Stormwater quality is a major focus for Melbourne Water. Carefully designed waterways and wetlands are being used to slow stormwater and filter pollutants in new residential and commercial developments. These structures are therefore essential to the management of stormwater flows and water quality in receiving water bodies (i.e. rivers, lakes and coastal waters).
Back in 2019 in my capacity as a Certified Professional Soil Scientist (CPSS – https://www.soilscienceaustralia.org.au/cpss/) and horticultural scientist, I was asked to prepare technical specifications for Melbourne Water’s assets. The brief required four soil specifications that could be applied to the entire metropolitan area. That was a challenging task given the huge diversity of soil types across Melbourne.
As it turned out, the majority of new Melbourne Water (MW) assets were in the most rapidly growing parts of the city, i.e., the north and west. These areas are sited on quaternary basalts which give rise to fine textured soils with substantial chemical imbalances. By fine textured, I mean that they are high in clay and silt; and by chemically unbalanced, I mean for the purposes of growing plants for wetland function and public amenity. Fine textures and chemical imbalance result in risks to drainage and consequent waterlogging.
The chemical imbalance in these soils is due to high to very high levels of sodium and magnesium and low levels of calcium.
Why is this a problem? Because calcium is the most important chemical element for soil structure. Soil structure is known as the soil’s aeration and drainage network. When sodium and magnesium are elevated and calcium is low, soil structure is very weak. Clay colloids in soils dominated by sodium (sodic soils) effectively dissolve (disperse) in the presence of fresh water (rain). This phenomenon is exacerbated by high magnesium and can result in waterlogging, death of plants, soil erosion, and turbidity and sedimentation of waterways.
The requirements of the MW specifications are therefore necessarily strict. However, this also means that achieving compliance can be difficult. This is why a caveat was included in the specifications that allows a CPSS to deem a soil compliant if a level of fitness-for-purpose can be achieved.
Happily, in most cases, a level of fitness-for-purpose can be achieved. However, this does require that the CPSS also has a sound understanding of plant growth requirements so the marriage between amended soils and healthy plants can be long and happy.
