Upper South East Dryland Salinity And Flood Management (USEDS&FM) Program

ISSUE: Didicoolum Drain - Preferred Option

By Michael Leak.
October 2005

In December 2002 the Upper South East Dryland Salinity and Flood Management Act was proclaimed.
In June 2003, the Government gave approval for an integrated package of environmental management initiatives and drainage construction works to develop the next phase of the USEDS&FM Program. This phase will cost $49.3 million consisting of $38.3 million provided in the National Action Plan for Salinity and Water Quality and an $11 million landholder levy, which has the option to be offset by participating in a biodiversity offset scheme.
The infrastructure component of the program consists of 665km of open drains being constructed of which 255 km of drains were previously constructed under Natural Heritage Trust. In October 2003 the Public Works Committee supported the Program's $20.9 million proposal for the construction of the remaining 410 kilometres of drain.
One of these drains is the 30km Didicoolum Drain located in the Marcollat Watercourse of the Central Catchment.

The original proposal was for a 2m deep drain located through the center of the flat.
The purpose of the drain was to intercept the groundwater table to reverse the salinisation that had already taken place and to reduce the risk of dryland salinity developing in areas currently not showing signs of salinisation but predicted to given the shallow saline groundwater system.
Providing a drain that intercepts and lowers the summer groundwater table means that less salt accumulates in the root zone over time and a greater capacity exists to flush root zone salts down the profile. Less root zone salinity means a greater number of more productive pasture options for landholders and better quality surface runoff for wetlands.
The original plan was also to restore the environmental value of the Marcollat Watercourse that has been significantly modified by landholders to improve local drainage, requiring restoration of sill levels, stock management, and more natural watercourse flow regimes.
Some landholders were satisfied with this plan including McGregor, Truscott and Nitschke.

Some Landholders along the Marcollat have always expressed a concern about groundwater drainage, including Max Verco (deceased), Prossers, and Heywood. More recently, other landholders have expressed similar concerns including Johnson, Moorhouse and Driver.
The concerns of these landholders can be summarized as follows;
- Bringing saline groundwater from southern properties via a groundwater drain through a productive area with better quality groundwater may contaminate the better quality groundwater.
- A groundwater drain will drain the better quality groundwater away from this area and therefore reduce productivity.
- A groundwater drain will reduce the soil moisture in the root zone that sustains a longer pasture growing season and therefore reduce productivity.
- Stock bores that currently access shallow groundwater of reasonable quality may need to be deepened post drainage and may then not be suitable for stock as the quality of the groundwater declines with depth.
- Wetlands may be adversely affected by a groundwater drain due to the interception of surface flows from the east and from lowering the groundwater beneath the watercourse.

The USE Program undertook a drilling, soil analysis and interpretation program to better understand and quantify these concerns. Three reports were produced as a result of these investigations.
- Report on Soils and Groundwater by Michael Durkay
- Report on Hydrogeology "Didicoolum Drain/Marcollat Watercourse Assessment" by GHD.
- Report on Agronomy "Pasture Report Marcollat Flat Proposed Drain" by Horizon Farming.
These reports generally concluded that a deep drain would not be the best outcome for the Marcollat Flat.
The reasons for this position were made principally by acknowledging that a groundwater drain as proposed would create the additional risks noted above.
The USE Program, in their technical review of these documents, did not fully support the findings of the reports. Whilst the issues identified in the reports were acknowledged by the review team, it was concluded there was inadequate assessment made of the observations and trends in water tables and the consequent dryland salinity risks that have been previously well documented in the USE.
A supplement to these three reports was issued to all landholders by the USE Program.

For more detail, read the entire report. [Adobe PDF 628 kB]