The classification of inland salt lakes in Western Australia

Abstract

Inland salt lakes in Western Australia have been used by the mining industry for the disposal of excess water generated during the mining process. However, the impact of these operations on the salt lakes is poorly understood. This is mainly due to the lack of information on the biota and chemistry for the lakes. The main aim of this project was to develop a classification system for inland salt lakes of Western Australia based on abiotic and biotic factors such as sediment and water quality, invertebrates and algae to determine lakes with unique or significant features. Water and sediments collected from the salt lakes were generally characterised by an alkaline pH, high salinity and the majority of lakes being dominated by sodium and chloride. Concentrations of some metals were also high, particularly in surface water. A high degree of variation in water and sediment quality was demonstrated both within and between the study lakes. In addition, these parameters were shown to be influenced by geography, geology, stage of the hydrocycle within which the lake was sampled and the occurrence of dewatering discharge. Biota in the salt lakes must be able to cope in a harsh environment, adjusting to temporary water regime, high temperature, and high salinity. As such, the species richness of these systems is generally low. Diatoms (a group of algae) and invertebrates were investigated among the biota. A total of 56 diatom species were recorded from 24 lakes. The most common species were Amphora coffeaeformis, Hantzschia aff. baltica and Navicula aff. incertata. These species were shown to have broad tolerances to environmental variations. Sediment chemistry explained variations in diatom community structure, with zinc, moisture content and cobalt having the greatest and negative influence.In terms of invertebrates, a total of 101 invertebrate taxa were recorded from 13 lakes in this study. Crustacea dominated and the greatest number of taxa was from the genus Parartemia. There were some differences in invertebrate community structure between lakes, most likely reflecting the high degree of speciation, and poor dispersal mechanisms of certain key species. Community structure was influenced by water quality, with phosphorus, bicarbonate and magnesium contributing to the variations in community structure. Among the 43 lakes chosen for this study a total of 17 lakes had received, or are currently receiving dewatering discharge. Sites receiving dewatering discharge generally reported higher concentrations of salts, nutrients and some metals in both water and sediments compared to natural lakes. Species richness of biota such as diatoms and invertebrates was lower at the lakes receiving dewatering discharge. However, the impact was generally localized within the pooled area of dewatering discharge. Also, despite these impacts, there appears to be signs of amelioration by flushing events. Currently there are no guidelines for water and sediment chemistry for inland salt lakes in Western Australia. Australian and New Zealand Environment and Conservation Council (ANZECC) guidelines are the most relevant available. Concentrations of cadmium, cobalt, chromium, copper, lead, nickel and zinc in surface water of the natural inland salt lakes were shown to exceed ANZECC guideline values.Comparison with the relevant ANZECC sediment guidelines showed that they were applicable to the salt lakes, with the exception of nickel and chromium which were naturally high in the salt lake sediments. Classification of data using multivariate analysis was done for both dry and wet phases of the hydroperiod. Six groups were delineated for the sediment and diatom data, and four groups were defined for the water quality and invertebrate data. It was common for sites from particular lakes to fall in more than one group as a result of the variability in these systems. There are a number of practical applications of this system for the mining industry and it may be used as a predictive tool for determining the impact of dewatering discharge and highlighting unique salt lakes within the Goldfields of Western Australia.