Introduction
Developing stable, functional, sustainable post-mining landscapes requires a holistic, integrated approach. In the past, rehabilitation has focused on individual components in isolation. Achieving long-term success requires combining landform evolution modelling (LEM), detailed landform cover design, and growing medium investigations. This integrated method allows creation of geoformed and managed environments that are resilient, ecologically functional, and meet set post-closure land use aims. This paper outlines the importance of this synergy, drawing upon examples from SGME experience.
Challenge of Land Rehabilitation
Disturbed lands, such as those from mining operations, present large rehabilitation challenges. Substrates like tailings can possess unique geochemical properties, including metal mobility, extreme pH levels, salinity issues, and nutrient shortfalls. Establishing a stable geoformed landform requires not only addressing surface erosion but also managing subsurface processes like infiltration, contaminant transport, and capillary rise, and promoting long-term vegetation growth needed for stabilisation and ecological function. Failure to address any one aspect can compromise an entire rehabilitation effort, leading to environmental risks and long-term management liabilities.
Integrated Approach: A Synergy of Disciplines
A sound rehabilitation plan relies on interplay between understanding starting materials, designing interventions, and predicting long-term performance:
Growing Medium Investigations: This is a foundational piece. It involves detailed characterisation of substrate (eg tailings, waste rock, and available soils). Key analyses include:
- Geochemistry: Assessing reactivity (like acid-forming potential), identifying mobile metals, and understanding background mineralogy.
- Physical properties: Evaluating texture, water holding capacity, and compaction potential.
- Fertility: Determining pH, electrical conductivity (EC), nutrient status, cation exchange capacity, presence of potentially toxic metals and plant-availability. Understanding a growing medium’s limits and risks (eg metal uptake by vegetation, and poor nutrient supply) informs cover design. Field trials are essential to test amendments (like fertilisers or organic matter) and different growing medium compositions or thicknesses.
Landform Cover Design: Informed by growing medium investigations, a cover system is designed to mitigate identified risks and support target post-closure land use (eg grassland or woodland). Key considerations include:
- Material selection: Choosing suitable materials (eg topsoil, subsoil, waste rock, and geosynthetics) based on availability and function.
- Layering and thickness: Designing profiles to achieve specific goals, such as limiting infiltration, preventing upward capillary rise of contaminants, providing root depth, storing plant-available water. Different designs might incorporate capillary breaks or varying growth layer thicknesses.
- Vegetation selection: Choosing species adapted to the climate, designed cover system, and chemical stresses discovered during growing medium investigations. Trials assessing direct seeding or volunteer species performance can be valuable. Effectiveness of different cover designs is assessed through monitoring key parameters like seepage rates and vegetation health.
Landform evolution modelling (LEM): LEM provides a predictive capability, assessing long-term geomorphic stability of designed landform. Using inputs like:
- Digital Elevation Model (DEM): Representing final landform shape.
- Material Erodibility Characteristics: Based on cover materials and vegetation cover.
- Climate Data: Including rainfall and evaporation patterns. LEM simulates surface water runoff, erosion, deposition processes over extended periods (eg 100 years or more). Models like LAPSUS can predict gully formation, estimate erosion rates, identify areas susceptible to instability. This modelling informs initial landform design (eg slope angles, drainage features) and verifies whether designed cover system and vegetation plan will provide erosion control over time.
Case Study: TSF Rehabilitation, NSW
The rehabilitation approach at a TSF in NSW provides a practical example of this integrated approach. Investigations characterised tailings as chemically unreactive but identified metal mobility concerns and properties challenging for plant growth (alkaline pH, nutrient deficiencies, and high total metals). Various growing medium options and cover designs, including direct seeding into tailings and constructed covers with varying layers and thicknesses, were trialled and assessed for parameters like seepage and vegetation metal uptake. Direct seeding established tolerant species like saltbush. Use of LAPSUS was important to evaluate long-term erosion potential of final landform design incorporating established vegetation. Modelling predicted low erosion rates and long-term stability, provided vegetation cover was maintained. This integration of detailed substrate analysis, cover performance trials, and predictive stability modelling demonstrated that a stable, functional landform trending towards a desired post-mining land use was achievable despite inherent challenges of tailings as a growing medium.
Benefits of Integration
Integrated landform design yields benefits:
- Risk mitigation: Identifies and addresses potential failure mechanisms (eg erosion, contaminant migration, and vegetation failure) early.
- Optimised design: Ensures cover systems are designed for specific substrate challenges and landform configuration.
- Enhanced sustainability: Increases likelihood of achieving long-term physical stability and ecological function.
- Cost-effectiveness: Reduces risk of rehabilitation failures and long-term remedial actions.
- Regulatory confidence: Provides robust evidence to support closure planning and demonstrate long-term performance predictions.
Conclusion
Development of sustainable, geoformed environments, in post-mining landscapes, cannot rely on a piecemeal approach. Integration of detailed growing medium investigations, informed landform cover design, and predictive landform evolution modelling provides a powerful, synergistic framework. By understanding substrate limits, designing mitigation measures through cover systems, modelling long-term geomorphic stability, practitioners can create resilient, functional landscapes that meet closure objectives and minimise long-term environmental risk. Our TSF case study from NSW exemplifies how this integrated approach translates into practical success, demonstrating a clear pathway towards sustainable land rehabilitation.