Townley, L.R., and Ellerbroek, D.A. (1995), BHPAC Spoil Hydrology Project: Water movement in the Possum Pit transect, prepared for Rust PPK, CSIRO Minesite Rehabilitation Research Program, 52pp. September. [Confidential]

This report provides the results of modelling of water movement in spoil in the Possum Pit transect. The objective of Task 090 of the BHPAC Spoil Hydrology Project was to demonstrate that our ¡°model(s) are capable, without further development, of modelling water movement for the Possum Pit transect¡±. This objective has been achieved.

In the specification for this Task, Rust PPK indicated their expectation that modelling would be based on the use of two models, SUTRA and AQUIFEM-N. During the course of the work, it became clear that:

  • SUTRA was not suitable for the task, and that a better choice for variably saturated flow modelling would be MDFW (Green, 1994, based on 3DFEMWATER).
  • Modelling with any variably saturated model would be extremely unlikely to produce worthwhile results, given (i) the nature of (preferential) flow processes in the spoil and (ii) the amount and quality of field data available.
  • AQUIFEM-N is well suited to the task ¡ª more easily than originally anticipated.

    In terms of the objectives of the Task, we have found that SUTRA and MDFW are unsuitable for modelling water movement for the Possum Pit transect, that it is unlikely that any variably saturated model could provide adequate results, and finally that it is sufficient to model water movement in the spoil with a saturated flow model, combined with an algorithm for estimating ("guesstimating") infiltration into the spoil.

    We have found that AQUIFEM-N is capable, without further development, of modelling water movement for the Possum Pit transect. The same may be true for many other saturated flow models, of which the most widely used is MODFLOW. The ability of a saturated model to simulate water movement for the Possum Pit transect will depend on its ability to handle the drying of the aquifer between significant recharge events.

    The ability of any model to simulate water movement accurately depends on the availability of appropriate field data. Field data at Possum Pit are limited in quantity, and some data appear to be of dubious quality. There are not enough data to allow a unique ¡°calibration¡± of a model of water movement in the spoil. However, the data do provide an indication of the magnitude of some critical aquifer parameters, and we have identified a method of determining the time scale of drainage from the spoil.

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