Recharge systems for protecting and enhancing groundwate

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TOPIC 6
Case studies / Region issues and artificial recharge case studies
lease. This demonstrates clear qualitative evidence of significant connection between the Wittenoom Dolomite and
Marra Mamba aquifers. Contour spacing is much higher in the dolomite than in the Marra Mamba, demonstrating
the higher regional transmissivity and storage in the karstic aquifer. These findings confirmed the conceptual
understanding of bedrock aquifer characteristics which could not be definitively evaluated through conventional
short term pumping tests.
In terms of quantifying the dewatering requirements of Marandoo, the most important outcome of the trial is the
dataset it provides for calibration of the groundwater model and for determining the influence of the West Angelas
Shale aquitard. The shale layer is the major control on the contribution of the dolomite aquifer to Marandoo
dewatering. Earlier calibration of the model was relatively insensitive to this layer.
Water levels, pumping rates and reinjection rates during the 44 day Trial formed a dataset which was used to calibrate
the shale layer properties. The calibration returned a horizontal hydraulic conductivity of approximately
0.75 m/day and a storage value of 0.0001 for the shale. These results represent a significant tightening of the shale
properties by comparison to earlier work. The calibration process with the Trial dataset was relatively sensitive to
the shale properties, and also highlighted the need for further geological exploration work to improve delineation of
the shale in some areas. The confidence in Marra Mamba dewatering estimates has been significantly improved by
provision of the Trial dataset to the modelling.
Vertical connection across the clay layer was investigated by review of the shallow bore hydrographs, particularly at
the pumping and re-injection sites. The influence of the trial was not seen in any of the shallow aquifer hydrographs
across the project area.
The vertical gradient at pumping bore TPB5 was increased markedly by the abstraction (from 0.053 m/m to
0.15 m/m). There was no break in the stable water level trend in shallow monitor bore OW24s, which is 5 m from
the pumping bore. Re-injection at TIB1a reduced the prevailing local vertical hydraulic gradient from strongly
downward to weakly downward (from 0.10 m/m to 0.019 m/m). The shallow monitor bore OW30s lies 5 m from
the re-injection bore and displayed a long-term downward trend which was uninterrupted by the trial.
It can now be concluded with confidence that the degree of widespread connection between the deep and shallow
aquifers is extremely low.
The ‘reverse-pumping test’
Aquifer parameters at TIB1a were derived by simply treating ‘draw-up’ as ‘drawdown’, and applying standard analysis
techniques to water level responses. The results were similar to results derived from earlier test-pumping of the
adjacent TIB1 bore:
2003 test-pumping: transmissivity = 3000 m 3 /d/m, storativity = 2.8 x 10 –3
2004 re-injection: transmissivity = 2200 m 3 /d/m, storativity = 1.2 x 10 –4
This technique must be applied carefully to ensure that re-injection results can be extrapolated to pumping performance.
It could not, for instance, be applied accurately to an unconfined aquifer, where the saturated thickness
will increase under re-injection as opposed to a decrease under pumping. This situation could also introduce inaccuracies
arising from vertical inhomogeneity.
ISMAR 2005 ■ AQUIFER RECHARGE ■ 5th International Symposium ■ 10 –16 June 2005, Berlin