Impact Assessment of the Proposed Didicoolum Drain, Upper South East, SA

Prepared for Department of Water Land and Biodiversity Conservation
By Stuart Richardson and David Poulsen, Resource and Environmental Management Pty Ltd .
November 2005

Entire report [Adobe PDF 4.9 MB]

EXECUTIVE SUMMARY

Shallow saline groundwater in the low lying agricultural land of the Upper South East region of South Australia has negatively impacted the productive capacity of the inter-dunal flats and the health of wetlands located along the western edge of the flats (such as the Marcollat
Watercourse). Groundwater drains can be an effective approach for the removal of excess shallow groundwater and salt, and maintain the watertable below the depth where salinisation and waterlogging of land can occur.

Resource & Environmental Management Pty Ltd (REM) was engaged by the Department of Water Land and Biodiversity Conservation (DWLBC) to undertake an assessment of the likely impacts of the proposed Didicoolum Drain on local groundwater levels beneath the Marcollat Flat. The area to
be serviced by the proposed drain comprises low lying inter-dunal plains located about 50 km northeast of Kingston S.E. on the road to Keith.

The purpose of this hydrogeological assessment and numerical modelling exercise was to provide answers to questions asked by landholders during the consultation process established by the Upper South East Program team for decision making for the proposed Didicoolum Drain. The work described in this report is intended to provide an objective understanding of the relative potential impacts of drainage and wetland management scenarios on the shallow aquifer underlying the inter-dunal flat. The model results are not intended to be an exact representation of present or
future physical conditions in the area modelled. Rather, the results are considered to be useful as a semi-quantitative illustration of the relative changes in groundwater conditions that can be expected from the various components of the proposed works package. The key components of
the works package proposed by DWLBC are:

• Construction of a 1.5 to 2.0 m deep drain (the Didicoolum Drain);
  
  
• Operation of a weir within the drain to maintain the watertable along the drain at pre-drain levels through the spring period; and
  
  
• Re-instatement of wetland sills at natural levels within Marcollat Watercourse.
  

This assessment has used existing hydrogeological information combined with analysis of new groundwater monitoring and slug testing data, collected by REM and DWLBC, to inform the development and use of a simplified numerical groundwater flow model to estimate the likely
extent of impact of the proposed drain and potential interactions with nearby features including wetlands.

The following key conclusions have been drawn from the results of the limited field investigations and groundwater modelling:

Aquifer properties

1. A compilation of the local hydro-stratigraphic data has shown that there is a complex mix of aquifer material within the inter-dunal flats that can be broadly characterised as being mainly limestone (with some clay content) on the eastern part of the flat and mainly clayey sand and
   sandy clay on the western part of the flat. There are sand layers on the western part of the flat which are likely to be capable of being drained.The extent to which the occurrence of the clayey sediment controls aquifer hydraulic conductivity appears to be variable.
   
   
2. The Aquifer testing (by slug recovery tests) indicates that the hydraulic conductivity of the shallow part of the aquifer on the inter-dunal flat along the Marcollat watercourse is lower (0.03 - 0.7 m/day) than measured in pumping aquifer tests elsewhere in the Upper South East (9 - 47 m/day). It is acknowledged that the reliability of estimates of hydraulic conductivity from slug recovery tests is lower than estimates from pumping aquifer tests, although local geological logs indicate that limestone on this flat contains some clayey material. For the purposes of groundwater modelling a range of hydraulic conductivity values were used.

Impact of the drain on groundwater levels

1. The proposed 1.5 m deep drain appears to not significantly lower summer (end of May) groundwater levels on either transect and will thereby minimise drainage impacts. This is because the summer water levels (estimated by the model) are generally deeper than 1.5 m on each of the transects. Based on these results it is expected that the 1.5 m drain will not provide significant benefit in the management of dryland salinity.
   
   
2. The model suggests that the 2 m deep drain will not significantly lower the summer groundwater level on the Kyeema transect (because of the greater depth to groundwater), but will reduce the level of the summer groundwater level on the South Reedy transect where groundwater is shallower. The latter scenario will reduce the evaporative demand that causes the accumulation of salt in soil and will therefore provide greater protection from dryland salinity.
   
   
3. The 2 m deep drain will be more efficient in lowering groundwater levels by keying into a more permeable part of the aquifer, especially at locations near North Swamp where the clay material extends to around 2 m bgl.
   
   
4. The proposed 2 m drain is unlikely to have a significant impact on stock bores located on the eastern side of the drain near the Kyeema transect, including through the summer months when the use of the bore would be to be higher. This is because the expected drawdown in the vicinity of the stock bores will be very small in comparison to the saturated thickness of the aquifer accessed by the stock bore.
   
   
5. The impact of the drain (as far south as South Reedy) under the low hydraulic conductivity (1m/day) scenario is less extensive laterally when compared with the high hydraulic conductivity (20m/day) scenarios.
   

Operation of the weir in the drain

1. The operation of a weir in the drain through spring maintains pre-drain groundwater levels to the start of summer at Kyeema, which means the drain is less likely to cause soil moisture losses. The operation of a weir has less impact to the south (near South Reedy, because of the grade), but still reduces the effect of the drain while in operation. It will be critical that there is agreement to the operational rules for the weir to avoid adverse impacts that could occur if the weir causes drain water to recharge back into the local groundwater system during late winter and spring, especially if the drains carry saline water. The intention of using the weir should be to mimic natural (predrain) variation in groundwater levels during spring and then remove the weir to allow the drain to operate at full capacity during the summer months.

Reinstatement of sills to natural levels within the wetlands

1. Reinstatement of sills to natural levels within the wetland basins and retention of water through the June to October period each year is likely to result in groundwater mounding beneath the western side of the inter-dunal flat. The mound is estimated to extend in both an easterly and westerly direction The impact will be greater if water is maintained in the wetlands through the spring and summer. The presence of the mound means that the drain does not have an adverse impact on groundwater levels near the wetland during winter periods, and it is considered that a drain is required to help alleviate the potential impacts of groundwater mounding on agricultural
   production.
   
   The model results indicate that the mounding dissipates by the end of May, however, it is likely that the model underestimates the true impact of reinstatement of sills to natural levels since it is likely that water will be retained within the wetland for a longer period than assumed, especially if additional flows are directed from the south. Extension of the period of inundation of the wetlands will result in higher groundwater levels during the summer period when evaporative demand is higher.
   
   
2. It is important that rules for operation of the wetland sills be agreed and designed to ensure environmental values are enhanced and to avoid adverse impacts to agricultural productivity. The following recommendations have been provided based on the key conclusions of this
   investigation:
   
   
   1. To provide maximum effect on the problem of dryland salinity and to off-set potential impacts from groundwater mounding associated with retention of water within the wetlands it is recommended that a 2 m deep drain be constructed along the full alignment of the Didicoolum Drain.
      
      
   2. The operational rules for sills on the wetland basins and for the weir need to be agreed by all stakeholders prior to construction of the drain. The wetland sills and weir should be operated to avoid adverse impacts to agricultural productivity, while providing environmental benefit.
      
      
   3. Further groundwater modelling should be completed using the models developed in this project to test the effect of a range of weir operation scenarios on groundwater levels and to inform the choice of an agreed set of weir operational rules.
      
      
   4. Further development of this modelling approach should include a more comprehensive assessment of the hydraulic conductivity of the aquifer with a pumping aquifer test.
      
      
   5. Further structured sensitivity analysis would be useful in assessing the impact of uncertainty and variability in model input parameters.
      
      
   6. A monitoring program be put in-place as soon as possible to allow for the collection of baseline groundwater information prior to construction of the drain and to provide a dataset that measures the impacts of the drain, wetland sills and weirs on groundwater levels and on agricultural productivity at targeted locations.
      
      
   7. The present conceptual understanding does not satisfactorily explain the step change in groundwater levels occurring to the west of the wetlands. More information is required to refine the model to address this abrupt change. This task should include the installation of a monitoring well below the Padthaway Formation (within the Bridgewater Formation) near the dune on the western side of the flat.
      
      
   8. To gain greater confidence in the potential performance of drains in the Upper South East it is recommended that a model be developed for the Fairview Drain that is similar to the models that have been prepared for the proposed Didicoolum Drain. Groundwater monitoring undertaken prior to drain construction and following drain construction at the Fairview Drain site provides a unique opportunity to check model predictions against actual performance. It is expected that this type of modelling can support future reviews of drain performance.

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For more detail, read the entire report. [Adobe PDF 4.9 MB]