surface soil and vegetation properties

8 SOILS EVOLUTION AND CONSTRUCTED SOIL PROFILES

9 SOILS EVOLVE Self armouring as a result of erosion Soil profile development

10 SOILS EVOLUTION PREDICTION Models for evolving soils recently developed (marm, SSSPAM). Some agricultural testing. Not tested at mine sites Key Existing Concepts Evolving surface armour due to water erosion Evolving soil profile due to physical, chemical and biotic processes Prediction of soil grading, water holding capacity, infiltration, organic matter. NB: Soils the media in which vegetation grows Future Extensions Vegetation feedbacks Soil and soil water chemistry

11 SOIL: BATTER EVOLUTION WITH SSSPAM

12 SOIL: RANGER EVOLUTION WITH SSSPAM

13 DERELICT LANDS APPLICATIONS Will soils get better or worse with time? Chemistry, erosion, water holding capacity Will amelioration efforts make problems better or is it better to let it continue to evolve naturally. Probably a function of how far the existing soils/spoils/wastes have already evolved.

14 FINAL PIT VOIDS VERSUS IN-FILLING VOIDS

15 THE ISSUE If final voids are left what is the water quality likely to be and what off-site (groundwater, air) impacts are likely long term? Voids are typically (but not always) a sink for on-site water but will concentrate salts due to evaporation. If final voids are filled offsite impacts are inevitable. What is the offsite impact of groundwater? Long-term groundwater quality has been a major uncertainty in Gateway Certificate assessments. Times of water equilibration years. Times of chemistry equilibration unknown. Ionic chemistry before and after equilibration uncertain.

16 Evaporation

17 Evaporation

18 Evaporation

19 RESULTS Water volume void water (m 3 x10 7 ) ML/day 3.6 ML/day 4.7 ML/day Salt concentration salinity (mg/l) years mg/L mg/L mg/L mg/L mg/L years

20 RESULTS salt from rainfall 3 % salt from spoil 39% salt from groundwater 58%

21 LONG-TERM SPOIL WATER CHEMISTRY Rapid equilibration but some long term trends we don t yet understand. Early AMD response.

22 CONCLUSIONS: THE THREE LONG- TERM FOCI Erosion, long-term waste containment, and landform stability. Widely used internationally in mining and nuclear waste industry for past 25 years. Soils and long-term ecosystem sustainability, Constructed soil profiles, nutrients, salts. Research tool ready for testing on mine sites. Final pit voids vs in-filled pits. Long-term term groundwater impacts. Lots of unknowns central to the fill-nofill argument that need to resolved.