The Department of Water (DoW) are constantly on the look out for effective ways to reduce nutrient inputs into groundwater systems and waterways. Using funding provided by the State Government’s Swan Canning water quality improvement plan (SWQIP), the DoW has been investigating how to reduce nutrient losses from new residential and commercial developments. This is particularly critical on Perth’s Swan Coastal Plain due to its largely sandy conditions, upon which agriculture and horticulture (think bucket loads of fertiliser!) was a historical main stay of the city’s growing population.
Most recently, the DoW has found success in trialling IMG, or Iron Man Gypsum: a by-product of mineral sands processing, to treat nutrients in groundwater. More specifically, using IMG as a soil amendment around subsoil drains discharging urban groundwater to clean it up before it heads downstream!
After four years of field-scale trialling at a turf farm in Ellen Brook, the material was found to be very effective at preventing dissolved Phosphorus from leaching into groundwater, and producing no other contaminants that might be a risk to the environment. Upon this basis, the DoW decided to try IMG in an urban development with a high watertable full of nutrient rich groundwater.
This led to a collaboration with Urban Quarter at their Abingdon development in Southern River. The development is located within the Canning River catchment in a classic high watertable/sandy grey Bassendean soil site mixed in with swamp deposits. As with many outer suburbs of Perth (no longer) the area saw more than 40 years of chicken/cattle farms, orchards and nurseries providing nice, juicy sources of Nitrogen and Phosphorus.
Unfortunately for our waterways, this agricultural legacy has resulted in an increased movement of nutrients as developments have been constructed in combination with subsoil drainage infrastructure (to avoid muddy backyards). So the clever scientists down at DoW came up with the brilliant idea of installing a blend of IMG/sand blend to encase subsoil drains, to try and capture some of the nutrients before they travel on their merry way down to the Canning River and supply the local algae population.
The material was provided free for trial use and trucked to site for installation in 2014. The unblended IMG was delivered to site before being mixed in batches and placed within excavated trenches in which subsoil drains were laid. The main premise of the design being that groundwater is sucked into the subsoil drain filters through the soil amendment for treatment before getting carried away and discharged downstream.
Monitoring bores were set up next to subsoil drains utilising both amended and non-amended fill for regular sampling post-construction. Pre-existing shallow monitoring bores were also used to establish background levels, and subsoil drainage discharge was also sampled on occasion. Samples were collected and groundwater levels measured throughout 2014-2015. Geotechnical properties of the amended fill was also tested to check compaction, permeability, and load.
Results from the first year of monitoring indicate that IMG blended fill has been very promising at treating nutrients in shallow groundwater, particularly dissolved Phosphorus and soluble organic Nitrogen. In addition, it could potentially be used under structures such as house foundations or pavements which could be very handy in areas of limited space. No more can be said until findings are officially published by the DoW!
OR you can find out more by heading down to the New WAter Ways Water Sensitive Cities Speakers Series event this Friday: Dr Brad Degens (Senior Soil and Water Scientist/Hydrogeologist, Department of Water) and Stuart Reside (General Manager, Urban Quarter) will be presenting findings from the Abingdon trial in detail then.
No registration required (it’s free!), just get down to the Department of Water’s Atrium Theatrette (168 St George’s Tce) at 12.30pm on Friday 26th August.
Subsoil amendments will be included in the next version of UNDO (Urban Nutrient Decision Outcomes) – DoW’s new web-based decision support tool which evaluates nutrient reduction decision for urban developments on the Swan Coastal Plan in south-west WA.
So here’s to DoW utilising new materials and technology to better manage our urban groundwater and river systems – let’s get behind more of that!