Date of Award

2013

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Natural Sciences

Faculty

Health, Engineering and Science

First Advisor

Professor Pierre Horwitz

Second Advisor

Professor Will Stock

Third Advisor

Dr Mary Boyce

Abstract

In the past decade the wetlands of the Swan Coastal Plain (SCP) region of Western Australia have been subject to increasing fire frequency and intensity. Whilst wetland sediment fires (also known as peat fires) on the SCP are not new phenomena, the increased frequency, duration and extent of combustion have been concomitant with an increase in urbanisation and reduction in average annual rainfall for the region. This has led to a decrease in ground- and surface-water levels which, in turn, has increased the susceptibility of the wetland sediments to ignition and combustion events. Increased wetland fire severity has resulted in the loss of large pools of organic matter as well as numerous geochemical changes in wetland sediments. The physical and chemical modifications of wetland sediments have implications for the water quality of these wetlands, particularly on the SCP where an intimate link between water quality and the underlying geomorphology can be demonstrated. Previous wetland sediment disturbance events, such as drought and dewatering, have led to the oxidation of sediments, which has resulted in the acidification, base cation leaching and metal contamination of both ground- and surface-waters. The buffering capacity is strongly linked to the underlying geomorphology. Wetlands on the highly-leached, poorly-buffered Bassendean dune geomorphic unit tend to acidify readily, whereas wetlands on the well-buffered, Spearwood dune geomorphic unit, generally tend to be less acidic and have the capacity to recover (i.e. return to near-neutral conditions). In recent times, some of the wetlands on the Spearwood dune system have remained acidic. This suggests that the buffering capacity of this system is finite and may be linked to the severity of the oxidation event. The physical, temporal and chemical nature of water quality response from dried, heated and combusted wetland sediments are not well understood nor are the processes that drive them. The aim of this research, therefore, was to identify and characterise the inorganic water quality responses to the combustion of organic-rich wetland sediments. The study examined post-fire sediment pore-water and downstream ground-water quality, and the short and long term temporal characteristics of these responses. A laboratory microcosm experiment was conducted to investigate the role of temperature and sediment heterogeneity on observed water quality responses. The porewater of burnt sediments differed greatly from that of unburnt sediments and was indicative of pyrite oxidation. There were also temporal changes associated with seasonal rainfall events and groundwater fluxes. Results of the long-term temporal analysis indicated the exhaustion of the in-situ buffering capacity of the wetland sediments, which resulted in the permanent acidification of the groundwater downstream of the burnt sediments. These patterns were partly obscured by transient buffering supplied by the ash created from the combustion of vegetative organic material and the influx of carbonate-rich groundwater. Laboratory microcosm analyses confirmed the inorganic hydrochemical signals, and the significance of sediment type; including parent geomorphology, in influencing the water quality response. The increased frequency, duration and extent of drying, heating and combustion of wetland sediments suggest an erosion of buffering, and thereby a loss of resilience for these wetlands, threatening their ecological integrity. This research enhances our understanding of the environmental impacts of wetland sediment fires and increases the potential for pre-emptive, rather than reactive management services.

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