Review comments for "Impact Assessment of the Proposed Didicoolum Drain, Upper
South East, SA" by Resource & Environmental Management Pty Ltd, July 2005
By
Michael Leak
20 September 2005
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Abridged Review
I am of the opinion that REM has a conflict of interest that must be declared openly and
up front in this report. Landholders with a specific objective for the Didicoolum Drain
and therefore the outcome of the decision making process have and continue to
engage the services of REM to advise them in regard to technical aspects. Other
landholders will be very sceptical of a report produced by REM that comments on the
impact of various drainage scenarios given REM's pre-existing relationships. In order
to adequately address this, my advice would be to declare this conflict and then add
that the report has been peer reviewed by independent hydrogeologists to confirm the
results.
I also believe the emphasis of the draft report is to prove that a 2m deep drain with a
weir would result in very little additional impact on the concerns of the landholders
opposed to the drain. This is not the reason that the report was commissioned and in
my view is an example of REM's conflict of interest in producing the report. As
discussed with you, McGregor is going to have to compromise on the depth for this
section of the drain. Most of my comments are to redress this emphasis in the report
and rephrase it to answer the following questions;
- What is the lateral impact of a drain that just intercepts the summer groundwater
table for the two transects at the end of winter (highest predrain g/w level) and
end of summer (lowest predrain g/w level)?
- What is the lateral impact of a drain that just intercepts the summer water table
for the two transects at the end of winter and the start of summer, assuming a
weir is installed at 27.2? Will this reduce impact for those that don't want the
drain? Would this create problems for those that want the drain? Ideally, this
weir would be operated to match the surrounding g/w levels.
- What is the impact of reinstating wetland sills in the watercourse on groundwater
mounding on the flat? Will the proposed drain reduce this impact?
- A small section can be added that considers the impact of a 2m drain for both
transects with the above scenarios.
I cant really comment on the assumptions made in the model and will refer to Glenn
Harrington's input to this. Glenn concludes that the report is technically sound,
contains sufficient background information and explanation of the approaches used,
and has conclusions that are well supported by the modelling results.
The issue of hydraulic conductivity concerns me in that we have slug tested the aquifer
to determine the hydraulic properties and found repeatability within the numbers, yet
we conclude that the results are not representative. This to me opens up a huge
challenge. I think the way in which you have dealt with this issue in the report is about
the best we could do. I wonder whether there are other references that could be added
that point to the inaccuracies of slug testing individual bores. I have also spoken with Michael Durkay about the slug test results and he makes comment that the aquifer properties tested were before the water tables rose with winter rains and he therefore
thinks that the properties we tested were below the proposed bed level of the drain and
therefore not realistic. He believes it would have been better to wait until the water
levels reached their highest/shallowest and slug test then to assess the aquifer
properties in the zone that we are interested in draining. Comment?
The model establishment appears to represent the changing drawdown profile at the
drain very well because the width of the slice has been reduced to improve the
accuracy. I wonder whether a similar boundary condition could have been established
on the western margin to better represent the step change that occurs in the
groundwater system as the flat meets the western range. There was no slug test data
obtained to model this so I know I'm dreaming if I expect this to be done. It would be
good (one day) to see the groundwater models actually represent what happens in the
field.
I would also have preferred to see the weir pool dynamically modelled as a falling level
matching the rate of natural spring/summer groundwater decline rather than as a static
level throughout the spring months. Again, I understand this to be a limitation of the
model and not a significant one at that.
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