Engineers Australia

Residential development on the New Volcanics: A case study of suspected barium contamination and soil amendment

Roger Wrigley, Robert van de Graaff and Simone Lang

Abstract

As Melbourne expands to the north and west in response to growing demand for residential accommodation, soils developed on the Lava Plains (Geological Survey of Victoria, 1967), otherwise known as the New Volcanics (Cochrane, Quick & Spencer Jones, 1999), will be exploited. These soils are commonly shallow, alkaline and duplex, whilst the interspersed grey cracking clays pose significant constraints on building foundations and urban infrastructure due to their high “shrink-swell” nature. Rock floaters, clay fissuring, calcareous layers, sodicity and high electrolyte levels can serve as additional impediments.

The aforementioned soil characteristics also constrain garden development, landscaping and revegetation due to the presence of relatively heavy B-horizons with poor internal drainage, a relatively flat landscape with poorly-defined surface drainage, shallow heavy finely-textured topsoils and shallow calcareous bands which limit root propagation. The range of native and exotic plant species that can be planted in this landscape is limited, despite the need to improve aesthetics in order to attract residents.

Whilst the geological characteristics of soils on the New Volcanics are well understood and their limited potential as plant growth media recognised, the impact of soil chemical properties on some foundations, plant growth and heavy metal availability is not.

This paper describes the outcome of an investigation into potential barium (Ba) contamination of a 15 ha site located 18 km west of the CBD which threatened to stop the proposed residential development. Barium, as a trace element, concentrates in intermediate and acid magmatic rocks and its occurrence is linked with alkali feldspar and biotite (Kabata-Pendias, 2001). In soils in the natural environment, barium will generally occur as barium sulphate (BaSO4) due to the ubiquitous presence of sulphate ions. Some paint pigments contain barium salts and thus barium levels can be elevated on derelict land (Bridges, 1987). Barium is also associated with radio-active fall-out and nuclear waste. Barium sulphate is widely used as a safe tracer in medical practice due to the insolubility of the compound.

In conducting a soil survey for residential development where barium levels are found to be high it is essential to determine if possible why the levels are high, how widespread is the area impacted and whether anthropogenic input is likely. Of particular interest are the distribution of barium concentrations with depth and the association of barium with appropriate anionic concentrations. Evidence of site disturbance or encapsulation is also important as is the prospect of pseudo-stratification.

Knowledge of the availability of heavy metals for plants can assist the assessment of risk to humans from food chain transfers and contact with soil barium; fortunately there are well-defined relationships between pH and soil electrolytes that dictate this availability.