Sources and chemical evolution of salts on the Australian continent

Abstract

This research investigates how the sources of salts in surficial waters and their chemical evolution are influenced by: distance inland from the coast, catchment lithology, catchment hydrology, climate, and agricultural and human activity. This project involved eight study areas in Australia: Acraman and Lake Eyre in South Australia, Yeelirrie and Disappointment in Western Australia, Natimuk and Colac in Victoria, Buchanan in Queensland and Kulgera in the Northern Territory.

The hydrochemistry of surficial waters sampled from the eight study areas indicates that both marine aerosol and rock weathering are predominant hydrochemical processes that contribute to the chemical composition of these surficial waters. The ³⁶C1 and ³⁴S isotope hydrochemistry data indicate that the marine aerosol is the principal source of chloride and sulphate in surficial waters. The rock-weathering component is most evident from the Na/Cl, K/Cl, Mg/Cl, Ca/Cl and HCO₃/CI chemical ratios and the alkali and alkaline earth normative carbonate salts of dilute waters - in terms of their departure from a marine hydrochemistry and a marine salt norm, respectively. The chemical ratios and salt norms for the surficial waters also reveal that the highly-modified marine hydrochemistries for dilute groundwaters chemically evolve to hydrochemistries for saline waters which are marine-like (are similar to marine chemical ratios, and have vestiges of the halite-carnallite-bischofite-anhydrite-keiserite-magnesite marine salt norm). However, the trend in Cl/Br, Na/Cl, K/Cl, Mg/Cl and Ca/Cl chemical ratios (from dilute to saline waters) and the presence of complex alkali and alkaline earth normative sulphate salts in saline waters indicates that rock-weathering reactions (such as calcite and dolomite dissolution and ion-exchange reactions) have contributed solutes at early stages in the chemical evolution of these waters. As these waters chemically evolve, their hydrochemistry evolves in response to additional hydrochemical processes (such as carbonate-, gypsum- and halite-formation and ion-exchange reactions).

The presence of clays in near-surface strata in all study areas contributes to the gradual decrease in cationic ratios (Na/Cl, K/Cl, Mg/Cl and Ca/Cl) due to ion-exchange reactions as dilute groundwaters chemically evolve to saline waters with marine-like hydrochemistries. The effect that ion exchange has on the chemical evolution of surficial waters is most evident for K+ ions. The findings of this project indicate that in the Colac study area, lake systems with the same basement lithologies but which have significantly different catchment hydrologies and hence significantly different groundwater contributions also have distinctly different hydrochemistries. This distinction is especially evident in the K/Cl ratios of their lake waters.

In the more temperate study areas (Natimuk, Colac and Buchanan), the chemical precipitation of Ca- and Mg-bearing carbonates from surficial waters also decreases their Ca/Cl, Mg/Cl and HCO₃/CI ratios with increasing salinity. However, in the more arid study areas (Yeelirrie, Acraman, Kulgera, Disappointment and Lake Eyre), calcretes and other soil carbonates have formed in drainage channels. The formation of dolomite is also common in these calcretes. Groundwaters flowing through such calcrete deposits commonly chemically evolve to have Ca/Cl and Mg/Cl ratios higher than those in waters of similar Total Dissolved Solids (TDS) in the more temperate study areas. The presence of near-surface marine sediments within a catchment (as for the Natimuk and Colac study areas) is also a potential source of Ca²+ and Mg²+ ions to groundwaters from their re­ solution of calcite and dolomite in the marine sediments.

In semi-arid and arid regions, gypsum is also commonly present in lake systems. Re-solution of this gypsum results in surficial waters having high Ca/Cl and SO₄/CI ratios compared to the marine hydrochemistry. This effect is most evident in the dilute to saline surficial waters in the Yeelirrie and Acraman study areas and in parts of the Natimuk study area (prior to halite saturation). In these study areas there is a significant presence of gypsum in the near-surface lithology, and biogenic sulphate reduction has a minimal impact on the pool of sulphate ions. However, in the more temperate study areas (Buchanan, Colac and in parts of Natimuk), where there is a more limited source of sulphate ions, biogenic sulphate reduction is the predominant hydrochemical process.

The low Mg/Cl ratios of Buchanan and Lake Eyre saline waters are largely attributed to the formation of palygorskite (a Mg-rich clay mineral). The high pH of the saline Buchanan lake waters, the high silica content of inflowing groundwaters and the sub-tropical semi-arid Buchanan climate are considered appropriate environmental conditions for the formation of palygorskite in lake deposits over approximately the last 1.6 Ma. Even though the palygorskite present in the Etadunna Formation at Lake Eyre formed during the Oligocene to the Miocene, the formation of palygorskite is likely to account for the significant depletion of magnesium in the saline waters of Lake Eyre and its subsurface brines.

The hydrochemistries of very saline waters in the arid study areas are significantly affected by halite crystallization and halite re-solution. Their relative predominance increases as a function of the aridity of the study area. A halite-deficient (hypersaline) component in near-surface Yeelirrie and Acraman groundwaters in arid study areas with no record of marine sediments is also suggested from the hydrochemistries of their groundwaters.

However, a hypersaline and dolomitic diagenetic characteristic of certain groundwaters in the temperate study areas, where marine sediments occur in the near-

surface strata (Natimuk and Colac), suggests that these groundwaters have acquired a marine connate component from re-solution of pore waters entrapped in the marine sediments. Although, the hydrogen and oxygen isotope hydrochemistry reveal that the waters in these aquifers are meteoric, there is hydrochemical and hydrogeological evidence in the Natimuk and Colac study areas to suggest that saline marine connate pore waters may be released from the weathering of marine sediments present in these groundwater systems.

This investigation has shown, from the hydrochemistry of lake waters in the more arid study areas, that bromide enrichment is indicative of hypersalinity. However, in the near-surface Yeelirrie and Acraman groundwaters, bromide enrichment may also occur by degradation of soil organic matter. Conversely, under different environmental conditions (such as experienced in the sub-tropical Buchanan study area), it is possible that bromide uptake by the biomass results in bromide depletion in near-surface waters.

The high nitrate concentration of groundwaters in the arid study areas has been attributed to biogenic sources in the near-surface soils and the lack of denitrifying activity. However, in the more temperate study areas, agricultural and human activity are responsible for low levels of nutrients in surficial waters.

The sulphur isotope hydrochemical data for the Australian continent suggest that close to the coast there is a significant sea-salt component in rainfall and marine aerosols. These data also suggest that along most of the east, west and south coasts of mainland Australia there is a significant non-sea salt sulphate (largely DMS-derived) component. However, the sea-salt sulphate component of the marine aerosol decreases with increasing distance inland. The sulphate component in marine aerosols, which reach the arid interior of Australia, is mostly of non-sea salt sulphate origin.

This investigation, involving the hydrochemistry of surficial waters from eight study areas widely distributed in Australia, has clearly shown that climate, catchment lithology, catchment hydrology, distance inland from the coast, and agricultural and human activity are important factors in determining the sources of salts in surficial waters and their chemical evolution on the Australian continent. Show more