Date of Award

2007

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Bachelor of Science Honours

School

School of Natural Sciences

Faculty

Faculty of Computing, Health and Science

First Supervisor

Ray Froend

Second Supervisor

Will Stock

Abstract

The waste product resulting from the alumina refining process consists of residue sand and mud which is laid on bauxite residue disposal areas (RDAs). Once the RDAs have been effectively solar dried they are revegetated to minimise deep drainage and potential groundwater contamination. In this study, two RDAs located at Alcoa's Pinjarra refinery were seasonally monitored to determine the effects of irrigation on soil water availability and plant water use. Three experimental Blocks were established by Alcoa, each with an irrigated and unirrigated site. Since then, irrigation has been applied to the three treated sites to maintain the root zone water holding capacity throughout the dry months (October - April). Soil samples were collected at 0.25 m intervals to a maximum depth of 3m and analysed for their gravimetric soil water content, soil water potential, soil particle size, pH and .electrical conductivity. Plant water potential, stomatal conductance and transpiration were measured for three species, Eucalyptus gomphocephala, Acacia cochlearis and Hardenbergia comptoniana. In addition, these measurements were also carried out at a native site for comparison between vegetation growing on the artificial RDAs, reliant solely on soil stored water and native vegetation, with access to deeper water sources. This study identified the RDAs to have sufficient soil water availability at both the irrigated and unirrigated sites to maintain seasonally high plant water potentials and contribute to only mild levels of plant water stress. All plants lowered their stomatal conductance in the dry months with the first rains of the wet season increasing stomatal activity and transpiration. Over the wet period, the unirrigated plants exhibited lower stomatal conductance and transpiration rates than irrigated plants, most likely due to their smaller leaf area. H comptoniana maintained the highest water potential of the three species, which may be attributed to significant leaf shedding in the dry months. This species also responded rapidly to the first rains with the establishment of new growth and higher rates of transpirational water losses through the wet period compared to E. gomphocephala and A. cochlearis. It appears that irrigation plays an important role in the rapid establishment of seedling biomass, however it has no significant effect on the level of water stress experienced by plants growing , on the RDAs. There is the possibility that the dense biomass and potential shallow rooting depth of plants growing on irrigated RDAs may reduce the ability of plants to withstand natural summer drought once irrigation has ceased and therefore reduce the capability of vegetation to minimise deep drainage and potential groundwater contamination in the long-term.

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