by Norah Kates, Water Quality Planner/Project Manager at King County, WA

Right now, rain is something that we are all (almost all anyway) wishing for.  Rain ends the fires and brings on the chanterelles.  If this year is anything close to normal though, by sometime in November we will be wishing for anything but rain.  When it does rain more and more communities are working to keep that rain from flooding streams and treatment plants by using green stormwater infrastructure.  Green stormwater systems use a combination of organics and sand to allow rapid infiltration and contaminant removal.  They are green systems and are an asset to communities.

In Washington State, the DOE currently restricts the type of organics you are allowed to use in these systems.  Biosolids in any way shape or form have been prohibited based on the perception that they will leach rather than remove metals and organics from these systems.  Northwest Biosolids and King County have supported research on this very topic.  Our work has focused on developing quantitative tools to predict performance of these systems.  The thought is that by using these tools, certain types of biosolids might be highly effective ingredients for bioretention soils.

The last research on this topic was conducted by Norah Kates who received her MS a few years back.  Her study was published in a peer review journal this summer.  Norah’s work focused on testing a wide range of organics and different water treatment residuals alone and in combination to see if we could predict phosphorus movement from these systems.  Previously we had tested two different soil extractions; the phosphorus saturation index (PSI) and the phosphorus saturation ratio (PSR) for their predictive abilities. Both of these use an extract to predict how much P will become soluble and how much Fe (iron) and Al (aluminum) will be there to bind it.   Norah took this several steps bigger by deliberately testing mixtures across a range of PSI and PSR values.  Her ingredients included three WTRs, two alum based and one iron based and many types of organics including a high iron cake from San Francisco, food yard compost, Tagro potting soil, a yard waste compost, and multiple biosolids from King County. 

In general, she found that the organics released a lot of phosphorus and the WTRs absorbed a lot of phosphorus.  By putting them together at the appropriate ratios, she was able to sharply reduce phosphorus release.  Certain organics such as Class B cake are unlikely to ever be used in these systems. However, her research showed that by combining organics with WTRs at ratios to meet specific values for the PSI and PSR, it is possible to limit the potential for phosphorus loss from these systems.  She also found that if you look at the denominator of the PSI or PSR- the binding capacity of the system, you can get even better predictive abilities for the mixtures.  The denominator includes the molarity of the ‘glue’ of the system.  Two mixtures with the same PSR or PSI may have different molarities in the denominator.  The one with the higher concentration of Fe and Al is likely to leach less P over time.  This work adds to and reinforces our previous work on this topic.  Hopefully we’ll start moving the needle forward to greater acceptance of what works in these systems instead of building them based on what we think might be a good idea. 

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