I spent a good part of last winter completing a literature review on bioretention media for a revision to the Minnesota Stormwater Manual. So when I attended the Low Impact Development (LID) conference in St. Paul, MN earlier this month, I was very interested to learn of a recent specification coming out of New Zealand: a 129-page technical report, Media Specification for Stormwater Bioretention Devices (Fassman et al 2013). I looked at it the very next day: what a goldmine! Where was this when I was doing my literature review?!
Co-written by an engineer and an ecologist / soil scientist, and backed by an extensive literature search, lab tests, personal communication with many other disciplines, and soil suppliers, it is the most comprehensive and well rounded publication I have seen on bioretention media to date.
Today I’ll be providing a brief overview of the scope of the report and highlighting a few of the things I liked the most about it. I will not give an overview of the results of the studies conducted in this report;those are well summarized in the report’s conclusions section.
Report Scope Overview
The investigation described in this report consists of three parts, a literature review and two stages of investigation.
A) Literature review
The literature review includes:
- a sampling of recommended bioretention media guidelines from North America, Australia, and New Zealand, including guidelines for aggregate and organic composition, media depth, ponding depth, hydraulic conductivity, particle size distribution, compaction, and media guidelines for pollutant removal.
- summary of trends in bioretention media over time; i.e. how bioretention media guidelines have evolved since some of the first bioretention applications in the 1990’s and why.
- a review of laboratory tests to date on performance of bioretention media and bioretention media with additives.
- a review of field tests to date of bioretention media performance.
B) Stage 1 Investigation
According to the authors of the report, “The aim of the first stage of research was to assess how candidate bioretention filter materials react to different mixing and compaction treatments and how their performance compares to criteria established from a literature review” (Fassman et al., 2013).
The authors searched for a media with the following design criteria:
- “Have a high enough hydraulic conductivity to allow for surface infiltration of stormwater meeting a specified time of water drawdown from a maximum ponding depth to prevent extended water ponding (i.e. minimum hydraulic conductivity).
- Have a low enough hydraulic conductivity to allow for stormwater to be retained in the media for a sufficient contact time to allow for pollutant removal mechanisms to operate (i.e. maximum hydraulic conductivity).
- Have chemical composition to remove pollutants.
- Provide plants with required nutrients and water to allow for sustained long term growth.
- Be structurally stable and maintain even flow through media (avoiding preferential flow)” (Fassman et al 2013).
In addition to meeting the above design criteria, the media also needed to meet the following assessment criteria:
- “Is readily available from New Zealand sources (preferably in the Auckland region).”
- Can be consistently supplied with adequate quality control” (Fassman et al 2013).
Stage 1 investigation included:
- Visiting suppliers
- Testing particle size distribution of the available aggregates
- Compaction testing of the selected bioretention media
- Testing hydraulic conductivity of media
- Testing chemical composition of selected aggregates and composts, including pH, organic carbon (C) %, total nitrogen (N)%, C:N ratio, Olson phosphorus (P), total phosphorus (P), anion retention, C:P ratio, Cation Exchange Capacity (CEC), base CEC saturation
- Testing the chemical composition of the same parameters for selected bioretention media, unleached and leached by water.
C) Stage 2 Investigation
The second stage was a performance test of the best media chosen during the first stage. Specifically, “the aim of the second stage was to carry forward the best candidate media from the first stage and conduct water quality tests. Water quality tests aim to provide estimation into the pollutant removal ability of the media, as well as developing an understanding of how the pollutants are being removed. Several of the mixes tested for water quality performance were also subjected to plant growth trials in a shaded glasshouse” (Fassman et al 2013).
Stage 2 included investigation of the following on pollutant removal efficiency:
- effect of media contact time on pollutant removal efficiency
- cumulative pollutant mass loading and breakthrough
- event based pollutant removal efficiency
- effect of media aging on pollutant removal efficiency
- effect of drying period between storm events
Throughout this report, it becomes evident how important it is to combine literature review with lab tests and multidisciplinary communication to study low impact development (LID). Here are a few examples.
Compaction testing to quantify the effects of compaction on soil-based vs. sand-based bioretention media
One of the big challenges in bioretention construction is that the same media can have widely varying hydraulic conductivity depending on the level of compaction. Though it is well know that compaction significantly influences hydraulic conductivity, very few bioretention guidelines to date specify how or whether the bioretention media is to be compacted. This study actually addresses this issue and studied in detail how water content influenced compaction and hydraulic conductivity of various media as well as the effect of compaction by light tamping vs. wetting and drying on various media.
The authors conclude:
“Compaction testing shows water content is an important property to consider when installing filter media as different water contents will produce significantly different densities of media under the same compactive effort. Compacted densities for media with a high proportion of compost can change by up to 26% as the water content is altered with infiltration rates being reduced by a factor of four. East Coast Sand and Woodhill Black Sand without organic content display no change in resulting compaction when water content is varied, indicating that the organic proportion is of most importance when it comes to compaction of sand:compost mixes. Most bioretention filter media will contain organic matter, and therefore it is important to specify a comprehensive compaction strategy which takes into account water content… Testing also shows the hydraulic conductivity of sand-based mixes does not significantly differ between light tamping compaction or wetting and settling compaction.” (Fassman et al, 2013, p. 102)
These tests enabled the authors to write a specification that will provide desired compaction levels with a predictable impact on hydraulic conductivity.
Thorough approach to evaluate available aggregates
Unlike most bioretention media reports, this report actually describes the mineralogical composition of the aggregates studied, as well as the chemical properties that affect pollutant reduction. Dialogues with suppliers proved helpful to determine quality control, availability, consistency of supply, and cost. The importance of talking to the suppliers was highlighted, for example, when the authors found out that one of the materials that best fit their criteria, scoria, was not likely to be available long term: “Unfortunately, although scoria was identified as a desirable aggregate, meeting with the supplier revealed that scoria production in the Auckland region in the future is uncertain, with no substantial new resources being available once Three Kings Quarry is exhausted (Winstone’s pers. comm. 2010)” (Fassman et al 2013, p. 66). Knowing this allowed the authors to drop this aggregate from further studies, and use their time and resources more productively studying other materials with a more certain long term supply.
Importance of multi-disciplinary communication
I was intrigued to read in the section on “Event Based Pollutant Removal Efficiencies” that media sorption can have different responses when exposed to high concentrations of phosphorous versus low concentrations of phosphorus. The authors found that four out of five media reduced phosphorus when exposed to high phosphorus concentrations; however, when exposed to low phosphorus concentrations, they instead leached phosphorus. The authors explain that “High concentrations of phosphorus may cause other ions to be displaced from media anion sites, to make anion sites available for phosphorus ions. For high concentrations of phosphorus, the ion usually displaced from the anion sites is the OH- ion, followed by Cl-, NO3- and SO42- (Parfitt 2011). The displaced ions may not have been displaced if instead it was a low concentration of phosphorus dosed” (Fassman et al 2013, pp. 80-81).
I’ve read a lot of research papers but had never seen a reference to Parfitt (2011) before, so I looked it up in the references section, and found that it was personal communication with a biogeochemist. I was impressed that the authors did not just do a literature search and lab studies, but also talked to a biogeochemist. Perhaps this is a no-brainer to some people, but how many people actually do this?
As the authors acknowledge themselves, this study did not include field studies, and research is showing more and more that, due to factors like the interactions of soil, vegetation, microbes, and pollutants over time, field study results are often very different from lab studies. For this reason, field studies are needed to confirm the results of the studies and findings from this technical report. However, preliminary tests like the ones described in this report can be invaluable to guide field studies to be as informative and efficient as possible.
This report is a great resource for stormwater professionals who want to get an overview of the “state of the science” of bioretention media worldwide, as well researchers studying bioretention media. It will also be invaluable for bioretention media suppliers to know exactly what the market is looking for.
Fassman, 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. Downloaded August 2013.
Nathalie Shanstrom is a landscape architect with The Kestrel Design Group.