At the Forefront of Bioretention Media Specifications: an Interview with Curtis Hinman

Curtis Hinman is a senior scientist with Herrera Environmental Consultants in Seattle, WA. Before joining Herrera, Curtis was faculty with Washington State University (WSU) Extension and the Department of Biological Systems Engineering, and was the University’s Green Stormwater Infrastructure Specialist.  With WSU he co-designed and was lead scientist for the WSU Low Impact Development (LID) Research Program which is one of the largest LID research facilities in the U.S. Mr. Hinman is the author of the “Low Impact Development Technical Guidance Manual for Puget Sound” and the “Rain Garden Handbook for Western Washington.”

I was thrilled to have the opportunity to talk to Curtis a few weeks ago about his research on bioretention media, since he is at the cutting edge of research on this topic. Our conversation has been edited and condensed. – Nathalie Shanstrom

Nathalie Shanstrom [NS]: What are the main research questions you are investigating, specifically in regards to bioretention media?

Curtis Hinman [CH]: Three to four years ago, both Herrera and Washington State University, where I worked prior to Herrera, identified pollutant leaching from bioretention soils, which was largely the result of the compost content (the mineral component in the bioretention media also contributed). Like most bioretention media specifiers, we were using compost because it offers many benefits: it is a local waste product, grows plants well, has great water holding capacity, and high cation exchange capacity (helpful for plant growth and pollutant removal).

However, these benefits were counterbalanced by initial, and in some cases long-term, flushing of nitrates, phosphates, and dissolved copper (*see note at the end of this blog for more information on the significance of these pollutants). At that time there was not a lot of research on the leaching characteristics of the individual media components. So I and some collaborators (Ed note:see note at the end of this blog for list of collaborators) decided to start an experiment that aims to do the following:

1) investigate the leaching potential of individual media components (with a weak acid extraction),

2) identify materials with low leaching potential,

3) create various blends of components with low leaching potential,

4) flush these blends with clean water to see “what comes out,” and

5) dose these blends with synthetic stormwater to see how well they capture pollutants.

Can you share your initial findings with us?

We are finding that compost is not the only media component that can leach nutrients, some sands can even leach phosphorus and copper.

It has long been well documented in agriculture literature that when you plow, nutrients can leach out with irrigation or rainfall, so it should be no surprise that bioretention media, which are also disturbed, can also leach nutrients.

What different media are you testing?

We are testing 8 different blends:

  • a control blend which is the 60% sand and 40% compost mix typically used in Washington,
  • a mix with 90% sand and 10% compost, with a polishing layer underneath,
  • 6 mixes composed of sand low in phosphorus with coconut coir. The coconut coir is not only low in phosphorus, it also has a high water holding capacity and adds structure. Various soil amendments are added to the sand/coir mixes, such as high carbon wood ash (like biochar, but burned in an oxygen environment), which leached low levels of nutrients and copper in the weak acid extractions.

We are currently in the process of reviewing lab data, and preliminary lab results seem promising, as some of the blends appear to be working well, i.e. they are not leaching nutrients and they are capturing pollutants. The next steps will be more lab tests and full scale field tests. One of the critical questions to be answered in the next phase is whether or not plants can thrive in these media blends.

Have you found any other organic materials with low leaching potential besides coir?

Coconut coir is the best material we have found so far. We tested peat and found high levels of nitrogen leaching, although peat does have great water holding capacity and high cation exchange capacity, which is great for metals. However, using peat is not realistic considering how controversial it is from a harvesting and sustainability perspective.

Have there been any surprises in your results so far?

One of the biggest surprises has been realizing how complex it is to develop a media that has an adequate infiltration rate, grows healthy plants, is affordable, is readily available, and captures multiple pollutants. And I’ve been studying bioretention media for 10 years! Anyone who has ever written a specification can probably relate to this answer! I am hopeful that we’ve found some materials that fit all these criteria, but there is more work to do.

How do you see bioretention soil specifications evolving/changing over the next 10 years?

I expect there will likely be multiple bioretention soil guidelines. Different guidelines will be developed for specific objectives and project conditions. Not all projects need high performance bioretention soils. For a raingarden without an underdrain in an area with adequate native soils, for example, a mix of existing soil with compost will likely be the most sustainable and cost effective solution. A bioretention basin with an underdrain in a phosphorus sensitive lake basin would need to follow different guidelines, aiming to minimize phosphorus leaching and maximize phosphorus removal.

When do you expect your results to be published and where should people look for them if they want to find them when they are published?

I anticipae that results will first be published March 2015 on Washington Department of Ecology (DOE)’s website. I also anticipate the findings being published in journal articles after publication on Washington DOE’s website.

If you want to follow Curtis’s work, you can do so here. Thanks, Curtis!

*This work is supported by a grant from Washington Department of Ecology to Kitsap County; Technical lead is Herrera Engineering; collaborators are Washington Department of Transportation, City of Seattle, City of Redman, and Seattle University.

**Nitrates are especially problematic for bioretention practices above drinking water sources. Phosphates are problematic in phosphorus sensitive lake basins, as they promote plant growth, which can deplete dissolved oxygen, leading to anoxic lakes. Dissolved copper is toxic to salmonids (including salmon, trout, whitefish) and therefore especially problematic in the Pacific Northwest. Research, including Hinman’s, has found dissolved copper concentrations in runoff from bioretention exceeding regulatory thresholds. Even very low dissolved copper concentrations (below 10 ppb) have been found to result in sub lethal effects to salmonids. Subsequent research, however, found no sub lethal effects on salmonids even from higher concentrations of runoff from bioretention systems, presumably because in runoff from bioretention systems, dissolved copper is bound to dissolved organic carbon.

Aaron Volkening / CC BY 2.0

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