The Role of Vetiver Grass in the Rehabilitation ofToxic and Contaminated Lands in Australia

Paul Truong and Dennis Baker , Resource Sciences Centre , Department of Natural Resources, Brisbane, Australia.

1. INTRODUCTION

There has been increasing concerns in Australia and world wide about the contamination of the environment by urban wastes and 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 .

Table 1: Investigation thresholds for contaminants in soils (Reference 1)

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 ceased 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) which has been widely known for its effectiveness in erosion and sediment control (4.), has also been found to be highly tolerant to extreme soil conditions including high metal concentrations (11,12,13). Research and field applications in Queensland have shown that vetiver grass is suitable for the rehabilitation of contaminated lands.

All the research and applications reported in this paper were conducted using the genotype Monto ( registered in Australia as Monto vetiver ), which is genetically equivalent to the majority of non-fertile genotypes such as Sunshine (USA), Vallonia (South Africa ) and Guiyang (China) ( R.P. Adams, Baylor University, pers.com..). Therefore the following results can be applied with confidence in China when the Guiyang cultivar is used.

2. TOLERANCE

2.1 Tolerance to low soil pH and Mn toxicity.

Experimental results from glasshouse studies where soil pH was modified by S and CaCO3 (from 3.3 to 8.0) are shown in Table 2. While very high levels of Mn up to 578 mgKg-1 were recorded, the Al concentration was very low (2.5 mgKg-1). The high extractable Mn concentrations in the soil would have a major effect on plant growth.

Table 2 shows that when adequately supplied with N and P, vetiver can grow in soils with extremely high acidity and Mn. Vetiver growth was not affected and no obvious symptoms were observed when the extractable Mn in the soil reached 578 mgKg-1, soil pH as low as 3.3 and plant Mn 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 Mn in plant tops when growing in mine spoils containing 106 mgKg-1 of Mn (10). Therefore vetiver which tolerates much higher Mn concentrations both in the soil and in the plant, can be used for the rehabilitation of lands highly contaminated with Mn.

Table 2: Effects of soil pH on soil Al and Mn levels, plant yield and plant Mn.

*Extractable Mn by 25:50 Soil/0.00 5M DTPA T = Trace

2.2 Tolerance to low soil pH and Al toxicity.

The application of S and CaCO3 produced a wide range in soil pH (from 2.1 to 7.4) for this experiment (Table 3). Both exchangeable Al and Mn concentrations in the soil were directly affected by soil pH but exchangeable Al gave greater response. It has been previously reported that in most cases, where both soil Al and Mn are high, yield reduction was due to Al toxicity rather than Mn (7). From the results presented in 2.1, vetiver growth in this trial was affected by high levels of Al in the soil, not by Mn.

Table 3 shows that when adequately supplied with N and P, Vetiver produced excellent growth even under extremely acidic conditions (pH = 3.8) and at a very high level of soil Al saturation percentage (68%). Vetiver did not survive an Al saturation level of 90% with soil pH = 2.0; although a critical level of Al could not be established in this trial, observation during the trial indicated that the toxic level for Vetiver would be between 68% and 90%.

Table 3: Effects of soil pH and Al concentrations at planting and harvest on vetiver yield

*Al Saturation%={Exch Al/Exch(Al+Ca+K+Mg+Na)}x100

**Extractable Mn in mgkg-1

2.3 Tolerance to soil salinity

Saline threshold level.

Results of glasshouse 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 4).

Saline tolerance level.

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. At the time of planting the area was completely bare and salt crystals could be observed on the lower part of the slope and soil salinity levels recorded, ranged from ECse = 3.85 dSm-1 at the top of the slope to 44.59 dSm-1 at the bottom of the slope.

Table 4: Salt tolerance level of Vetiver grass as compared with some crop and pasture species grown in Australia.

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 5), reaching a height of 60cm in eight weeks (12).

In a column experiment in the glasshouse, it was established that the high salt tolerance of vetiver is due partly to its deep rooting characteristics which enable it to escape the high salt concentrations on the soil surface (12). These results are supported by observation in Fiji, where vetiver is found growing in highly saline soils, on tidal flats next to mangrove swamps.

Table 5: Soil salinity levels corresponding to different species establishment.

2.4 Tolerance to strongly alkaline and strongly sodic soil conditions

A coal mine overburden sample used in this trials was extremely sodic, with ESP (Exchangeable Sodium Percentage) of 33%. Soil with ESP higher than 15 is considered to be strongly sodic (7). Moreover, the sodicity of this overburden is further exacerbated by the very high level of magnesium (2400 mgKg-1) compared to Ca ( 1200 mgKg-1) (Table 6).

Results from added soil amendments show that while gypsum had no effect on the growth of vetiver, N and P greatly increased its yield. DAP (di ammonium phosphate) application alone at 100 kgha-1 increased Vetiver dry matter yield 9 times from 5.4 to 48.9g per pot. Higher rates of gypsum and DAP did not to improve vetiver growth further. These results were strongly supported by field observations.

Table 6: Chemical analysis of the coal mine overburden.

3. TOLERANCE TO HEAVY METALS

3.1 Tolerance levels.

A series of glasshouse trials was carried out to determine the tolerance of vetiver to high soil levels of heavy metals. Table 7 shows the treatments of the five metals tested. These levels were chosen to represent the range commonly found in contaminated sites in Queensland.
In a hydroponic trial, it was found that most vascular plants are highly sensitive to heavy metal toxicity (Table 8) (3) and most plants were also reported to have very low threshold levels for As, Cd and Ni in the soil (Table 10) (2). Results of these trials confirm that vetiver is highly tolerant to these heavy metals.

Arsenic:

Results in Table 10 show that vetiver yield was significantly reduced when soil arsenic level at 250 mgKg-1 . These results did not establish the toxic threshold for vetiver, but it is likely to be between 100 and 250 mgKg-1, an extremely high concentration compared to the threshold of 0.02 - 7.5 mgKg-1 reported (Tables 8 and 9). Arsenic soil levels higher than 20 mgKg-1 are required by government authorities in Australia to be rehabilitated (Table 1). Vetiver is therefore a highly suitable species for rehabilitating such sites (11).

Table 7: Induced soil heavy metal levels and chemicals used.

(1) As Di sodium methyl arsenate (DSMA) (4) As copper sulphate (CuSO4)

(2) As Cadmium sulphate (CdSO4) (5) As nickel chloride (Ni Cl2)

(3) As Dipotassium chromate (K2CrO4)

Table 8: Threshold levels of the heavy metals in hydroponic trial (3)

Table 9: Threshold soil levels of some heavy metals (2)

Cadmium:

Results from Table 10 indicate that vetiver growth was significantly affected by soil Cd levels at and higher than 60 mgKg-1. Although its yield was reduced by 48% at 120 mgKg-1, vetiver maintained a slow and continual growth up to that level in the soil which is extremely high compared to with the threshold of 1.5 mgkg-1 (Table 9). Vetiver can be established in most contaminated industrial wastes and gold mine tailings sites with Cd contamination in Queensland (11).

Copper:

Copper is an essential plant nutrient but becomes toxic to plants at high levels in the soil. Results indicate that the critical soil level for vetiver is between 50 and 100 mgkg-1 which is very high compared with the threshold of between 0.5 and 8.0 mgkg-1 (3). Reasonable growth continued at a soil Cu level of 100 mgKg-1.

Chromium:

Vetiver growth is significantly affected by soil Cr at 600 mkg-1. Its critical level in the soil is between 200 and 600 mgKg-1 which are extremely high compared to the threshold of between 0.5 and 10.0 mgKg-1 reported (Table 8) (11).

Nickel:

Although nickel is considered a trace element for plants and is a constituent of the important enzyme urease, it is extremely toxic to plants at high concentrations (5). The toxic threshold level of Ni in the soil has been reported between 7 and 10 mgKg-1 for most plants (2), but in this series of trials, 58% of growth still occurred at soil concentrations of 100 mgKg-1.

Table 10: Dry matter yield of vetiver as affected by various levels of As, Cd, Cu, Cr and Ni in the soil.

*Different alphabets indicate significant difference

3.2 Plant tops and roots concentrations of heavy metals

The heavy metal contents of plant tops and roots presented in Table 10 confirm the extremely high tolerance of vetiver to these elements . For As, the toxic content for most plants is between 1 and 10 mgKg-1, for vetiver levels up to 72 mgKg-1 are tolerated. Similarly for Cd, the toxic content for vetiver is 48 mgkg-1 and for other plants between 5 and 20 mgkg-1. An impressive finding was that while the Cr and Ni toxic contents for vetiver were 18 and 347 mgKg-1 respectively, growth of most plants was affected at the content between 0.02 and 0.20 mgKg-1 for Cr and between 10 and 30 mgKg-1 for Ni. Vetiver had similar tolerance to Cu as other plants at 15 mgKg-1 ( 5,6).

3.3 Distribution of heavy metals in the plant

Although the data for root contents were not complete due to the shortage of root material, results in Table 10 indicate that while vetiver retains most of the Cd, Cr and Cu in the root, a greater proportion of Ni and possibly As were translocated to the tops.

3.4 Tolerance to some other heavy metals

Early results of further study on vetiver tolerance to heavy metals indicated that vetiver is also highly tolerant to high levels of mercury, selenium, lead and zinc in the soil.

3.5 Applications

The above results have established that vetiver is an excluder (or index plant) to some heavy metals and an accumulator to others. When vetiver is an accumulator of As and Ni in the shoot, its use in the rehabilitation of contaminated lands will have both positive and negative implications.

On the positive side, vetiver will not only provide a very effective means of stopping the spreading of these heavy metals from these sites, but also when harvested and removed from the sites and disposed of safely elsewhere, the level of As and Ni in the soil can be gradually lowered with time.

On the negative side, these heavy metals in the plant shoots can enter the food chain and would become a health risk if animals are allowed to graze on the rehabilitated sites.

4.0 REHABILITATION OF CONTAMINATED LANDS

Old landfills, and industrial waste dumps such as tanneries, tick control sites etc are usually contaminated with heavy metals such as As, Cd, Cr and Hg. As these heavy metals are highly toxic to humans, the movement of these metals off-site must be controlled.

The erosion at an old landfill site near Brisbane is a great concern to the local community as contaminated materials and leachate polluted adjacent ground and water courses. The landfill was capped with 1m of topsoil and successfully rehabilitated with local vegetation but the sides (70% slope) which could not be easily topsoiled were bare of vegetation and were highly erodible.

Rehabilitation works was carried out by planting vetiver rows on the slopes at 1m VI (Vertical Interval) spacing. For leachate control, vetiver was planted 0.5m apart over the entire area of 3 x 20m at the toe of the slope where leachate appeared. Although the landfill was contaminated (Table 11), vetiver established easily and grew well with N and P application at planting. The slopes were completely stabilised within 12 months and local vegetation established naturally between the hedge rows. During the same period, leachate export was reduced substantially during the wet season and was eliminated during the dry season. When the slope was stabilised native trees and shrubs were planted to complete the rehabilitation works. In this application vetiver is a pioneer plant.

Table 11: A typical heavy metal profile of the old landfill near Brisbane.

* values exceeds permitted levels.

5. REHABILITATION OF ACID SULFATE SOILS (ASS).

ASS have severely affected productivity of coastal sugarcane farms in Queensland and New South Wales in Australia. In north Queensland erosion of drainage channel banks of ASS on a cane farm was persistent and severe. Several attempts in the past to stabilise these banks with various plant species had failed due to plant death. Vetiver grass was planted along the banks of these channels late in 1995 and after 8 months, although not fully mature, vetiver had successfully stabilised these banks. Table 12 indicates that actual ASS exist and all samples analysed have high to very high potential ASS. Vetiver was successfully established on these soils without any fertiliser and reached a height of 80cm after 8 months. Vigorous growth occurred when DAP (300 kg/ha) was applied at planting (13).

6. REHABILITATION OF MINING SITES

6.1 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 6). Vetiver was established successfully on these soils and has stabilised the mound slopes (20%) and promoted the establishment of other sown and native pasture species. Similar successful results were also obtained in a gold mine overburden site.

Table 12: Acidity and aluminium levels of the ASS at Babinda.

TAA = Total Actual Acidity; TPA = Total Potential Acidity

6.2 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 this site where the substrate was saline, highly sodic and extremely low in N and P. The substrate contained high levels of soluble S, Mg and Ca. Plant available Cu, Zn, Mg and Fe were high (9).

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. Vetiver survival was significantly increased by mulching but fertiliser application by itself had no effect. Mulching and fertilisers together increased growth of vetiver by 20tha-1 which was almost 10 times higher than that of marine couch. The results confirm the findings from glass house trials.

6.3 Gold mine tailings

Fresh tailings: Fresh gold tailings are typically alkaline (pH = 8), low in plant nutrients and very high in free sulphate (830 mgKg-1 ) and total S (1-2%). Vetiver was established and grew very well on these tailings without fertilisers, but growth was improved by the application of 500 kg/ha of DAP.

Old tailings - Due to high S content, old gold mine tailings are often extremely acidic (pH 2.5-3.5) and low in plant nutrients. Revegetation of these tailings is very difficult and often very expensive, the bare soil surface is highly erodible. These tailings are often the source of contaminants, both above ground and underground to local environment.

Trials were conducted on two old (8 year) tailings sites, one is typified by a soft surface and the other with a hard crusty layer. The soft top had pH of 3.6, sulphate at 0.37% and total S at 1.31%. The hardtop had pH of 2.7, sulphate at 0.85% and total S at 3.75%. Both materials were low in plant nutrients. Less lime was needed to increase pH in the soft top than with the hard top material. For example to bring pH to 6.4, only 10tha-1 of lime was needed for the soft top while the hard top required 40tha-1. (Table 13). Furthermore on the soft top site, vetiver yield was significantly improved by 5tha-1 of lime and further increase was obtained up to 15tha-1 of lime. Conversely, with the hardtop material, significant yield was obtained only at 30 tha-1 of lime, higher rate of liming had no effect on vetiver growth.

On both materials, higher rates of fertiliser up to 500 kgha-1 of DAP did not improve growth. The expected interaction between liming and DAP application did not occur. This may be due to interference of high liming rate on the availability of other plant nutrients such as P. Further research on this aspect and the potential acidity present in these soils is in progress.

Table 13. Effect of liming rates on tailings acidity and vetiver growth

7. Conclusion

From the above it can be concluded that vetiver can play a major role in the rehabilitation of toxic and contaminated lands. For successful application of vetiver a full understanding of the chemical properties of the soil requiring rehabilitation is needed for best results.

REFERENCE

1. Australian and New Zealand Guidelines for the Assessment and Management of Contaminated Sites (1992). Australian and New Zealand Environment and Conservation Council, and National Health and Medical Research Council, January 1992 .

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3. Bowen, H.J.M. (1979). Plants and the Chemical Elements. (Ed.). Academic Press, London.

4. Greenfield, J.C. (1989). Vetiver Grass: The ideal plant for vegetative soil and moisture conservation. ASTAG - The World Bank, Washington DC, USA.

5. Kabata, A. and Pendias, H. (1984). Trace Elements in Soils and Plants. CRC Press, Florida.

6. Lepp, N.W. (1981). Effects of heavy metal pollution on plants. Vol.1: Effects of trace elements on plant functions. (Ed.) Applied Science Publication. London.

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8. Northcote, K.H. and Skene, J.K.M. (1972). Australian Soils with Saline and Sodic Properties. CSIRO Div. Soil. Pub. No. 27.

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10. Taylor, K.W., Ibeabuchi, I.O. and Sulford (1989). Growth and accumulation of forage grasses at various clipping dates on acid mine spoils. J. Environ. Sci. and Health A24: 195-204.

11. Truong, P.N. and Claridge, J. (1996). Effects of heavy metal toxicities on Vetiver growth. Proc. First Int. Vetiver Conf. Thailand (in press).

12. Truong, P.N., Gordon, I. and Baker, D. (1996). Tolerance of Vetiver grass to some adverse soil conditions. Proc. First Int. Vetiver Conf., Thailand (in press).

13. Truong, P.N. and Baker, D. (1996). Vetiver grass for the stabilisation and rehabilitation of acid sulfate soils. Proc. Second National Conf. Acid Sulfate Soils, Coffs Harbour, Australia pp. 196-198.