Why Brisbane water tastes earthy after rain — and what removes it

What’s actually causing it — the science in plain terms

When Brisbane water tastes or smells earthy, grassy or musty, the culprit is almost always one or both of two naturally occurring organic compounds:

• MIB (2-Methylisoborneol): A terpenoid alcohol produced by certain algae (particularly blue-green algae/cyanobacteria like Anabaena) and some photosynthetic bacteria. The human nose can detect MIB at extraordinarily low concentrations — as little as 5–10 nanograms per litre (nanograms are billionths of a gram). This extreme sensory sensitivity is why even trace amounts produce strong taste and odour perceptions.
• Geosmin: Another naturally occurring compound produced by cyanobacteria and Actinomycetes bacteria. Geosmin is literally the compound that gives soil and rain their characteristic earthy smell. It is detectable by the human nose at concentrations as low as 5 ng/L.

Both compounds are entirely safe to drink. They are not contaminants in any health sense — they are naturally produced biological metabolites at concentrations that cause sensory responses but no toxicological effect. Queensland Health confirmed the water remained safe throughout both the January 2022 and December 2024–January 2025 taste events.

Emeritus Professor Jurg Keller from the University of Queensland’s Australian Centre for Water and Environmental Biotechnology, quoted by the ABC during the 2022 event: “These compounds are not ‘dirt’ and don’t come from dirt either. They are actually produced by algae and/or photosynthetic bacteria.”

When and why it happens — the trigger conditions

MIB and Geosmin events in Brisbane follow a consistent seasonal and weather pattern. The combination that triggers significant events:

1. Warm weather: Higher water temperatures in Wivenhoe Dam and the Brisbane River accelerate algal and bacterial growth.
2. Heavy rainfall followed by sunlight: Rain introduces nutrients into the dam. Subsequent sunlight and calm water conditions allow algae to bloom.
3. Still water: Reduced wind and flow creates stratified, calm water where algae can concentrate near the surface.

This combination occurs most frequently in summer (December–February), which explains why the major events in Brisbane — January 2022 and December 2024–January 2025 — both occurred in the post-wet-season period. The 2024 event specifically followed hot, rainy conditions in late December that increased algal levels in the raw water supply to the Mount Crosby Water Treatment Plants.

Why conventional water treatment can’t fully remove it

This is the frustrating reality that Seqwater openly acknowledges: standard water treatment processes — coagulation, flocculation, sedimentation and filtration — are very effective at removing algae (the organisms themselves), but are not effective at removing MIB and Geosmin (the compounds algae produce).

The compounds are present in dissolved form in the water at nanogram concentrations. They don’t respond to the physical separation processes used in conventional treatment. They pass through filters that would easily remove the algae that produced them. Seqwater’s response during events is operational — increasing releases from Wivenhoe Dam to flush and dilute raw water concentrations, routing more water through North Pine and Gold Coast plants, and moving water around the SEQ grid to blend and reduce concentrations. This takes weeks, not days.

Research led by UQ in collaboration with Seqwater is developing biological treatment approaches using moving bed biofilm reactors (MBBR), which achieved 84% MIB removal and 87% Geosmin removal in pilot studies. These are not yet deployed at treatment plant scale in Brisbane.

What actually works at home

Seqwater’s official suggestions — chilling the water and adding lemon — have a modest effect. The colder temperature mildly suppresses taste perception, and lemon’s citric acid partially masks the earthy flavour. These are helpful but not transformative.

A carbon block filter is the most effective household intervention:

• Standard GAC (granular activated carbon): Some reduction — approximately 40–60% — but effectiveness varies significantly depending on carbon quality, cartridge age, and contact time. Professor Keller’s comment: “Some water filters that contain activated carbon will be able to reduce these compounds somewhat, but it depends on how well the removal capacity is still in these filters.” A saturated or old GAC cartridge provides very little benefit.
• Quality carbon block (fresh cartridge): A high-quality carbon block filter in good condition can reduce MIB and Geosmin by 80–90%. The key is contact time — carbon block forces more intimate contact between water and carbon media than loose granular carbon. And critically: the cartridge must be fresh. An exhausted carbon cartridge treats neither chloramine nor taste compounds effectively.
• Activated carbon + RO: The carbon pre-filter stage in RO systems is highly effective at removing organic taste compounds. If you have an RO system, MIB/Geosmin events will be essentially unnoticed.

Will it happen again?

Yes — with reasonable certainty. MIB and Geosmin events are a recurring feature of surface-water supplies in subtropical and tropical climates. Seqwater confirms they “regularly monitor” for these compounds and that they are “sometimes present in drinking water” at detectable levels. Significant taste events in Brisbane have now occurred in January 2022, January 2025, and multiple other lesser events in between.

Climate patterns that produce warmer, wetter summers with more intense rainfall events are associated with increased frequency and intensity of algal bloom conditions. Brisbane residents living near the Mt Crosby supply area should consider this an ongoing seasonal variable rather than an exceptional event.