Geochemistry of acid brines: Lake Tyrrell, Victoria, Australia

Abstract

Pore water, groundwater and sediments were collected from Lake Tyrrell to study the evolution of brines in an acidic environment. Lake Tyrrell is a 150-km 2 inland salt playa. Acid-saline groundwater enters the playa through springs along the lake margin and is evapo-concentrated past halite precipitation. The brines were found to contain sea salts, but some solutes do not follow the seawater evaporation curve. Processes that occur in neutral-alkaline systems (e.g., gypsum formation) occur here, but precipitation of alunite (KAl 3(SO 4) 2(OH)6), jarosite (KFe 3(SO 4) 2(OH) 6) and Fe-oxides also affect solute behavior. Processes that occur in the recharge area and along the groundwater flow path (K uptake, calcrete and gypcrete formation) affect the brine's chemical signature. Regardless of the chemical variability found among the samples, solutes have evolved from a common source (sea salt). Acidification, which appears to occur early in the evolution of the groundwater, is related to pyrite oxidation and may be enhanced by precipitation of Fe-oxides, alunite and jarosite, and evaporation. Acidity is maintained in the system because of the lack of buffering capacity of aquifer minerals. We suggest that this acid brine from Australia is another end-member in the Eugster-Jones-Hardie model for the evolution of water by evaporation in closed basins.