Acidsulpahte

Vetiver Grass Hedges for Water Quality Improvement
in Acid Sulfate Soils, Queensland, Australia
Paul Truong1, Geoffrey Carlin2, Freeman Cook2, and Evan Thomas3
1
Veticon Consulting, Brisbane, Queensland, Australia
2
CSIRO Land and Water, Brisbane, Queensland, Australia
3
Gold Coast City Council, Gold Coast, Queensland, Australia
Abstract: Acid sulfate soils are weak mechanically andtherefore highly erodible, if drain banks are not
properly stabilised, they are 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
mono-sulphides and metal oxides to accumulate due to the high iron, aluminium and other metal
concentrations in drainage waters.
Innorth Queensland, the establishment of vetiver grass has been shown to control channel bank
erosion, lower frequency of drain maintenance, trap sediments in runoff water and reduce acidic loading
by exposing less acid sulfate soil in the drain wall to oxidisation and leaching.
A trial was established in south Queensland to demonstrate the effectiveness of vetiver hedges in
water qualityimprovement by stabilising highly acidic drains banks and trapping nutrients, sediment,
agrochemicals and cane trash. Intensive monitoring and sampling of the section of drain planted with
vetiver, a drain section without and the contributing run-off has demonstrated the economic and
environmentally effectiveness of vetiver grass in improving water quality in term of pH, trash
input, sediment anddissolved oxygen etc.
Key words: vetiver, water quality acid sulfate, erosion, pH, trash, sediment, dissolved oxygen
Email contact: Paul Truong or Geoff Carlin

1

INTRODUCTION

The formation of coastal acid sulfate soils has occurred in Australia over the past ten thousand
years. During that time the sea level rose and sulfates in the seawater mixed with land sediments
containing organicmatter 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 sulphides that are located within these soils are exposed to air,
they oxidise and produce sulfuric acid. Urban, agricultural and recreational developments in ASS canresult 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 have many other environmental impacts.
Early agricultural practices in ASS used for sugar cane production established deep drains. It was
thought that the deep drainswould remove excess ground water from the fields and prevent water logging
of the cane. Recent field studies and model development by Cook et al . (2000b) suggests that most 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 drain
systems may requirea reasonable capital investment and could take years to implement.
An alternative approach to stabilising the existing drain networks from drained ASS fields is the

schemes. Vetiver grass has been used successfully to stabilise banks of drains on ASS in Babinda north
Queensland (Truong and Baker, 1996) and more recently it was planted as a hedge in the riparian zones
adjacent to drains inthe experimental trial that is being established at Pimpama, Queensland. Drain and
stream bank erosion is controlled by the Vetiver’s vigorous and extensive 3 to 4 m deep, root system that
could be fundamental in decreasing freshly exposed ASS. The stiff stems of the thick vetiver hedge slow
the movement of runoff waters, trapping silts, sediments and cane trash that may otherwise enter major...