Optimising ecosystem services of (urban) soils
Where soils are used to mitigate effluent or stormwater they must provide key ecosystems services of water detention, water retention, and contaminant removal (Figure 1).
Detention lowers the peak rate of discharge to surface waters, helping reduce stream erosion and flooding. It may also enhance groundwater recharge. Retention reduces the volume of effluent or stormwater entering surface or ground water. Retention improves contaminant removal by allowing time for plant uptake (with evapotranspiration), adsorption to soil particles, or contaminant transformation (denaturing chemicals to innocuous derivatives). Plants rely on this retained water to sustain them between rainfall events.
Resource consents for areas (or devices) receiving effluent / stormwater should specify minimum soil criteria to increase the certainty that an acceptable level of performance can be achieved. Soil physical criteria include minimum (and maximum) infiltration. They may also require a minimum water storage volume (within a defined depth). Sometimes favourable conditions are assumed if soil textures are coarse, i.e. sand to sandy loam, or meet specific particle- size distributions. Natural soils may be modified, or new soils engineered, to meet specific performance criteria. The growing media used in rain gardens and swales are examples. In cities with few natural sandy soils, such as Auckland and Wellington, growing media are manufactured.
In Auckland, adding up to 50% sand to Ultic Soils (silty clays) to create sandy loam led to slumping, infiltration < 10 mm/h (below the minimum standard), and extended periods of anaerobic conditions. Wellington and Christchurch also report problems selecting suitable media. The soil physics laboratory at Landcare Research, Palmerston North, has developed a three-step method for screening potential rain garden media. This builds on joint research with the University of Auckland for Auckland Council (Fassman-Beck et al. 2013). First, the range of moisture contents a medium can contain is established. Second, the susceptibility of the medium to a standard level of compaction is measured at several moisture contents, including likely delivery moisture content and a relatively high moisture content. A resilient rain garden mix maintains a similar bulk density under a range of moisture contents. Finally, the permeability and water stored for plant growth in the medium are measured.
Suitable media have infiltration rates >50 mm/h and high water retention (at 100–1500 kPa tension). Pumice sand is an ideal base for rain garden mixes. It can hold up to four times more water than ‘standard’ sands (‘pumice’ vs ‘aggregate’ columns in Figure 2). Adding 20% v/v organic matter (composts) does not necessarily increase retention of pumice but slightly improves retention of sands. This is because the additional water is in pores drained at >1500 kPa (nominal wilting point) or tightly held to organic particles. Adding 20% v/v sandy-textured allophane (P+20%A) or an Allophanic Soil (P+20%T) to improve metal and P attenuation did not change retention. Measuring retention is useful to show arborists that very coarse textured media can supply as much water as their standard landscaping mixes, while being resilient to compaction. Pumice-based mixes also have much higher detention than standard tree mixes, providing greater stormwater benefits.
Auckland Council 2011. Rain garden construction guide. Stormwater Device Information Series: Auckland Council.
Fassman-Beck EA, Simcock R, Wang S 2013. Media specification for stormwater bioretention devices. Prepared by Auckland UniServices for Auckland Council. Auckland Council Technical report TR2013/011.
Simcock R, Dando J 2013. Mulch specification for stormwater bioretention devices. Prepared by Landcare Research for Auckland Council. Auckland Council Technical Report TR
John Dando & Robyn Simcock — Landcare Research