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Modelling saturated/unsaturated water and phosphorus movement through an amended soil matrix

Cock, Peter Lawrence (2002) Modelling saturated/unsaturated water and phosphorus movement through an amended soil matrix. PhD thesis, Murdoch University.

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Nutrient enrichment, particularly by phosphorus, is a problem in wetlands and waterways on the Swan Coastal Plain in Western Australia. The on-site disposal of domestic sewage via leach drains significantly contributes to this problem. Leach drains are of particular concern due to the shallow water tables and the inability of most of the soil to attenuate nutrients.

A system was designed for on-site treatment of domestic sewage that greatly reduces nutrient loadings without the need to provide reticulated sewage. The Ecomax system is innovative in that it uses a lined pit to maximise contact between the wastewater and an amended soil matrix. The amended soil is a combination of silica sand and residue from the Bayer process (Red Mud), this mix has a documented ability to absorb nutrients. Though the nutrient absorption abilities of the Red Mud were known the characteristics of the system design were untested. This project involved the monitoring of pilot scale trials of the Ecomax system, and development of numeric models to allow the simulation of system performance and nutrient absorption life.

Two distinct tasks were undertaken in the project: monitoring of a field site and development of numeric flow and transport models. A variety of long term and short term measurements were taken at the field site, these were; inflow volumes, reservoir water levels, outflow volumes, in situ water levels, inflow water quality, outflow water quality and in situ soil and pore water phosphate content. In the numeric modelling both the saturated/unsaturated flow and transport models were developed from first principles.

Using data normalisation and overlay techniques it was shown that a stable low conductivity zone is quickly established at the base of the infiltration surface in the leach drain. Analysis of the long-term water level and inflow data indicated that after the first few months the test cells did not undergo any further reduction in infiltration capacity. The infiltration characteristics of the test cells are supported by established theory and published results.

The flow model showed that the movement of water through the cells could be explained by unsaturated flow theory. It was predicted that flow from the cells would be entirely by unsaturated flow, this was supported by in situ water level measurement. A simple model for predicting long-term water levels for a constant input volume was developed and shown to be generally applicable to the cells at the test site.

The transport model adapted the Barrow Shaw equation for batch reactions for use in the convection dispersion equation, until now this had not been satisfactorily applied in this context. In the short term (5 years) the transport model estimates was in good agreement with phosphate measurements taken at the field site. This gives confidence in the modelling prediction that the cells will be able to retain phosphate for at least 20 years.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): UNSPECIFIED
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