Sustainable Agriculture

Tidal Floodgate, with Float Arm

The Tweed River is brackish (salty) as far upstream as Murwillumbah. Historically, floodgates were installed in creeks and drains to prevent high tides from flooding arable land with brackish water. This action inadvertently lead to a range of environmental issues such as acid sulfate soil pollutant export and reducing fish nursery habitiat.

In 2000 Council, in partnership with landholders and state government agencies, commenced a program to improve tidal flow and fish passage through floodgate modification in drains and creeks on the floodplain. Improving tidal flow in cane drains helps to buffer the impact of acid sulfate soil discharge, significantly reducing pollutant export to waterways.

Thirty-nine high priority floodgates have been modified so far, delivering significant environmental outcomes and even a few unexpected benefits to landholders, such as reducing the time and cost associated with controlling weeds in cane drains, which are reduced / displaced when drains are regularly flushed with brackish water.

Unvegetated Cane Drain – Costly to
Manage and Environmentally Damaging

Maintaining cane drains often requires soil disturbance to remove sedimentation and rectify bank slumping.

In 2007 Council, in partnership with state government agencies and local landholders commenced a program to plant out cane drains with native ground cover and small trees to stabilise the banks, discourage weeds and prevent topsoil runoff.

A Greenbank in Action

Known as ‘green banks’, vegetated drain banks require little maintenance and so the soil is not disturbed, drastically reducing acid sulfate soil impacts.

A Greenbank with Three Years of Growth

Nearly 30km of green banks have been planted on the floodplain in areas such as Johnsons Creek Condong, Blacks Drain South Murwillumbah, McLeods Creek Duranbah, Leddays Creek Tumbulgum and Mooball Creek Wooyung.

In 2005 Council, in partnership with state government agencies and local landholders commenced a field levelling and drain in-filling program to reduce acid sulfate soil runoff to local waterways.

Historically, cane paddocks have had varying degrees of undulation, requiring large numbers of field drains to export surface water to the adjacent river or creek. These drains require regular cleaning, which disturbe the soil and generated acid sulfate and other pollutants, which end up in the river.

Levelling cane paddocks reduces the number of field drains required, because excess water can flow off the paddock naturally. This increases the amount of land available for production while reducing soil disturbance associated with drain maintenance. This in-turn reduces acid sulfate soil runoff to the waterways.

Nearly 40km of field drains have been filled across the floodplain including farmland at Eviron, Bray Park, Christies Creek, Murwillumbah, Kynnumboon, Tygalgah and Chinderah.

For many years, Blacks Drain at South Murwillumbah has been identified as a major source of acid sulfate related pollutant (e.g. iron, aluminium, sulfuric acid) export to the Tweed River.

In 2008 Council was successful in obtaining $100,000 from the NSW Environmental Trust, Urban Sustainability Grant to reduce drain depth and increasing drain width to retain drainage capacity without disturbing the acid sulfate soil layer. These works have been very successful, improving a previously unusable pasture while preventing further oxidation and transport of sulfidic materials to the Tweed River.

Blacks Drain Photo Report (1.26mb)

Farmers Composting Workshop
March 2010

In 2010 Council, in partnership with Tweed Landcare, Northern Rivers Food Links and local landholders, conducted a number of on-farm trials to compare the benefits of organic compost and manures with conventional fertilisers.

Compost Application

Farmers from across the shire have each received up to 20 tonnes of compost to apply to their soil. Visual observation by farmers indicates that the trials have been very successful and soil sampling to verify these observations are underway.

On-farm Compost Production

Paddock Remodelling to Reduce
Soil Runoff During Rain

The Cudgen Plateau is famous for its fertile, volcanic soil and Cudgen Creek is famous for turning the colour of the soil after heavy rain.

Native Plants Help to Keep
the Earth Berm Stable

Through the Sustainable Agriculture Program, Tweed Shire Council is working with local landholders to help keep the soil on the paddocks.

This is being achieved with minor modifications to the interface between paddocks and drainage lines. Swales, or earth berms along drainage lines direct runoff into small settling ponds which slow the flow, giving the muddy water time to settle out. Farmers can then retrieve the soil that would have otherwise ended up in downstream waterways.

The University of NSW and Tweed Shire Council in partnership with NSW Cane Growers Association and NSW Sugar Milling Co‐operative are undertaking another Australian Research Council (ARC) Linkage project ; LP110100480 ‘Exploiting Natural Processes to Effectively Remediate Acidified Coastal Environments’ commenced in 2011 with the main aims to undertake:

Research which identifies acid sulfate soil (ASS) hotspots within 3 catchments in the Tweed Shire and then the most suitable remediation strategies to reduce ASS discharge on a catchment scale;
On‐ground remedial works identified in (1) above; and
Field measurements to determine the efficacy of the remedial works in reducing the discharge of problematic contaminants (particularly iron and aluminium) from the 3 catchments.

The major field research undertakings in 2011 focused on two catchments, namely Blacks Drain (Figure 1 - 2)and Christies Ck (Figure 3). In the former catchment, research was aimed at determining how effective remedial works conducted in 2009 have been in reducing the discharge of iron and aluminium from the upper catchment area. In the latter catchment, previous research was utilised with limited further sampling to identify problematic ASS areas and potential techniques to improve water quality from this catchment.

For some of the key findings, please refer to the Annual Progress Report for 2011 (1.31mb).

Figure 1: Comparison of the quantity of (dissolved) iron exported from Blacks Drain, after a rainfall event, in 2008
prior remediation work (9 days of measurements) and in 2011 post remediation work (6 days of measurements). Note the dramatic
decrease in the load of Fe exported (kg/hr) during 2011.

Figure 2: ARC 11-14 Linkage project partners reps at Blacks drain from the right:
Rick Beattie (NSW Sugar), Craig King (Cudgen Drainage Union - Cane Farmer), Dr Jacqueline Stroud (UNSW),
Robert Quirk (NSW Cane Growers Association), Yliane Yvanes-Giuliani (UNSW), Dr Richard Collins (UNSW),
Sebastien Garcia-Cuenca (TSC) not pictured

Figure 3: Location of Christies Creek major drainage lines and the subcatchments (designated as sites on the Figure)
monitored for the export of iron and aluminium (Kinsela and Collins, 2011).

Acid Sulfate Soil Research by
UNSW and Tweed Council

Between 2005 and 2008 Council co-hosted a research project by the Australian Research Council with the principal objective of developing innovative, scientifically-sound, practicable, floodplain management techniques to reduce the impacts on estuary and coastal water quality from Acid Sulfate Soils drainage products.

Acid Sulfate Soil Research by
UNSW and Tweed Council

The results of the study highlighted that there was no evidence of extensive saline intrusion into cane as a result of tidal flushing, that tidal flushing helps to maintain higher groundwater levels next to the drain and so result in lower Aluminium and Iron concentrations and that the upper catchment is the main source of contaminants discharged from Black’s Drain.

It was recommended for remedial work to focus on certain localised drains in the upper catchment. Remedial work commenced in 2008 and is due for completion in 2011 as part of a project funded by the NSW Environmental Trust.

Monitoring Equipment for the GhG Study

In 2006 Council co-hosted a research project by the Australian National University to compare greenhouse gas emissions from acid sulfate soils and non-acid sulfate soils in cane paddocks.

Preliminary results indicates that emissions of CO₂ from ASS are in the top of the range for agricultural soils, emissions of N₂O from N-fertilised soils appear to be much higher than expected from agricultural soils and that ASS are sources of atmospheric CH₄ particularly when very wet when their emissions are comparable with rice fields and wetlands.

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