image of demonstration against extension of the Didicoolum drain

Report on Soils and Groundwater
MARCOLLAT REGION, UPPER SOUTH EAST SOUTH AUSTRALIA

Prepared for Department of Water, Land and Biodiversity Conservation
By Michael Durkay
Liquid Gold Hydrology Services
August 2004

EXECUTIVE SUMMARY

This Soils and Groundwater Review document is one of a number of reports that have been commissioned by the Upper South East Dryland Salinity Management Group to investigate the potential local impact of the proposed deep drain construction. In particular, it focuses on sub-soil and groundwater issues of the Marcollat Flat in the upper southeast of South Australia.

The Marcollat area has been long regarded by the local community as having complex soil and groundwater issues although the results of the study suggest that it can be simplified somewhat.

Overall, the sub-soils are predominantly calcareous in the east and clayey on the western side of the flat. High pH values (>9) are typical of the entire study area whereas the electrical conductivity (EC) of the 1:5 soil solutions varies considerably. A significant trend prevails with the EC results showing values that are very low in the east but relatively high on the western side of the Marcollat Flat. It would appear from the investigation that although a salinity problem is not present in the limestone dominated sub-soils of the area it does exist in the clayey and topographically lower regions of the Marcollat watercourse.

The high pH and EC values of the clay subsoils suggest that sodicity may also be an issue if groundwater levels are altered. However, it is not within the scope of this study to fully make that assessment and should be dealt with separately.

The groundwater monitoring program has produced results showing similar trends evident with the soils investigation. Groundwater quality in the east is good but deteriorates towards the west with the most saline water found in the watercourse. Groundwater from the eastern side of the flat has often been used for irrigation on Kyeema without obvious deterioration of the topsoils. Pumping of this water produces a typical draw-down of the watertable towards the east but results in mounding to the west. The implication here is that the hydraulic conductivity is less to the west than to the east which is compatible with the soil characteristics.

Extensive inundation was evident on the clayey soils but was virtually non-existent on the limestone dominant soils during the latter part of the monitoring period. The obvious moisture infiltration variation further supports the difference in hydraulic conductivity apparent across the Marcollat Flat. It therefore follows that a considerable difference in the drainage characteristics of the soils also exists between east and west.

Assessments of drain influence in relatively homogeneous soils have been completed on the Fairview Drain. A study at the same location was also conducted concurrently with this investigation and found that the deep drain was presently having a minimal impact on adjacent watertables. If, however, the drain was producing any effects on the watertable then it was on the eastern side. Watertable fluctuations in the drained areas were typical of those in undrained localities and could be closely correlated with rainfall records for Padthaway from the Bureau of Meteorology.

The implication of the findings of the study, along with other available data, is that although, for the most part, the Marcollat Flat fits the general regional mould developed for surface and groundwater management, a number of local issues may need to be addressed before a drain is constructed in its intended location. Major points for consideration are:

  • The east-west regional groundwater flow is likely to produce the greatest effects towards the east of a north-south orientated drain such as is intended
  • Soils to the east of the flat do not have a salinity problem
  • Groundwater to the east is of good quality
  • Waterlogging is not a major problem on the eastern side
  • It is these waters and soils that are most likely to be affected by removal of groundwater with extreme wetting and drying cycles and an associated deterioration of soil and groundwater quality
  • Soils to the west of the flat are saline and experience a high degree of waterlogging due to their low infiltration capacity and low hydraulic conductivity
  • A groundwater drain is unlikely to have a significant impact on watertables or soil salinity levels due to the clayey characteristics of the soils
  • If waterlogging issues on the western side of the flat are to be addressed then it needs to be in the vicinity of the problem rather than to the east in a topographically higher region.

Read the entire report. [Adobe PDF 2.2MB]