VETIVER GRASS TECHNOLOGY FOR
MINE TAILING REHABILITATION
Paul N. V. Truong
Resource Sciences Centre, Queensland Department of Natural Resources
(Paper prepared for the First Asia-Pacific Conference on Ground and Water Bio-engineering, Manila April 1999)
The Vetiver Grass Technology (VGT) was first developed for soil and water conservation in farmlands. While this application still plays a vital role in agricultural lands, vetiver grass unique morphological, physiological and ecological characteristics has a key role in the area of environmental protection.
Unique morphological characteristics include a massive finely structured and deep root system capable of reaching 3-4m in the first year. In addition vetiver is tolerant to extreme climatic variation such as prolonged drought, flood, submergence and extreme temperature. It is also tolerance to wide range of soil pH, from 3.0 to 10.5, highly tolerant to soil salinity, sodicity, acidity, Aluminium and Manganese toxicities, and heavy metals such as Arsenic, Cadmium, Chromium, Nickel, Lead, Zinc, Mercury, Selenium and Copper in the soil.
In Australia vetiver has been successfully used to stabilise mining overburden and highly saline, sodic, magnesic and alkaline (pH 9.5) tailings of coal mine and highly acidic (pH 2.7) and high arsenic tailings of gold mines.
In South Africa vetiver has been used very effectively to stabilise/rehabilitate "slimes dams". Rehabilitation trials at de Beers diamond mine slimes dams confirms that vetiver can survive in very harsh environments where surface temperature of the black kimberlite exceeds 550C. Wastes and slimes dams from platinum and gold mines have also been successfully rehabilitated with Vetiver.
There has been increasing concerns in Australia and world wide about the contamination of the environment by by-products of rural, industrial and mining industries. The majority of these contaminants are high levels of heavy metals which can affect flora, fauna and humans living in the areas, in the vicinity or downstream of the contaminated sites. Table 1 shows the maximum levels of heavy metals tolerated by environmental and health authorities in Australia and New Zealand.
Concerns about the spreading of these contaminants have resulted in strict guidelines being set to prevent the increasing concentrations of heavy metal pollutants. In some cases industrial and mining projects have been stopped until appropriate methods of decontamination or rehabilitation have been implemented at the source.
Methods used in these situations have been to treat the contaminants chemically, burying or to remove them from the site. These methods are expensive and at times impossible to carry out, as the volume of contaminated material is very large, examples are gold and coal mine tailings.
If these wastes cannot be economically treated or removed, off-site contamination must be prevented. Wind and water erosion and leaching are often the causes of off-site contamination. An effective erosion and sediment control program can be used to rehabilitate such sites. Vegetative methods are the most practical and economical, however, revegetation of these sites is often difficult and slow due to the hostile growing conditions present which include toxic levels of heavy metals.
Vetiver grass (Vetiveria zizanioides L.), due to its unique morphological and physiological characteristics, which has been widely known for its effectiveness in erosion and sediment control (Greenfield,1989), has also been found to be highly tolerant to extreme soil conditions including heavy metal contaminations (Truong and Baker,1998).
Table 1: Investigation Thresholds for Contaminants in Soils ( ANZ, 1992)
*Maximum levels permitted, above which investigations are required.
This paper highlights research results which show the wide ranging tolerance of vetiver to adverse conditions and heavy metal toxicities, and also field applications in Australia and South Africa where vetiver grass is highly effective in the rehabilitation of mining waste particularly contaminated tailings. All the research and applications reported in this paper were conducted using the genotype registered in Australia as Monto vetiver, but DNA typing has shown that Monto is genetically identical to the majority of non-fertile genotypes such as Sunshine (USA), Vallonia (South Africa ) and Guiyang (China) (Adams and Dafforn, 1997). Therefore the following results can be applied with confidence when these cultivars are used for mine rehabilitation.
2.0 TOLERANCE TO ADVERSE SOIL CONDITIONS
2.1 Tolerance to High Acidity and Manganese Toxicity.
Experimental results from glasshouse studies show that when adequately supplied with nitrogen and phosphorus fertilisers, vetiver can grow in soils with extremely high acidity and manganese. Vetiver growth was not affected and no obvious symptoms were observed when the extractable manganese in the soil reached 578 mgKg-1, soil pH as low as 3.3 and plant manganese was as high as 890 mgKg-1. Bermuda grass (Cynodon dactylon) which has been recommended as a suitable species for acid mine rehabilitation, has 314 mgKg-1 of manganese in plant tops when growing in mine spoils containing 106 mgKg-1 of manganese (Taylor et al, 1989). Therefore vetiver which tolerates much higher manganese concentrations both in the soil and in the plant, can be used for the rehabilitation of lands highly contaminated with manganese.
2.2 Tolerance to High Acidity and Aluminium Toxicity.
Results of experiments where high soil acidity was induced by sulphuric acid show that when adequately supplied with nitrogen and phosphorus fertilisers, vetiver produced excellent growth even under extremely acidic conditions (pH = 3.8) and at a very high level of soil aluminium saturation percentage (68%). Vetiver did not survive an aluminium saturation level of 90% with soil pH = 2.0; although a critical level of aluminium could not be established in this trial, observation during the trial indicated that the toxic level for vetiver would be between 68% and 90% (Truong, 1996; Truong and Baker,1996).
2.3 Tolerance to High Soil Salinity
Results of saline threshold trials showed that soil salinity levels higher than ECse = 8 dSm-1 would adversely affect vetiver growth while soil ECse values of 10 and 20 dSm-1 would reduce yield by 10% and 50% respectively. These results indicate vetiver grass compares favourably with some of the most salt tolerant crop and pasture species grown in Australia (Table 2).
In an attempt to revegetate a highly saline area (caused by shallow saline groundwater) a number of salt tolerant grasses, vetiver, Rhodes (Chloris guyana) and saltwater couch (Paspalum vaginatum) were planted. Negligible rain fell after planting so plant establishment and growth were extremely poor but following heavy rain during summer (nine months later), vigorous growth of all species was observed in the less saline areas. Among the three species tested, vetiver was able to survive and resume growth under the higher saline conditions (Table 3), reaching a height of 60cm in eight weeks (Truong,1996). These results are supported by observation in Fiji and Queensland, where vetiver was found growing in highly saline tidal flats next to mangrove.
2.4 Tolerance to Strongly Alkaline and Strongly Sodic Soil Conditions
A coal mine overburden sample used in this trial was extremely sodic, with ESP (Exchangeable Sodium Percentage) of 33%. Soil with ESP higher than 15 is considered to be strongly sodic (Nothcote and Skene, 1972). Moreover, the sodicity of this overburden is further exacerbated by the very high level of magnesium (2400 mgKg-1) compared to calcium ( 1200 mgKg-1) (Table 4).
Results from added soil amendments show that while gypsum had no effect on the growth of vetiver, nitrogen and phosphorus fertilisers greatly increased its yield. DAP (di ammonium phosphate) application alone at 100 kgha-1 increased vetiver dry matter yield 9 times. Higher rates of gypsum and DAP did not to improve vetiver growth further. These results were strongly supported by field observations.
Table 4: Chemical Analyses of the Coal Mine Overburden.
* ESP = Na % of total cations
3.O TOLERANCE TO HEAVY METALS
3.1 Tolerance Levels and Shoot Contents of Heavy Metals.
A series of glasshouse trials was carried out to determine the tolerance of vetiver to high soil levels of heavy metals. Literature search indicated that most vascular plants are highly sensitive to heavy metal toxicity and most plants were also reported to have very low threshold levels for arsenic, cadmium, chromium, copper and nickel in the soil. Results shown in Table 5 demonstrate that vetiver is highly tolerant to these heavy metals. For arsenic, the toxic content for most plants is between 1 and 10 mgKg-1, for vetiver the threshold level is between 21 and 72 mgKg-1. Similarly for cadmium, the toxic threshold for vetiver is 45 mgkg-1 and for other plants between 5 and 20 mgkg-1. An impressive finding was that while the toxic thresholds of vetiver for chromium is between 5 and 18 mgkg-1 and that for nickel is 347 mgKg-1, growth of most plants is affected at the content between 0.02 and 0.20 mgKg-1 for chromium and between 10 and 30 mgKg-1 for nickel. Vetiver had similar tolerance to copper as other plants at 15 mgKg-1 (Kabata and Pendias,1984; Lepp, 1981).
Table 5: Threshold Levels of Heavy Metals to Vetiver Growth
NA Not available
3.2 Distribution of Heavy Metals in the Vetiver Plant
Results in Table 6 show that the distribution of heavy metals in vetiver plant can be divided into three groups:
The important implications of these findings are that when vetiver is used for the rehabilitation of sites contaminated with high levels of arsenic, cadmium, chromium and mercury, its shoots can be safely grazed by animals or harvested for mulch as very little of these heavy metals are translocated to the shoots. As for copper, lead, nickel, selenium and zinc their uses for the above purposes are limited to the thresholds set by the environmental agencies and the tolerance of the animal concerned.
In addition, although vetiver is not a hyper-accumulator it can be used to remove the some heavy metals from the contaminated sites and disposed off safely else where, thus gradually reducing the contaminant levels. For example vetiver roots and shoots can accumulate more than 5 times the chromium and zinc levels in the soil (Table 6).
Table 6: Distribution of Heavy Metals in Vetiver Shoots and Roots.
BQ Below Quantification
4.0 REHABILITATION OF MINE TAILINGS IN AUSTRALIA
4.1 Coal and Gold Mines Overburden
The overburden of open cut coal mine in central Queensland is generally highly erodible. These soils are usually sodic and alkaline (Table 4). Vetiver has established successfully on these soils and stabilised the spoil dump with 20% slopes and promoted the establishment of other sown and native pasture species. Similar successful results were also obtained on a gold mine overburden site.
4.2 Coal Mine Tailings
In an attempt to rehabilitate an old coal mine tailings dam, (surface area of 23 ha and capacity of 3.5 million cubic metres) a trial was set up to select the most suitable species for the rehabilitation of this site. The substrate was saline, highly sodic and extremely low in nitrogen and phosphorus. The substrate contained high levels of soluble sulfur, magnesium and calcium. Plant available copper, zinc, magnesium and iron were also high. Five salt tolerant species were used: vetiver, marine couch (Sporobolus virginicus), common reed grass (Phragmites australis), cumbungi (Typha domingensis) and Sarcocornia spp. Complete mortality was recorded after 210 days for all species except vetiver and marine couch. Vetivers survival was significantly increased by mulching but fertiliser application by itself had no effect. Mulching and fertilisers together increased growth of vetiver by 2 tha-1 which was almost 10 times higher than that of marine couch (Radloff et al). The results confirm the findings from glass house trials.
Fresh tailings: Fresh gold tailings are typically alkaline (pH = 8-9), low in plant nutrients and very high in free sulphate (830 mgKg-1 ), sodium and total sulfur (1-4%). Vetiver established and grew very well on these tailings without fertilisers, but growth was improved by the application of 500 Kgha-1of DAP.
Vetiver is now being trialed for a large-scale application to control dust storm and wind erosion on a 300ha tailings dam. When dry the finely ground tailings material can be easily blown away by wind storms if not protected by a surface cover. As gold tailings are often contaminated with heavy metals, wind erosion control is a very important factor in stopping the contamination of the surrounding environment. The usual method of wind erosion control in Australia is by establishing a vegetative cover, but due to the highly hostile nature of the tailings, revegetation is very difficult and often failed when native species are used. The short term solution to the problem is to plant a cover crop such as millet or sorghum, but these species do not last very long. Vetiver can offer a long term solution by planting the rows at spacing of 10m to 20m to reduce wind velocity and at the same time provide a less hostile environment (eg shading and moisture conservation) for local native species to established voluntarily later.
Old tailings - Due to high sulfur content, old gold mine tailings are often extremely acidic (pH 2.5-3.5), high in heavy metals and low in plant nutrients. Revegetation of these tailings is very difficult and often very expensive and the bare soil surface is highly erodible. These tailings are often the source of contaminants, both above ground and underground to the local environment. Table 7 shows the heavy metal profile of gold mine tailings in Australia. At these levels some of these metals are toxic to plant growth and also exceed the environmental investigation thresholds (ANZ, 1992).
Field trials conducted on two old (8 year) gold tailings sites, one is typified by a soft surface and the other with a hard crusty layer. The soft top site had a pH of 3.6, sulphate at 0.37% and total sulfur at 1.31%. The hard top site had a pH of 2.7, sulphate at 0.85% and total sulfur at 3.75% and both sites were low in plant nutrients. Results from both sites indicated that when adequately supplied with nitrogen and phosphorus fertilisers (300Kgha-1of DAP) excellent growth of vetiver was obtained on the soft top site (pH=3.6) without any liming. But the addition of 5tha-1 of agricultural lime significantly improved vetiver growth. On the hard top site (pH=2.7) although vetiver survived without liming, the addition of lime (20tha-1) and fertiliser (500kgha-1 of DAP) improved vetiver growth greatly.
Table 7: Heavy Metal Contents of a Representative Gold Mine Tailings in Australia.
NA Not available
4.4 Bentonite Tailings
Bentonite mine tailings (reject) is extremely erodible as they are highly sodic with Exchangeable Sodium Percentage (ESP) values ranging from 35% to 48%, high in sulphate and extremely low in plant nutrients. Revegetation on the tailings has been very difficult as sown species were often washed away by the first rain and what left could not thrive under these harsh conditions. With adequate supply of nitrogen and phosphorus fertilisers vetiver established readily on this tailings, the hedges provided erosion and sediment control, conserved soil moisture and improved seedbed conditions for the establishment of indigenous species..
Rehabilitation trials conducted by De Beers on both tailings dumps and slimes dams at several sites, have found that vetiver possessing the necessary attributes for self sustainable growth on kimberlite spoils. Vetiver grew vigorously on the alkaline kimberlite, containing run off, arresting erosion and creating an ideal micro-habitat for the establishment of indigenous grass species. Rehabilitation using vetiver was particularly successful on kimberlite fines at Cullinan mine where slopes of 35 degrees are being upheld. It is clear that vetiver is likely to play an increasingly important role in rehabilitation and, as a result of this, nurseries are being established at several mines (Knoll, 1997).
At Premier (800mm annual rainfall) and Koffiefonteine (300mm rainfall) diamond mines where surface temperature of the black kimberlite often exceeds 55oC, at this temperature most seeds are unable to germinate. Vetiver planted at 2m VI (Vertical Interval) provided shades that cool the surface and allowing germination of other grass seeds (Grimshaw pers.com.).
Vetiver has also been used successfully in the rehabilitation of slimes dams at the Anglo American platinum mine at Rastenburg and the Velkom, President Brand gold mine (Tantum pers.com.)
From the research results and applications presented above, VGT is highly suitable for the rehabilitation of contaminated mining wastes and tailings. For successful application of vetiver a full understanding of the chemical properties of the materials requiring rehabilitation is needed for best results.
Dr. Paul Truong, Principal Soil Conservationist, Leader Bio-Engineering and Land Rehabilitation Group, Resource Sciences Centre, Queensland Department of Natural Resources, Brisbane, Australia
Dr. Truong has over 20 years experience in the use of vegetation for erosion and sediment control, land stabilisation and rehabilitation in tropical and subtropical Australia. In the last 10 years he has concentrated on the application of the Vetiver Grass Technology for the above purposes. His pioneering research & development on Vetiver Grass Technology ( VGT) have led to the extension of VGT beyond its original application in soil and water conservation in farmlands into the fields of flood erosion control, environmental and infrastructure protection, and mine rehabilitation. He has won several awards for his role in R & D of VGT from the World Bank and The Vetiver Network.