Didicoolum Drain/Marcollat Watercourse Hydrogeological Assessment

Prepared for Department of Water, Land and Biodiversity Conservation
By GHD Pty Ltd.
July 2004

Introduction
Background
Scope of work
Site visit
Conclusions from data review
Entire report [Adobe PDF 346 kB]

INTRODUCTION

GHD Pty Ltd (GHD) was engaged by the Department of Water, Land and Biodiversity Conservation (DWLBC) to conduct an assessment of hydrogeological conditions in the vicinity of the Marcollat Watercourse and Didicoolum Drain.

This commission resulted from discussions held with Mr Rob Kemp of DWLBC, in which it was stated that the proposed Didicoolum Drain will pass through properties of both low and high agricultural value, with the reasons for the variation in land capability being unknown. In a meeting held on 16 February 2004, it was stated that there was a need to consider the effects of installing the drain on groundwater, land, soils and the wetlands of
the Marcollat Watercourse.

GHD was then invited by DWLBC to tender for the hydrogeological investigation in the Marcollat Watercourse and agricultural flat in the vicinity of the proposed Didicoolum drain. A proposal was submitted to DWLBC on 27 February 2004, and approval obtained on 19 March 2004 by the receipt of contract documentation.

BACKGROUND

Much of the native vegetation throughout Australia has been cleared for agriculture. This includes large tracts of land in the upper south east of South Australia. Significant results of land clearance have included successful agricultural (farming) enterprises followed in many areas by salinisation, land degradation and then a loss of productivity. This is mainly due to land clearance leading to increased infiltration recharge of rainfall, rising groundwater levels, the mobilisation of salts in the soil profile, waterlogging, inundation and the concentrating of salt at, or near, the land surface through evaporation. This has also led to an increase in discharge of saline ground water as base flow to streams, resulting in additional biodiversity loss.

In the upper south east of South Australia, there is a perception that both salinisation and flooding of what was once useable land has occurred, and that drains are required to restore the functionality of the land.

The Upper South East Combined Councils Salinity Committee was formed, and investigations carried out in 1990 and 1991. In 1993, an Environmental Impact Statement titled “Upper South East Dryland Salinity and Flood Management Plan” was released, and a $36 million (1993) drainage scheme proposed. The scheme included:

The excavation of 270km of drains
The directing of surface water to downstream wetlands
The protection of low lying land from salinisation
The protection of native vegetation
The disposal of drainage water (groundwater) to the ocean
Limited discharge of saline surface and groundwater to the Coorong.

Since that time, information has continued to be gathered, including the publishing of a supplement to the EIS (1994), a review of the EIS on behalf of the Nature Conservation Agency, and a number of reports and papers from within government agencies relating to flow levels, pondage levels, agriculture and vegetation.

With respect to the Didicoolum drain, opinions of personnel both within the government agencies and on the land are divided, with the premise that the installation of the drain would provide the desired benefit being challenged. In particular, it has been stated that the land to the south of the Kingston – Bordertown road is salinised and of low productivity, whereas the land to the north of the road is of low salinity and is highly productive. A major concern (as stated) is that the drain will result in a loss of productivity of the land to the north of the road, and may not provide a significant benefit to the land south of the road.

This commission is intended to provide an overview of the likely outcomes of installing the Didicoolum Drain.

SCOPE OF WORK

The request for proposal sent by Michael Leak (DWLBC) on 16 February 2004 included a request to achieve the following objectives:

To understand site specific conditions and trends in the aquifer
To draw conclusions on the effect of groundwater on wetlands and woodlands of the Marcollat Watercourse
To draw conclusions on how groundwater impacts agricultural production in the Marcollat Flat
To predict the effects of the drain on the aquifer, agronomy, and the environment
To provide possible engineering solutions to mitigate potential impacts of the drain on groundwater, agronomy and the environment

It was also agreed that GHD would:

Assess the differences between the northern (fresh) and southern (saline) areas
Provide comments on the effects on the watercourse and wetlands of constructing the drain
Comment on the relative positioning of the drain in the flat
Comment on shallow versus deep drains and the potential impact on the wetlands and watercourse

To meet these objectives, we visited the site, discussed issues with DWLBC personnel and landholders, and reviewed geological, hydrogeological, agricultural, and environmental information.

Activities including the site visit and the data review are described below, and a series of discussion sections are also presented, followed by a response to each stated objective.

SITE VISIT

The Marcollat Watercourse and portion of the proposed Didicoolum drain within the subject area were inspected on 22 and 23 March 2004.

This included travel along the Harper Range, including the inspection of a road cutting (to examine the soil profile), and traversing the flat plain from east to west, noting the presence of subdued calcareous outcrops, variations in pasture, the presence of shallow drains and old watercourse meanders and lunettes. Lagoons were also briefly inspected and the presence of reeds on the western margin of some lagoons noted (indicating possible groundwater discharge from the Woolumbool Range), along with the loss of understorey vegetation, the presence of old natural flow channels and the excavation of connecting and diversion channels. These channels have been constructed between wetland lagoons along the Marcollat watercourse and also to the east on "Willalooka".

These channels have been installed to drain the land to the east of the watercourse on a local scale and those along the watercourse would have probably been installed to speed up the flow of water through the study area.

Also noted in the area to the south of the Kingston - Bordertown road was the presence of salt tolerant grasses (puccinellia and tall wheat grass) nearer the watercourse with clovers and other grasses more abundant to the east.

Discussions were also held with Dean and Sue Prosser and Tim Truscott, landholders to the north of the Kingston-Bordertown road. The abundance of limestone at or near the land surface on the Prosser property was noted, as was the presence of abundant clover, and a pump equipped water supply well was also started to demonstrate the high yield obtainable from the shallow unconfined aquifer.

The floodplain of the Marcollat watercourse was inspected on the Truscott property, the most northerly in the study area, and the impact on land of the impoundment of water at the Jip Jip weir discussed. The drain on the Brinkworth property immediately to the north of the study areas was also visited, as it is intended that the Didicoolum drain discharge to this structure.

CONCLUSIONS FROM DATA REVIEW¹

The information reviewed and details described above have led to the following
conclusions:

The interdune flats between the Harper and Woolumbool Ranges consist of clayey soils near the western margin and more limestone rich sediments further east
To the north of the Kingston - Bordertown road, the Harper Range peters out, resulting in the presence of a large flat area consisting of limestone soils
The limestone soils are of higher permeability and lower salinity than the clay soils further south near the watercourse. These soils are likely to significantly dewater if a drain is cut into them significantly below the water table, which could lead to a loss of production on the Truscott, Prosser, Johnston, Verco and parts of McGreggor properties. This effect could also occur on other properties containing limestone soils cut by the drain and intercepting groundwater
In general, the clayey soils are of low permeability and high salinity. In this type of soil, the dewatering effect of the drain is unlikely to extend far from the drain. Hence, the salt trapped in these soils may not be removed by drainage, and hence agricultural production is unlikely to significantly increase as a result of the installation of the drain.
The Marcollat Watercourse wetlands are sustained by water from the Woolumbool Range, by flows from the south and by sheet flow from the east. Wetlands in the south are more often fed by flow from the south, whereas the wetlands such as Kyeema rely to a large extent on flows from the east.
The installation of a drain will reduce flows from the east reaching the wetlands and could have a detrimental impact on ecology.

¹For details of data reviewed, see pp. 7-8 of the entire report. [Adobe PDF 346 kB]

Read the entire report. [Adobe PDF 346 kB]