Vetiver Grass Hedges for Control of Runoff and Drain ...
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CSIRO LAND and WATER Vetiver Grass Hedges for Control of Runoff and Drain Stabilisation, Pimpama Queensland By G.D. Carlin , P. Truong , F.J. Cook , E. Thomas , L. Mischke , K. Mischke 1 2 1 3 4 4 1 2 3 4 CSIRO Land and Water, 80 Meiers Road, Indooroopilly, QLD 4068 NR&M, 80 Meiers Road, Indooroopilly, QLD 4068 Gold Coast City Council, PO Box 5042, Gold Coast MC, QLD 9729 Norwell Road, Pimpama, QLD CSIRO Land and Water, Brisbane Technical Report 46/03, February 2003
Transcript of Vetiver Grass Hedges for Control of Runoff and Drain ...
C S I R O L A N D a nd WAT E R
Vetiver Grass Hedges for Control of Runoff
and Drain Stabilisation, Pimpama Queensland
By G.D. Carlin , P. Truong , F.J. Cook , E. Thomas ,
L. Mischke , K. Mischke
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CSIRO Land and Water, 80 Meiers Road, Indooroopilly, QLD 4068
NR&M, 80 Meiers Road, Indooroopilly, QLD 4068
Gold Coast City Council, PO Box 5042, Gold Coast MC, QLD 9729
Norwell Road, Pimpama, QLD
CSIRO Land and Water, Brisbane
Technical Report 46/03, February 2003
© 2003 CSIRO, Gold Coast City Council and Environment Australia (Natural Heritage Trust). This work is copyright. It may be reproduced subject to the inclusion of an acknowledgement of the source. Important Disclaimer: CSIRO Land and Water, Gold Coast City Council and Environment Australia (NHT) advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO Land and Water, Gold Coast City Council and Environment Australia (NHT) (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. ISSN 1446-6163
Australia’s Oceans Policy
Coastal Acid Sulfate Soils Program (CASSP)
Vetiver Grass Hedges for Control of Runoff and Drain Stabilisation, Pimpama, Queensland
FINAL TECHNICAL REPORT
February 2003
G.D. Carlin1, P. Truong2, F.J. Cook1, E. Thomas3, L. Mischke4, K. Mischke4 1CSIRO Land and Water, 80 Meiers Road, Indooroopilly, QLD 4068
2NR&M, 80 Meiers Road, Indooroopilly, QLD 4068 3Gold Coast City Council, PO Box 5042, Gold Coast MC, QLD 9729
4Norwell Road, Pimpama, QLD
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ABSTRACT ................................................................................................................. 3
INTRODUCTION ....................................................................................................... 4
PROJECT OBJECTIVES .......................................................................................... 4
METHODS................................................................................................................... 5
VETIVER ..................................................................................................................... 5 DRAIN WEIR STRUCTURES ......................................................................................... 6 RUNOFF WEIR STRUCTURE......................................................................................... 8 SITE SURVEY .............................................................................................................. 8 INSTRUMENTATION..................................................................................................... 9
Multipoint Sensor Chamber................................................................................... 9 SENSORS................................................................................................................... 11
Combination Sensor ............................................................................................ 11 Turbidity Probe.................................................................................................... 11 Pressure Transducers .......................................................................................... 11 Pumping Sampler ................................................................................................ 11
RESULTS AND PERFORMANCE......................................................................... 12
INITIAL PLANTING .................................................................................................... 13 ESTABLISHMENT....................................................................................................... 13 GROWTH................................................................................................................... 14 CLIMATE CONDITIONS AND RUNOFF EVENTS ........................................................... 17 AN EFFECTIVE BIO-FILTER ........................................................................................ 22 CONTROLLING BANK EROSION.................................................................................. 24 LOWERING OF THE WATER TABLE IN THE DRAIN DUE TO TRANSPIRATION AND UPTAKE
OF NUTRIENTS........................................................................................................... 25 ADOPTION BY THE SUGAR INDUSTRY IN NSW .......................................................... 26
CONCLUSION .......................................................................................................... 26
ACKNOWLEDGEMENTS ...................................................................................... 26
REFERENCES .......................................................................................................... 27
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Abstract Acid sulfate soils are weak mechanically and therefore highly erodible, if drain banks are not properly stabilised they become prone to collapse, dumping into the drains eroded soil and sediments, which are highly acidic and loaded with heavy metals and nutrients. Low flow velocities in the drains allow iron monosulfides and metal oxides to accumulate due to high iron, aluminum and other metal concentrations in drainage waters. The trial established vetiver grass along a section of drain and was compared to an unplanted control section. It was hoped the trial would establish the effectiveness of vetiver grass in controlling stream bank erosion, lowering the frequency of drain maintenance, trapping sediments in runoff water and reducing the acidic loading by exposing less acid sulfate soil in the drain wall to oxidisation and leaching. Due to climatic conditions, this trial has not conclusively shown that vetiver planting in the riparian zone on the drain banks would effectively improve water quality, however the trial demonstrated that vetiver planting has effectively controlled drain bank erosion and is an effective bio-filter during small runoff events. The data collected also suggests that pH improves in a site planted with vetiver and this could be attributed to the vetiver filtering sediments through a build up of organic material at the base of the vetiver plants as the organic material may have an absorption affect on acid generating chemicals present in the runoff water. The reduction in iron (Fe) concentration that was also observed during the trial may be attributed to the filtering by the vetiver of the precipitated Fe out of the water column. These results are only tentative as the events were small and without replication. Future research on the use of vetiver hedges in acid sulfate soils is required.
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Introduction The formations of coastal acid sulfate soils in Australia over the past ten thousand years are of interest here. During this time the sea level rose and sulfates in the seawater mixed with land sediments containing organic matter and iron oxides. The resulting chemical and biochemical reactions produced large quantities of iron sulfides in waterlogged sediments and formed what is known as acid sulfate soils (ASS) (Cook et al., 2000a). When the iron sulfides that are located within these soils are exposed to air, they oxidise and produce sulfuric acid. Urban, agricultural and recreational developments in ASS can result in exposure of sulfides and drainage export to ecosystems of acidic water loaded with heavy metals. This acidic discharge may result in fish kills (Brown et al. 1983, Easton 1989), deterioration of concrete structures and has many other environmental impacts. Early agricultural practices in ASS areas used for cane production established deep drains. It was thought that the deep drains would remove excess ground water from the paddocks and prevent water logging of the cane. Recent field studies and model development by Cook et al. (2000b) suggests that most clayey ASS have low hydraulic conductivities and redesigning drainage systems to minimise drain depth and maximise drain spacing should reduce the amount of acidity exported from ASS. The redesigning of drainage systems may require a reasonable capital investment and could take years to implement. An alternative approach to stabilising the existing drainage networks from disturbed ASS fields is the use of vetiver grass. This approach offers a simple solution that may complement future drain redesigning schemes. Vetiver grass has also been used successfully to stabilise the banks of drains on ASS in Babinda north Queensland (Truong and Baker 1996). Drain and stream bank erosion is controlled by the vetiver’s vigorous and extensive 3 m to 4 m deep root system that could also be fundamental in decreasing freshly exposed ASS. The stiff stems of the thick vetiver hedges have slowed and distributed the movement of runoff waters, trapping silts, sediments and cane trash that may otherwise enter major waterways. Research conducted in cane land in Mackay and cotton farms in Emerald indicated that Vetiver hedges are very effective in trapping nutrients and agrochemicals in runoff water (Truong et al. 2000).
Project objectives The project was designed to determine whether vetiver grass could be successfully grown and if the vetiver could control stream bank erosion. Once established the vetiver would lower the frequency of drain maintenance by trapping sediments in runoff water and hence reducing acidic loading by exposing less acid sulfate soil in the drain wall to oxidisation and leaching. Intensive monitoring and sampling of the section of drain planted with vetiver, a drain section without vetiver and the contributing runoff were preformed to demonstrate the economic and environmentally effectiveness of vetiver grass in managing existing drainage networks. Specifically, we aimed to test the method on an existing drained ASS site, with the objectives of:
• stabilising the riparian zones adjacent to drains • raising pH in drain discharge (currently pH 3 to 4); and • reducing the sediment concentration in drain waters by 20%.
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The project serves as a demonstration project to promote environmentally sensitive agricultural practices, targeted at the local cane growing community in particular and to land users on drained coastal ASS in general.
Methods The experiment endeavoured to show the effectiveness of the vetiver grass hedge by comparing the water quality of a vetiver planted section (figure 1), an unchanged control section and the contributing runoff waters.
Figure 1. Experimental layout showing the vetiver planted on a 45 m section of existing drain.
Vetiver Vetiver grass (Vetiveria zizaniodes L.) is native to South and South-East Asia where its primary use was for soil and water conservation. Due to its special characteristics, vetiver has also been used very effectively for applications such as steep slope stabilisation and environmental protection (Truong 1999). A sterile cultivar has been selected and registered in Queensland as Monto Vetiver, which does not self-seed, is non-invasive and has neither runners nor rhizomes. It can be eliminated easily by uprooting or Glyphosate spray. Vetiver has several characteristics that make it more suitable than other grasses for erosion control (figure 2) and can be grown in a range of soil and climatic conditions.
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Figure 2. Vetiver Plant Sketch. Silt loaded runoff water is slowed down by the plants dense stems (A). Silt then drops out of the water behind the hedge (B) and reduced sediment water (D) is discharged. The dense root system (C) binds and stabilises the soil while the tall thick grass (E) acts as a windbreak, trapping airborne debris (cane trash).
Vetiver can tolerate acidic, sodic, alkaline and saline soils that can even be loaded with very high levels of aluminium, iron, manganese and other heavy metals. It has a deep, dense spongy root system (up to 3 m) that binds soils together and is very tolerant to extreme climatic conditions. Vetiver can withstand frost (-10 degrees C.), heat (50 degrees C.) and drought, but needs to be established in areas with an annual rainfall greater than 450 mm (Truong 1996). It is resistant to most pests, nematodes and diseases. It can be slashed, trafficked and can withstand burning while green. It does however require full sun, and is very sensitive to shade that retards growth.
Drain Weir Structures The two weir structures constructed from 3 mm thick sheet aluminium (figure 3) where installed to determine the discharge volumes from the vetiver planted and control sections of drain. The structures matched the cross section of the drain and were driven 200 mm into the drain banks and bottom. The construction material, aluminium, was chosen over stainless or galvanised steel for its weight and cost advantages. Construction using soil or concrete was also unacceptable due to the possibility of extra site disturbance. The aluminium sheets where installed by first positioning them in the correct position in the drain (figure 1). A section of folded galvanised sheet was placed onto the top of the weir section and a sledgehammer used to drive the sheet into the drain cross section. This was reasonably successful, with some buckling of the sheet, which was subsequently straightened. A temporary brace of aluminium angle riveted to the side of the aluminium sheet may improve the installation technique.
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The discharge Q (m3/ hr for a 1200 V-notch weir) can be calculated using the discharge equation (Eqn. 1) (Grant and Dawson 1995) from flow height H (m) through the V-notch. With the control section weir positioned 300 mm lower then the vetiver planted weir, a maximum discharge flow of 424 m3 / hr can discharge from the control before impacting on the vetiver planted section.
( ) 5.28606HQ = (1) It should be noted that the weir’s discharge is not entirely accurate as it is affected by the downstream water height. Water leaving the weir crest (figure 3) is called the nappe and as long as air flows freely beneath the nappe, the nappe is aerated and the flow rate is accurate (Douglas 1995). The vetiver trial site is within a few metres of sea level and a small drainage event (40 mm rain in 2 hrs) can cause a significant rise in surrounding waterways causing backing up within the cane drains. The accuracy of weir measurements will always depend on the intensity and duration of the rainfall event.
Figure 3. Aluminum drain weir front view schematic, dimension in mm.
Pressure transducers located behind and upstream of the weirs gave water level measurements and allows for an approximate total discharge to be calculated.
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Runoff Weir Structure The runoff weir structure (figure 4) is constructed from 2 mm thick stainless steel sheet and is similar in construction to the drain weirs, but it is quarter of the size. The complexity and cost of measuring the total discharge from the cane paddocks that impact on the trial site would far outweigh the entire project budget, hence a small section of 6 cane rows was measured. This gave some indication of the quality of water entering the trial drain section. The accuracy of runoff measurements is also affected once the drains overtopped and the cane paddocks begin to flood.
Figure 4. Stainless steel runoff V-notch weir supported by eight besser blocks. Flow from cane rows is directed through the weir.
A pressure transducer was also located behind and upstream of the runoff weir to determine the height of water discharging through the weir and in the paddock.
Site Survey A site survey (figure 5) of the drain has been carried out using a dumpy level. Eight readings of drain cross sections where taken every 5 m of drain length. Two more surveys were to be carried out as the project progressed. The variation between surveys was to have assisted in determining the sediment build up in the vetiver planted and control section of drain. It was hoped that any drain bank erosion might also be noted. These later surveys where not performed as negligible drain bank erosion occurred due to the climatic conditions.
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Figure 5. Site survey of cross sections of drain with vetiver planted weir to the middle right.
Instrumentation The project budget did not allow for real time water quality monitoring and sampling of all 3 points: the section of drain planted with vetiver; the drain section without vetiver (control section); and the contributing runoff. If conventional methods where used, each point would require a combination sensor (pH, electrical conductivity, dissolved oxygen and temperature), turbidity and a pumping sampler. To determine the effectiveness of the vetiver trial all 3 sampling points needed monitoring, so an innovative system (Multipoint Senor Chamber) was devised for this purpose.
Multipoint Sensor Chamber
The Multipoint Sensor Chamber (figures 6 and 7) enables the sampling of multiple points. A Campbell Scientific data logger (CR10X) is used to control the overall operation of the sensor chamber. A logger program (figure 8) has been written to control pump relays, select pinch values (controlling the sampling points) and to take readings from the sensors.
Figure 6. Multipoint Sensor Chamber enables water quality monitoring from multiple sources using one set of sensors.
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Figure 7. Top view of the Multipoint Sensor Chamber with turbidity (left), combination sensor and float switch (right)
The number to sampling points and the distance to them limits the desired sampling frequency of the Multipoint Sensor Chamber. The vetiver site had two sample points (vetiver planted and control section of drain) sited 20 m from the sensor chamber and one (runoff) sited 10 m from the sensor chamber. The peristaltic pump with a 2 m suction lift delivered 8 litres/minute. Rinsing the intake line from the sampling point 20 m away, filling the sensor chamber and taking the reading took 4-6 minutes while the sampling point, 10 m away required 3-5 minutes. The system was designed to take water quality readings from each of the 3 sample points every 20 minutes, with at least a 3 minute rest for the pump. Pinch valves were used to control the sampling lines. These are air operated and controlled by a 12VDC solenoid valve. The compressed air for the pinch valves was supplied from a compressor with a receiving tank of 100 L with an initial pressure of 80 psi that was used as a reservoir. This allowsws 4000 pinch valve switches to be made before the air pressure drops below the switching threshold of 20 psi. This would have allowed the experiment to run for at least 18 hours in the case of a compressor failure. Alternatively, the system could have been run from a small 12VDC compressor. Select a sampling point Close sample chamber drain Run pump in reverse X minutes to discharge the line (from sampling point) Pump forward X minutes then reverses X minutes to rinse the line Pump forward until a float switch in sensor chamber indicates sensors are covered. Continue forward pumping for Y minutes to flush sensors, allowing excess water to discharge through overflow Trigger the pumping sampler if the absolute value of a delta change in height is greater then Z If sampler is triggered then pause until sample is taken Turn off pump Take sensor readings Open sample chamber drain Run pump in reverse X minutes to discharge the line (from sampling point)
Figure 8. Pseudocode for data logger program.
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Sensors Two sensors were obtained for the trial, a combination sensor and a turbidity probe. Both the sensors fit neatly into the 150 mm PVC used for the Multipoint Sensor Chamber (figure 7). As there was weeks or even months between runoff events in which the system lay idle, a biocide was used to keep the sensors in working order.
Combination Sensor
The combination sensor used for this trial was a Greenspan CS4-1200 that measured Electrical Conductivity (EC), Dissolved Oxygen (DO), Temperature and pH. The combination sensor has SDI-12 (serial data interface at 1200 baud) output that allowed connection to the SDI-12 serial / digital network widely used in hydrological and environmental monitoring (Campbell Scientific 1997). The EC was set at the highest range possible (0-60000 uS) as the drains were often inundated with seawater. The combination sensor had a diameter of only 65 mm and fitted neatly into the Multipoint Sensor Chamber. The response time of the DO sensor made it unsuitable for use in the Multipoint Sensor Chamber.
Turbidity Probe
The turbidity probe that was used was a Seapoint Turbidity Meter. The Seapoint Turbidity Meter detects light scattered by particles suspended in water, generating an output voltage proportional to turbidity or suspended solids (Seapoint 1995). The operating range was selected by two digital lines that are controlled by the data logger, thereby selecting the appropriate range and resolution for measurement of clean to very turbid waters. The optical design confines the sensing volume to within 5 cm of the sensor and with a diameter of only 25 mm making it suitable for use in the Multipoint Sensor Chamber. The Multipoint Sensor Chamber was covered and placed in an electronics cabinet (figure 9) also improving the accuracy of the Turbidity Meter by minimising the effect of external light.
Pressure Transducers
Three Greenspan PS700 pressure transducers where used for monitoring the water heights of the three sampling points. The pressure transducer measurements taken from the runoff site where used to control most of the experiment and triggered the pumping sampler. The pressure transducers in the drain assisted in determining the discharge volumes though the weirs and the accuracy of measurements during flooding.
Pumping Sampler
An ISCO 3700 pumping sampler was used to take samples from the three sampling points. The data logger monitored the runoff and drain pressure transducers at 60 second intervals to determine when to sample from each sampling point. The data logger then triggered the pumping sampler to take a 1-litre sample. The sample was then analysed to determine heavy metal (particularly aluminium and iron) and nutrient concentrations.
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Figure 9. Multipoint Sensor Chamber mounted in electronics cabinet, pluvio, pressure transducers, runoff and vetiver planted section weirs.
Results and Performance New vetiver planting was completed on schedule in November 2001. With only occasional initial watering, almost 100% establishment was achieved despite the dry weather. In fact, the entire project was subject to record drought and the driest period in more than 150 years for south east Queensland (figure 10).
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With very minimal intervention the vetiver maintained a lush and vigorous growth throughout this drought, reaching over 2 m high at the end of January 2003. To sustain this enormous amount of growth during this extremely dry period, with a root system more than 3 m deep, the Vetiver plant must have obtained an adequate supply of moisture and nutrients from the drain or subsoil. This resulted in the lowering the water table and
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prevented deep drainage of plant nutrients and agrochemicals, particularly nitrate, from entering the drain.
Initial Planting (January 2001) The initial vetiver planting used a side-mounted plough on the rear of a tractor (figure 11) to create a planting trench. A PVC tube was then used to make a hole and the pre-potted vetiver plants where dropped in with minimal effort. Part of this initial planting was used for the vetiver trial site.
Figure 11. Initial planting of vetiver trial, November 2001.
Establishment The remaining Vetiver planting was completed on schedule in November 2001, with only occasional initial watering; almost 100% establishment was achieved despite the dry weather (figure 12). The old planting part (January 2001) of the trial was trimmed back to 30cm height and fertilised to encourage new growth.
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Figure 12. Good establishment despite the dry weather
Growth Despite the drought the growth of the November 2001 planted vetiver was very vigorous, reaching 50 cm tall after four weeks (figure 13) and over 100 cm after two months in some sections (figure 14).
Figure 13. November 2001 planting shows excellent growth after seven weeks of drought conditions.
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Figure 14. November 2001 vetiver plants at equal height by the eighth week.
Although small gaps existed between plants within the hedge, both sediment and trash started accumulating along the hedge within three months of planting (figure 15).
Figure 15. Less than 3 months after planting the hedge started trapping sediment and trash
The older January 2001 vetiver planting grew vigorously and reached over 1.5m tall (figure 16).
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Figure 16. Less than three months after trimming, the old hedge reached over 1.5m tall in some sections
Growth was so rapid that the hedge became fully functional just five months after planting (figure 17).
Figure 17. Fully functional hedge five months after the November 2001 vetiver planting despite the drought conditions with neither effective rain or watering.
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Five months after planting vetiver plants formed a thick hedge and proved very effective in:
• controlling bank erosion; • trapping sediment and pollutants in runoff water; • stopping cane trash from reaching the drain; and • lowering of the watertable in the drain due to transpiration and uptake of
nutrients.
Climate Conditions and Runoff Events Since the vetiver project site was established the Gold Coast has suffered its longest period of drought on record, with only three minor runoff events (figures 18, 19 & 20) observed from the fields adjacent to the study drain during the project period.
Vetiver Event 28-29 Oct 2002
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Vetiver Event 13-17 Nov 2002
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Vetiver Event 24th-26th Dec 2002
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The only part of an event that produced enough runoff to sample was during the 15-16th of November (figure 19). This event occurred after only 40 mm of rainfall but followed a 80 mm rainfall event only 2 days prior. During the event it is estimated that there was 17.3 mm of actual runoff, which represents 43% of the rainwater. The catchment area from which the runoff occurred is approximately 22000 m2 and represents only 40% of the total catchment area contributing to the drain.
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The data collected during the November 15-16th event gave only speculative indication as to how effective the vetiver would be under runoff events. The data does show that pH (figure 22) improves in the site planted with vetiver and this could be attributed to the vetiver filtering sediments through a build up of organic material at the base of the vetiver plants. The organic material may also have an absorption affect on acid generating chemicals present in the runoff water. The reduction in iron (Fe) concentration (figure 26) may be attributed to the filtering by the vetiver of the precipitated Fe out of the water column. Although minor, this reduction is evident, as the concentration of Fe in runoff has increased. As the event is only minor, this is speculation, but would be worthy of further research.
Vetiver Event 13-17Dec2002
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Vetiver Event 13-17 Nov 2002
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02 0
1:50
15/1
1/20
02 0
5:50
15/1
1/20
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9:50
15/1
1/20
02 1
3:50
15/1
1/20
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7:50
15/1
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02 2
1:50
16/1
1/20
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1:55
16/1
1/20
02 0
5:55
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9:55
16/1
1/20
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3:55
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02 1
7:55
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02 2
1:55
Time
pH
0
50000
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300000
350000
400000
Cu
mu
lati
ve R
un
off
Vo
lum
e L
Q L
R-pH
V-pH
NP-pH
Figure 22. pH of runoff (R) vetiver planted drain section (V) and control (NP) section of drain.
Vetiver Event 13-17 Nov 2002
050
100150200250300350400450500
14/1
1/20
02 0
0:35
14/1
1/20
02 0
4:40
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9:05
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3:50
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02 1
7:50
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02 2
1:50
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1:50
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5:50
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9:50
15/1
1/20
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3:50
15/1
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7:50
15/1
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02 2
1:50
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1:55
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5:55
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9:55
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1:55
Time
Tu
rbid
ity
FT
U
0
50000
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400000C
um
ula
tive
Ru
no
ff V
olu
me
L
Q L
R-TrbFTU
V-TrbFTU
NP-TrbFTU
Figure 23. Turbidity of runoff (R) vetiver planted drain section (V) and control (NP) section of drain.
21
Vetiver Event 13-17 Nov 2002
0
500
1000
1500
2000
2500
3000
3500
13/1
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02 2
0:40
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0:40
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4:45
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9:10
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1/20
02 1
3:55
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7:55
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1:55
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1:55
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5:55
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9:55
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3:55
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7:55
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02 2
1:55
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2:00
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6:00
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0:00
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2:00
Time
EC
mS
/cm
0
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300000
350000
400000
Cu
mu
lati
ve R
un
off
Vo
lum
e L
Q L
R-EC
NP-EC
V-EC
Figure 24. Electrical Conductivity EC of runoff (R) vetiver planted drain section (V) and control (NP) section of drain.
Vetiver Event 13-17 Nov 2002
95
96
97
98
99
100
101
13/1
1/20
02 2
0:40
14/1
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0:40
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4:45
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9:10
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3:55
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7:55
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1:55
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1:55
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5:55
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1/20
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9:55
15/1
1/20
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3:55
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1/20
02 1
7:55
15/1
1/20
02 2
1:55
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1/20
02 0
2:00
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1/20
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6:00
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0:00
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4:00
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2:00
Time
DO
Sat
%
0
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400000
Cu
mu
lati
ve R
un
off
Vo
lum
e L
Q L
R-DOsatp
V-DOsatpe
NP-DOsatp
Figure 25. Dissolved oxygen in runoff (R) vetiver planted drain section (V) and control (NP) section of drain.
22
Vetiver Event 13-17 Nov 2002
0.245
0.25
0.255
0.26
0.265
0.27
0.275
0.28
0.285
13/1
1/20
02 2
0:40
14/1
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0:40
14/1
1/20
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4:45
14/1
1/20
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9:10
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1/20
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3:55
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1/20
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7:55
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1/20
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1:55
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1/20
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1:55
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1/20
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5:55
15/1
1/20
02 0
9:55
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1/20
02 1
3:55
15/1
1/20
02 1
7:55
15/1
1/20
02 2
1:55
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1/20
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2:00
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6:00
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0:00
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1/20
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4:00
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8:00
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2:00
Time
Fe
mg
/L
0
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400000
Cu
mu
lati
ve R
un
off
Vo
lum
e L
Q L
R-Fe mg/L
V-Fe mg/L
NP-Fe mg/L
Figure 26. Iron in runoff (R) vetiver planted drain section (V) and control (NP) section of drain.
An effective bio-filter The vetiver hedge formed a thick bio-filter strip trapping eroded materials from both cane fields and access roads (figure 27). Eroded soil washed down from the channel banks and roads into the drain increased the levels of acidity, nutrients and agrochemicals in the drain water that is eventually flushed out to the sea. In addition the trapped sediment also contains other pollutants, which further pollute the water by increasing BOD and COD.
Figure 27. Vetiver, an effective bio-filter trapping trash and sediments after the runoff event in November 2002.
23
After the harvesting of the cane in September 2002 we where able to measure the amount of trash that entered the drain (figure 28). This is due mainly to the fans on the harvester that allow trash to become airborne and as the harvester turns at the end of the rows the trash is blown into the drain. A small amount of trash was blown into the drain after harvesting but its impact was insignificant when compared to the effects of the harvester.
Figure 28. Impact of cane trash on control section of drain after harvesting. The trash was collected from the vetiver and control sections of drain and weighed (table 1). A small amount was taken from the surrounding paddock and a sub sample from the drain sample was then used to calculate the percentage moisture. The trash from the field contained 3.5% moisture while the sample from the drain contained 80% moisture. From the measurements taken we where able to show that the vetiver section of drain prevented 98% (Carlin pers com.) of cane trash made airborne during harvesting from entering the drain. This figure is only relevant for the climatic conditions that existed during and for 5 days after harvesting. Vetiver Planted Drain Control Drain Total Wet Weight 2.5kg 126.0kg Total Dry Weight 0.5kg 25.2kg
Table 1. Dry and wet weights of trash collected from the Vetiver and Control sections of drain five days after harvesting.
Under the green cane harvesting practice, a large quantity of cane trash is left on the ground after harvest. This trash is often either washed down or blown into the surrounding drainage network, reducing its flow capacity and increasing its carbon content and acidity, COD and BOD levels of the water. Due to drought conditions, a significant runoff event was not observed and it can only be presumed that vetiver would reduce runoff induced trash from entering the drains, although the small event during November gave some encouraging visual results (figure 29).
24
Figure 29. Vetiver, an effective bio-filter trapping trash and sediments after the small runoff event in November 2002.
Controlling bank erosion Vetiver was very effective in controlling channel bank erosion during the trial. Figure 30 shows the excellent outcome of vetiver planting on an eroding section of the channel, and figure 31 shows the contrast between planted and control section of the trial.
Figure 30. Vetiver was planted on this eroded section in November 2001 (left) and it was completely stabilised six months later (right).
25
Figure 31. Exposed and highly erodible banks of the control section (left) compared to the well stabilised vetiver section of the trial (right).
In addition to the direct effect of vetiver in stabilising the edged of the channel, vetiver planting also promoted the establishment of other plants on the steep batters. This establishment stopped the erosion of the exposed and steep slope, preventing the collapse of the highly acidic soil into the channel stream (figure 32).
Figure 32: Establishment of other plants provided further protection to the steep and highly erodible slopes
Lowering of the water table in the drain due to transpiration and uptake of nutrients
Despite the record drought, the driest in more than 150 years in southeast Queensland, vetiver maintained a lush and vigorous growth, reaching over 2 m high at the end of January 2003. To sustain this enormous amount of growth during this extremely dry
26
period the vetiver plant would have obtained adequate supply of moisture and nutrients from the drain or subsoil, resulting in lowering the water table and preventing deep drainage of plant nutrients and agrochemicals, particularly nitrate. The uptake of groundwater by vetiver may also limit the flow of groundwater to the drains. Major acidic discharge events in coastal areas occur after periods of heavy rainfall that break an extended dry period. The acidic event peaks as floodwaters recede and ground waters enter the drain. Vetiver may prove useful not only as a bio-filter but also in reducing the groundwater flow to drains, decreasing the levels of acidity, nutrients and agrochemicals in the drain water that are otherwise eventually flushed out to the sea.
Adoption by the sugar industry in NSW As a result of the field days, conferences and meetings both farmers and researchers in the sugar industry have recognised the effectiveness of the Vetiver System in improving drain water quality. One landowner in Murwillumbah plans to use vetiver extensively for this purpose in the near future. Researchers at Wollongbar Research Station have established trials to determine the effectiveness of the deep and extensive root system and thick top growth of vetiver in reducing agrochemical pollution.
Conclusion Due to the drought, this trial has not conclusively shown that vetiver planting on the riparian zone of the drain banks would effectively improve water quality by reducing acidity and nutrient levels in the drain water. However, this trial has clearly demonstrated that vetiver planting has effectively controlled drain bank erosion and is an effective bio-filter during small runoff events. The data collected suggests that pH improves in a site planted with vetiver and this could be attributed to the vetiver filtering sediments through a build up of organic material at the base of the vetiver plants. The organic material may have an absorption affect on acid generating chemicals present in the runoff water. Also, the reduction in iron (Fe) concentration may be attributed to the vetiver filtering the precipitated Fe out of the water column. Of course, this is speculation but these results suggest that future research on the use of vetiver hedges in acid sulfate soils is required to confirm these tentative conclusions.
Acknowledgements The authors would like to thank the support of Environment Australia’s Coastal Acid Sulfate Soils Programme, Gold Coast City Council, CSIRO Land and Water, Veticon Consulting and L. and K. Mischke for providing expertise and funding for this trial.
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References Brown, T.E., Morley, A.W., Sanderson, N.T. and Tait, R.D. 1983. Report on a large fish kill resulting from natural acid water conditions in Australia. J. Fish., 22, pp335-350 Cook, F.J., Hicks W., Gardner, E.A., Carlin, G.D. and Froggartt, D.W. 2000a. Export of acidity in drainage water from acid sulfate soils. Marine Pollution Bulletin (In Press). Cook, F.J., Rassam, D.W., Carlin, G.D. and & Gardner, E.A. 2000b. Acid flow from acid sulfate soils: Measurements and modelling of flow to drains. In Sustainable Environmental Solutions for Industry and Government. 3rd Queensland Environmental Conference, The Institute of Engineers, Australia, 25-26 May, 2000. Campbell Scientific. 1997. CR10X Operators Manual, Campbell Scientific, Utah, U.S.A., OV-22 Easton, C. 1989. The trouble with the Tweed. Fishing World, 3, pp58-59. Grant, D.M. and Dawson, B.D. 1995. ISCO Open Channel Flow Measurement Handbook, Fourth Edition, ISCO Environmental Division, Nebraska, U.S.A., 3, p33. Seapoint Sensors. 1995. Seapoint Turbidity Meter Users Manual. Seapoint Sensors Inc., New Haven, U.S.A., p3 Truong, P. and Baker, D. (1996). Vetiver grass for the stabilisation and rehabilitation of acid sulfate soils. In Proceedings, Second National Conference on Acid Sulfate Soils, Coffs Harbour, Australia. pp 196-8 Truong, P.N. (1999). Vetiver Grass Technology for land stabilisation, erosion and sediment control in the Asia Pacific region. In Proc. First Asia Pacific Conference on Ground and Water Bioengineering for Erosion Control and Slope Stabilisation. Manila, Philippines, April 1999.
