Building a recreational lake sounds simple enough. Dig a basin, let the water in, and enjoy the boating, skiing, and community events that follow. But in regions like Central Western Queensland, the soils beneath our feet can create challenges that go far beyond the surface. One of the biggest culprits? Sodic soils.
What Is Soil Sodicity?
Soil sodicity refers to the presence of high levels of sodium ions in the soil compared to other positively charged ions like calcium or magnesium. When sodic soils get wet, sodium causes clay particles to repel each other, a process called dispersion.
This is different from slaking, where soil aggregates break apart quickly when wet due to trapped air. Dispersion is a chemical problem, not just a physical one, and it can lead to:
- Bare, scalded patches with little vegetation
- Erosion and tunneling
- Ponded “milky” water after rainfall
- Poor infiltration, or soils that swing between boggy and rock-hard
Implications of Sodic Soils in Engineering and the Environment
Sodic soils affect both how we build and how landscapes function. Their impacts are tied to fundamental soil properties such as soil texture, soil structure, and soil salinity.
Engineering Implications
For engineers, sodic soils create a range of structural and construction challenges. A high exchangeable sodium percentage (ESP) weakens soil structure by disrupting the bonds between clay platelets, which reduces overall stability in earthworks, embankments, and lake floors. At the soil surface, dispersion can cause crusting and rapid erosion, particularly under heavy rainfall or wave action.
These soils also have reduced load-bearing capacity, making structures such as dams, levees, and roads more susceptible to cracking and settlement. On top of that, sodic clays absorb water unevenly, complicating compaction and moisture control during construction. Together, these factors mean that working with sodic soils requires careful design, monitoring, and often additional stabilisation measures.
Environmental Implications
Sodic soils have far-reaching environmental impacts that go beyond engineering challenges. When clay particles disperse, they clog soil pores, reducing infiltration and slowing water movement through the surface soil. This often leads to ponding, waterlogging, or increased runoff. As erosion accelerates, fertile surface soils are stripped away, degrading both farmland and natural ecosystems.
In saline–sodic soils, dissolved salts can temporarily hold clays together, but this stability is fragile. Rainfall or irrigation leaches the salts, lowering electrical conductivity and exposing sodicity, which triggers severe clay dispersion and erosion over time. Vegetation then struggles to recover, as poor soil physical properties restrict root growth and leave bare surfaces vulnerable to further degradation.
Management Outlook
In both engineering and environmental contexts, the goal is to improve soil structure to resist dispersion. Options include:
- Applying soil amendments like gypsum or lime (where viable).
- Adding organic matter to stabilise soil aggregates.
- Designing water bodies and embankments with soil risks in mind.
- Monitoring salinity and sodicity changes over time to prevent delayed failures.
But, How Do We Know If We Have Sodic Soils?
Sodic soils can be hard to identify by appearance alone. Bare scalded patches, poor plant growth, or crusting on the soil surface may raise suspicion, but these signs are not always reliable. To confirm soil sodicity, proper testing of soil properties is essential.
Material Testing for Sodicity
Diagnosing sodic soils takes more than field observation, proper material testing is essential. Engineers and soil scientists typically use a combination of laboratory methods and field assessments to evaluate soil properties. The key indicator is the Exchangeable Sodium Percentage (ESP). In Australia, soils are generally classified as sodic when ESP exceeds about 6%. At this point, excessive sodium ions cause clay particles to disperse, breaking down soil structure and reducing stability.
Supporting tests such as pH and Electrical Conductivity (EC) help determine whether sodicity is linked with salinity or strong alkalinity, while assessments of soil texture and clay content indicate how vulnerable the soil is to dispersion. Simple field techniques like the Emerson Aggregate Test can also reveal whether soil aggregates remain stable or disperse when wet, providing a practical check on laboratory results.
Sodic Soil Distribution in Australia
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In Australia, sodic soils occur in many landscapes, especially in low rainfall environments where dissolved salts are not easily leached from the soil profile. They are common across Queensland, New South Wales, Victoria, and particularly Western Australia, often forming sodic duplex soils with dispersive clay beneath sandy or loamy topsoils.
These soils can be adversely affected by rainfall or irrigation. When soil dries, aggregates harden and restrict root penetration, while under intense rainfall, dispersion reduces water infiltration and causes surface crusting that limits root growth and crop yields.
Case Study Crumb: Hughenden Recreational Lake
Hughenden, a small town halfway between Townsville and Mt Isa, set out to build a 20-hectare recreational lake to revitalise the community and create a new tourism drawcard. Designed for watersports and events (but not fishing or town supply), the lake promised big social benefits, but the soils told a different story.
The Challenges
- Highly sodic alluvial clays and mudstone on the lake floor
- Natural presence of gypsum and high calcium/sulphate levels, making gypsum treatment unfeasible
- Very soft mudstone with low strength
- Limited construction water (the site was bone dry)
- A tight budget, ruling out expensive lining options like HDPE
The Dilemma
On paper, Hughenden’s sodic soils should have dispersed severely. Yet laboratory tests showed an Emerson Class of 4, suggesting moderate stability; likely because natural soil salinity was helping hold the clay together. That seemed reassuring at first, but it came with a catch: once exposed to fresh water, salts could leach out, leaving the soils vulnerable to long-term dispersion.
The Solutions Considered
Gypsum is the standard fix for sodic soils, but testing at Hughenden showed ESP values above 25% and naturally high calcium sulfate levels. With gypsum already present in the soil, further application offered no benefit.
Other ideas were explored. A saltwater lake for clay stability, or synthetic liners to seal the basin, but both were unsuitable due to recreation needs and cost. Instead, engineers turned to soil conditioning and compaction, ripping the lake floor, adding moisture, and compacting it in layers. Over time, organic matter in the sediments was expected to help stabilise the soils.
Final Thoughts
Soils may look uniform on the surface, but their chemistry can make or break major water projects. In Western Queensland, understanding sodicity is not just an academic exercise, it’s the difference between a thriving recreational hub and a leaking, unstable basin.
The Hughenden Recreational Lake highlights that with testing, tailored remediation, and smart compromises, even challenging soils can be turned into a solid foundation for community success.
Download the PDF here.