K. Schroeder1, C. G. Rufaut2*, C. Smith3, D. Mains2 and D. Craw2
1 School of Environmental Sciences, University of Otago, PO Box 56, Dunedin, New Zealand.
2 Geology Department, University of Otago, PO Box 56, Dunedin, New Zealand.
3 Geography Department, University of Otago, PO Box 56, Dunedin, New Zealand.
This study- a glasshouse trial
This study addressed the issues of temporary phytostabilisation of tailings material. By means of a greenhouse trial, the most useful species and amendment regime for use on an actual mine site were identified. Gold mine tailings were used as the principal growing medium, with organic matter and fertilizer amendments. This study set out to examine (1) the feasibility and efficiency of establishing a plant cover directly on tailings using commercially available grass and legume seeds, (2) how the plants respond to additions of commonly used soil amendment products, namely compost and fertilizer and (3) levels of metal uptake in plants grown in tailings, particularly arsenic. Topsoil from an undisturbed location within the mine site is included to serve as a comparison to the tailings data.
The compost treatments and plant species tested.
Four treatments were tested (1) tailings + fertilizer, (2) tailings + greenwaste compost, (3) tailings + biosolid blended compost, (4) tailings + no amendment. Greenwaste compost was blended from green garden waste, bark, pumice and a mix of organic fertilizers, whereas the biosolid-blend was a mix of green waste, bark, pumice and slow release fertilizers derived from biosolids.
Five different plant species were chosen for the pot trial based on their ability to tolerate high salt concentrations, have a high sulphur demand and an ability to exist in an optimum pH range. The species included three grasses: Barley (Hordeum vulgare), Rye Corn (Secale cereale), and Italian Ryegrass (Lolium multiflorum), and two legumes: Red Clover (Trifolium pratense) and Lucerne (Medicago sativa).
All species were sown directly into the potted substrates.
Seed germination and increases in plant height measurements were taken regularly over the first 4 weeks and overall biomass measurements were made at the end of the study (10 weeks later).
The tailings, topsoil, greenwaste and biosolid blended composts were analysed for nutrient content (K, Na, Ca, Mg, Organic Carbon, Total N, Olsen P); heavy metal concentrations (Cu, Ni, Cd, Pb); cation exchange capacity (CEC); organic matter content and pH. Electrical conductivity, particle size, bulk density and macroporosity were determined on the tailings and topsoil only.
The winners- Barley and Rye Corn
Barley and Rye Corn were found to offer the best potential for successful temporary phytostabilisation of the Macraes tailings dam surfaces. These two grasses initially germinated well, grew relatively quickly, flowered and seeded in the unamended tailings substrate. However, the successful and sustainable long term use of tailings as a complete growing medium without some kind of amendment appears doubtful. The early flush of plant growth observed in the first two weeks was followed by increasingly frequent symptoms of nutrient deficiency and shoot tip necrosis. The inability of tailings to retain a non-limiting supply of water in the surface layers for early plant establishment is a fundamental restriction. Gold mine tailings typically restrict root growth and make plants vulnerable to desiccation, and the high salt concentration of the tailings from Macraes would also add additional stress.
Poor performance of N-fixing plants
The performance of nitrogen-fixing Red Clover and Lucerne in the unamended tailings was poor relative to the grasses. Although there is a strong benefit in establishing N-fixing plants on mine substrates the unique characteristics of the fresh tailings at the Macraes mine site does seem to support their early establishment. There are a number of factors that may be responsible: legumes rank amongst the most sensitive plants to arsenic toxicity. Nitrogen-fixers also tend to require high levels of available P which was exceptionally low in the tailings material.
Benefits of compost
The pot trial demonstrated that nutrient and physical limitations to plant growth inherent in Macraes tailings were overcome through application of organic compost material. Average biomass per pot was up to four times greater in compost-amended tailings than unamended tailings. It also showed that any effects of As toxicity were negligible in the compost-amended tailings, with plants of all species growing vigorously, including the legumes.
The benefits of adding organic matter to this tailings system are two fold:
- The chemical status of the tailings is improved by the addition of nutrients in a slow release organic form. Secondly, the physical status is improved by the organic matter acting as a store for water, and improving the structure of the tailings resulting in increased water holding capacity, macroporosity, infiltration, and a reduction in bulk density.
- Biosolid-blended composts (derived from sewage sludge) resulted in additional benefits ofincreased nutrient status compared to greenwaste compost. However these sewage based organic materials can act as a conduit for contaminants (such as heavy metals) to enter the system. For this reason, there is a growing body of evidence cautioning against the perceived benefits of adding biosolids compost to terrestrial systems.
Very high rates of fertiliser suppresses plant germination and growth
In this study, the amount of mineral P fertilizer (equivalent to 800 kg P/ha) was applied in order to intentionally trigger As mobilization. A subsample of water leachate from one fertilised pot per plant species showed that consistently higher levels of As were detected in comparison to pots of unamended tailings. A high application rate of phosphate fertilizer to tailings greatly suppressed plant germination and growth in all species. It is likely that phosphate-induced mobilization of As resulted in phytotoxic conditions that caused the suppression of all plant germination and growth.
- Seed germination and plant growth were influenced by both the chemical status of the tailings and the physical status, and in particular the water holding capacity.
- A one-off addition of organic matter in the form of compost resulted in increased plant growth and biomass in the short term, thus achieving a successful temporary plant cover.
- There was no evidence that a single application of biosolid compost elevated substrate metal levels above statutory guidelines, even within our high application rate. However, heavy metal accumulation via repeated applications of biosolid products (Cd, Zn, Pb) and phosphate-based fertilisers (Cd) will need to be monitored if their use becomes standard practice for mining company's to achieve temporary phytostabilisation.
- Elevated concentrations of As were identified in plant material principally grown in partial tailings. Uptake of As may be even higher in pure tailings; a hypothesis that requires further investigation.
- The pot trial findings in this paper need extrapolation to the field situation using more cost-efficient and practical amendment rates. Long term trials are important to establish strategies for initiation of temporary and sustainable vegetation cover on mine tailings without high contamination of heavy metals.