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Sediments

There are no published accounts about the geochemistry and mineralogy of precipitates in WA drains. However, there have been reports of high concentrations of heavy metals and rare earths in drain discharge, raising concerns about possible pollution in ‘downstream’ wetlands. This concern seems to have developed from the recent recognition that acid groundwater is widespread in the southwest of WA.

A group led by CRC LEME (Co-operative Research Centre for Landscape Environments and Mineral Exploration) and including the WA Department of Agriculture and the WA Department of Environment has been researching this heavy metal issue as part of the Engineering Evaluation Initiative (EEI). Although their study focusses largely on the Avon Catchment, at YYCMG’s request, the study has been extended to include parts of the Yarra Yarra Catchment. Much of the information we have collected from pits and bores over many years has been made available to the group and, in return, some of their findings are available to YYCMG.

In their preliminary survey of drains in the Avon Catchment, the EEI group identified three broad types of precipitate as potential hosts for heavy metals, namely

  • red crusts and gels (iron oxyhydroxides)
  • white films and gels (aluminosilicates)
  • black ooze/mud (monosulphides).

We have recognised examples of each of these types in existing drains in the Yarra Yarra Catchment. In addition, some drain-wall crusts of carbonate (e.g. lime), sulphate (e.g. gypsum) and halide (e.g. salt) are known from other areas to contain trace concentrations of metals as impurities.

Samples from Yarra Yarra, as well as other parts of the wheatbelt, are being examined by an independent group at the University of Western Australia in Perth. Although this research is at only an early stage, it is clear from mineralogical work completed so far that some of the hosts are in fact hydrous or amorphous species. That is, they have no well-defined crystalline structure, but have instead a temporary and precarious existence, which depends on immediate environmental conditions (such as temperature, humidity, acidity and/or exposure to air). Amorphous phases are unstable and can change chemically (releasing whatever trace elements they include) to form stable minerals. Many of these minerals, such as iron oxides, are relatively stable in rainwater, but become soluble (along with their cargo of metals) under strongly acid conditions.

A more-detailed knowledge of the chemistry of drain water and precipitates will allow us to devise effective management strategies and plan for any problems that might arise from the composition of drain sediments. For example, if a drain is blocked and allowed to dry out, black monosulphide muds will react with oxygen in the air to generate acid. When flow resumes, the water will become considerably more acidic and capable of carrying higher concentrations of most trace elements, including heavy metals such as copper and cadmium. Another research finding that has management implications is that many of the minerals that are known to carry metals have specific requirements for oxygen (or the lack of it). This means, for example, that solubility might be enhanced in strongly reducing conditions, such as those produced by decomposing matter. For this reason alone, quite apart from hydraulic considerations like maintaining an unobstructed flow and removing scale (iron oxide) build-up on walls, it is important to clean drains periodically.

 




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