Victor Frankenstein would feel right at home as a stormwater engineer. His success at cobbling together random body parts to create a living man/monster would put him in good stead to build a bioretention cell – with gravel, underdrain pipes and clean-outs, mulch, and plants all assembled from various origins to create a living stormwater practice. If this stormwater creature had a brain, it would certainly be the engineered soil media, as the media is responsible for processing pollutants, draining properly, and growing the plants.

If this is the creature that is lurking in our landscapes, is it a sustainable one? Put another way, why are we importing large volumes of an engineered soil media into our practices at great cost and use of fossil fuels, and are there alternatives to this approach?

This question has stirred my curiosity for many years. In the early days of bioretention (1990s), I worked for Albemarle County, Virginia, and we were eager to adopt this promising technology. In those days, our field experience working with contractors was often not so good, particularly when it came to the mixing of the soil media components (topsoil, sand, compost) at construction sites. The process suffered from a critical lack of materials quality control and consistency, and inspectors had several bad scrapes with contractors due to this. As bioretention applications continued to expand, there was interest from a local quarry to market a pre-mixed recipe. This ended up being a great advance, as the product, at a known unit cost, could be specified by design engineers and included in construction bids. The origin of this vendor-supplied soil mix was to address the aforementioned issues: quality control and consistency.

Since that time, there has been an evolving fascination in the stormwater profession with this soil media and continued tweaking of the recipe. Beginning with percentages of topsoil, sand, and compost, the mixes became increasingly more sandy with less compost – and for good reason, as monitoring studies demonstrated a consistent pattern of nutrients leaching out from the earlier generation of practices. The mix recipe continues to change, with variability existing between states and even jurisdictions.

Also, as LID became embedded in state and local stormwater standards and design manuals, demand for the manufactured soil mixes increased significantly. At this point, it appears to be a robust market as well as a challenging product for vendors to deliver, given the changeability and prescriptive (and sometime conflicting) nature of the standards. From a performance standpoint, monitoring studies indicate that the soil media does an admirable job of processing some pollutants (especially in their particulate forms) as well as providing impressive levels of runoff volume reduction.

So, why the question at the head of this article. . .doesn’t the evidence demonstrate that it is a sustainable practice to use such soil mixes?

Over the years of inspecting and designing practices, I watched the excavators scoop out the existing site soils to surprising depths, haul it away in large dump trucks, and then have more dump trucks roll in with the replacement “black gold” from a vendor. I also stewarded some project budgets where the soil media became the juggernaut of the cost estimate spreadsheet. I admit to scratching my head at times, wondering if it was all worth it in terms of environmental trade-off.

Furthermore, the horticulturalists and landscape professionals were getting into the game. Through teaching the stormwater sections of multiple Chesapeake Bay Landscape Professional (CBLP) certification classes (https://cblpro.org/), I interacted with many talented landscape professionals who are leaning heavily towards native plant communities for stormwater practices. How can we utilize native plant designs when we are wholesale removing and replacing an integral part of those communities – the very soil that the plants are growing in?

Recently, I came across a monitoring report from Villanova University, not only this year’s NCAA basketball champ, but one of the leading stormwater research institutions, under the steady and creative guidance of Dr. Robert Traver. The report included the following statement in the Findings section:

Life Cycle analysis – A life cycle approach is needed for SCM (stormwater control measure) evaluation of ancillary benefits. For example, the embodied energy and pollutants produced when quarrying sand (energy, carbon, etc.) and producing mulch for the Bioinfiltration Rain Garden negated the environmental benefits for the first two years of its life. Quantified life cycle benefits (or avoided impacts) during the operation phase of the system suggest that continued environmental performance of rain gardens and other green infrastructure offer services that not only offset adverse life cycle impacts but provide net benefits. . .(Wadzuk et al., 2017, p. 32).

That seems encouraging – that the environmental ledger sheet tips to the positive after a couple of years. Still, the question nagged at me: could we, at least in some cases, avoid creating such a deficit in the first place?

To further satisfy my curiosity, I performed some simple math on one of my own recent projects: about 1,100 square feet of bioretention with 24 inches of soil media and an underdrain. My rough calculations indicated an energy footprint of about 4,900 pounds of CO2. For that project, purchasing and hauling the soil media constituted just over 25% of the project budget.

As for modeling an appropriate native plant community that would be suited to growing in a very sandy soil media, I was very fortunate to have the expert guidance of Devin Floyd from the Center for Urban Habitats (https://centerforurbanhabitats.com/). Devin modeled a plant community based on a nearby floodplain that exhibited soils more aligned with those of the imported soil media. Devin is also a local guru and advocate for using a natural plant community approach to landscape design so that these projects can also support a conservation biology mission.

As can be seen in the photo below, the approach seems to have led to some happy plants!

After giving this some thought, I decided that a case could be made for both using the engineered soil media or perhaps dispensing with it.

The Case For Using Engineering Soil Media

  • For bioretention, it’s not just the rain falling on it that has to be managed. There is a drainage area — often consisting of several acres of parking lot, rooftop, and managed landscapes – that is diverted to the practice. This fact alone creates sophisticated hydrologic demands on the system. The soil media has to handle this inflow, pass it down through the soil column, and remove pollutants, all the while providing enough of a growing media for vegetation. This is a tricky set of circumstances that requires careful design and attention to standards.
  • As noted, the vendor-supplied soil media works with regard to pollutant removal and runoff reduction. The vendors I am familiar with have been conscientious in keeping pace with evolving soil media standards in various state stormwater manuals, and have done lots of research to improve the product. Most of us in the profession likely do not appreciate fully the complexity of mixing methods, sourcing materials, and quality control that goes into delivering this product to the marketplace.
  • In many cases, it’s required. Particularly for regulated new development and redevelopment projects, the specifications require not only a soil media that meets the standards, but very specific depths and installation methods. Even design guidance for some small-scale or residential projects require the engineering soil media. Using the engineered media and associated standards leads to predictable outcomes with regard to compliance, pollutant removal rates, and costs.
  • It is a good thing that there is a vendor industry out there, as newer soil media specifications may become more sophisticated, adding reactive elements (e.g., iron, aluminum, biochar) to remove soluble forms of phosphorus and/or enhance water holding capacity (see, for instance, the resources from the Chesapeake Stormwater Network: http://chesapeakestormwater.net/?s=performance+enhancing+devices).

The Case For Doing Without Engineered Soil Media

  • For some smaller practices or those with relatively small drainage areas, sustainable landscaping principles would suggest conducting a site assessment, taking soil samples, and designing a system based on the site conditions rather than engineering those conditions away. As documented in this article, this approach would certainly save some money as well as reduce the carbon footprint. The designer can still choose to amend existing site soils, but that is still far different than excavating to three or four feet and replacing the soil.
  • Some academic work is suggesting that we may be over-designing (and over-draining) our practices with automatic resort to underdrains and engineered soil media, and that more sustainable approaches are possible. Modelling indicates that in-situ soils can be retained for many Hydrologic Soil Group A, B, and sometimes C soils, and that good pre-design testing of saturated hydraulic conductivity and infiltration rates is essential (Lee et al., 2016).
  • On a more empirical level, those that have experimented with this approach (including the aforementioned Mr. Floyd) report that very good results are achieved by expanding the practice footprint to 3 to 4 times the standard design, even with Piedmont clay soils. The expanded footprint helps spread the water out into a vegetated landscape and provides a larger surface area for the processes of evapotranspiration, adsorption, and exfiltration. This also provides an expanded footprint for native vegetation, which has ancillary benefits for the ecosystem. While you’re at it, try establishing a dense planting scheme and do away with the mulch, especially after the first growing season (more carbon footprint reductions). This could emerge as an interesting hybrid between conservation landscaping and bioretention.
  • This approach also allows the designer to create a plant community that would be suited for the particular site, including its soil, instead of creating an imported landscape adapted to a uniform, sandy, engineered soil media.

I can relate to Victor Frankenstein, as he simultaneously loved and questioned the wisdom of his creation. Fortunately for our stormwater profession, we have the opportunity to continue to innovate and improve on our “sustainable” stormwater creatures.

References

Lee, R.S., Traver, R.G., and Welker, A.L. 2016. Evaluation of Soil Class Proxies for Hydrologic Performance of In Situ Bioinfiltration Systems. Journal of Sustainable Water in the Built Environment, 2016, 2(4):04016003.

Wadzuk, B., Traver, R., Komlos, J., Smith, V., Welker, A., and Ward, A. 2017. Project Report: VUSP – PADEP: Best Management Practice National Monitoring Site, Year 13 – 2016, Nonpoint Source National Monitoring Program, Clean Water Act Section 319. Villanova University College of Engineering, for: U.S. Environmental Protection Agency, Pennsylvania Department of Environmental Protection. http://www.villanova.edu/VUSP


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