Date of Award

2000

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Communications, Health and Science

First Advisor

Dr Pierre Horwitz

Second Advisor

Dr Mark Lund

Abstract

Acidification (pH< 4.5) was detected at Lake Jandabup, a shallow, seasonal wetland on the Swan Coastal Plain. Western Australia. Concern has been raised because populations of invertebrate species have declined or disappeared since acidification, potentially compromising the status of the lake as a nature reserve. The Jake bed is underlain with pyrite (feS2), which oxidises when dry producing sulfates, hydrogen ions and ferrous iron. Being seasonal, Lake Jandabup would be expected to be naturally acidic, but the more recent trend has been a prolonged (> 1 year) and extreme acidification (pH < 4.0). This study endeavoured to explain why the Jake may have undergone an acidic change, why it may have recovered and whether it will happen again. It was hypothesised that the acidification may have been caused by prolonged drying of the lake causing excessive oxidation of pyrite which produced more acid than the wetland could neutralise through buffering. The hypothesis was tested through laboratory and field experiments. The laboratory experiment involved taking intact sediment cores from around the lake, drying them for various lengths of time, and measuring pH, sulfate, total iron, total alkalinity and total aridity upon rehydration. Dried sediment from diatomaceous earth under Fe(OH)3 precipitate gave the lowest pH of less than 4.0. High sulfates, low alkalinity and low pH were interrelated. While drying in its own right produced low pH, generally there was no significant difference between drying times for release of selected variables. The field experiment involved the establishment of small in situ mesocosms to test for the effects of isolation of water on acid production and buffering capacity. Over an 18 week period between April and August 2000 weekly physico-chemical properties were taken inside and outside each in situ core and monthly, sulfate, total iron, total acidity and total alkalinity were tested. The field experiment showed an acidity producing area in the south western comer of the wetland in the diatomaceous sediment where iron hydroxide precipitate (an indication of acidification) occurs. Generally high sulfates and low total alkalinity were associated with low pH. providing evidence for pyrite oxidation. Over the study period the pH appeared to recover from <4.5 pH to 6.0 pH units. It is though that this recovery is only temporary as the pH increase can be attributed to dilution by rain and groundwater inflow. Longer term recovery will depend on future water levels at the Jake and ensuing that acid producing sediment remain anoxic. The best management option therefore is to prevent oxidation of pyrite by keeping the acidity producing sediments wet, however there are other biological and chemical processes that rely on a seasonal water regime and if disrupted could affect the functioning of Lake Jandabup. A compromise needs to be made between controlling acidification and maintaining a seasonal water regime at the lake.

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