Is there an Acid Demon Beneath Lake McLarty?

PHCC’s Rick James examines lake bed sediments at a dry Lake McLarty

Lake McLarty is an “ephemeral” wetland – meaning that it naturally dries out for a few months most years.  Due to Climate Change, these dry phases have been longer and this has the potential to affect the quality of the lake’s water and sediments when the first flows of autumn interact with the dry lake bed sediments.

The recently published State of the Climate 2020 report produced by the Bureau of Metrology and CSIRO paints a worrying picture for the south-west.  Since 1970 our April to October rainfall has declined around 16% with the Winter May to July rainfall declining by 20%.  For ephemeral wetlands like Lake McLarty this means more frequent and extended dry phases with the potential for lake bed sediments to dry out to deeper depths than has been the case in the past.

The term “Acid Sulfate Soils” is applied to soils containing iron sulphide minerals such as pyrite.  The iron sulfides are formed naturally by bacteria under anaerobic (oxygen-free conditions) and where there is an abundance of sulfate, organic matter such as decaying vegetation and iron.  They are typically found in distinct soil layers that, if kept wet, are harmless.  However when these layers dry out and are exposed to oxygen through deep drying and cracking of the overlying sediments, a chemical reaction can occur which results in the formation of sulfuric acid.  It’s not unusual for iron sulfide layers to occur in lake bed sediments on the Swan Coastal Plain, and as long as the dry phases are relatively short and these layers remain waterlogged and undisturbed then there’s no problem.  However, if exposed to the air due to episodes of extended drying then any resulting acid will be released from the soil into the lake water as it re-fills, reducing the water quality with potential impacts across the whole wetland ecosystem. .One indicator of the activation of Acid Sulfate Soils is a decrease in pH of the water, which is usually accompanied by an increase in the concentrations of nutrients and metals such as iron, aluminium and in some cases arsenic which are mobilised from the sediments by the increasing acidity of the water.

It’s known that soils containing iron sulfide layers do occur in the Lake McLarty area.  A comprehensive program of soil sampling in the lake bed and surrounds will commence soon during the hot, dry season of Boonaroo at this important Ramsar-listed wetland to determine the distribution, depth and thickness of these layers under the lake.  This work will identify the potential scale of the problem if lake bed sediments continue to dry out more than they have in the past.  Armed with this information, management options will be developed to help deal with this emerging threat to Lake McLarty and the valuable habitat that it provides for shorebirds, including those migratory species that travel thousands of kilometres to visit us from the Northern Hemisphere each year.

This work is being delivered by PHCC through its ‘Saving Lake McLarty Phase 1: Addressing acidification, hydrology and habitats’ project funded by the WA State NRM Program.