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Are Rain Gardens Mini Toxic Cleanup Sites?

Here's what happens to the pollutants swept up in stormwater runoff.

Lisa Stiffler on January 22, 2013 at 10:20 am

This post is part of the research project: Stormwater Solutions: Curbing Toxic Runoff

oily puddle

Oily puddle, Flickr user Banalities.

If you’re concerned about water pollution, you’ve likely heard this message: The water that gushes off our roofs, driveways, streets, and landscaped yards is to blame for the bulk of the pollution that dirties Puget Sound and numerous Northwest waterbodies. You probably also know about the most popular stormwater solutions, including rain gardens and other green infrastructure that soak up the filthy water, cleaning it before it reaches sensitive waterways that are home to salmon, frogs, orcas, and other wildlife.

But those two ideas taken together are making some people anxious. If stormwater is the source of such devastating amounts of petroleum and heavy metals, won’t the rain garden in my front yard become a mini toxic waste site that could harm children and pets?

So what exactly is in stormwater? Washington’s Department of Ecology has identified runoff as the prime source of the mercury, lead, copper, petroleum and other dangerous chemicals getting into Puget Sound (Table ES-1).* Ecology officials estimate that more than 400 pounds of lead, for example, are being washed into Puget Sound via the stormwater that flows from residential areas.

By tonnage, though, the most significant stormwater pollutants are dirt, oil and grease, nitrogen-containing compounds and phosphorus — not the heavy metals and other stuff that’s scarier from a human health perspective (Table 15). And for the pollutants that you wouldn’t want to come into direct contact with — lead, cancer-causing petroleum pollutants, etc. — nearly all are found in super small concentrations in stormwater.

For residential runoff sampled in the Puget Sound area, pollutants in stormwater were below levels of concern for everything except PCBs and phthalates (a family of plasticizing chemicals added to countless consumer items including lotions, perfumes, and soft plastics such as shower curtains; Table 12), but even these were at tiny concentrations.

The problem isn’t so much with the level of pollution in a given bucket of residential runoff, but the fact that the thousands of miles of roads and countless rooftops create so danged much of it. And yet the pollution is there, so what happens to it when it soaks into a rain garden or similar green stormwater infrastructure? Does it stay in the rain garden, or percolate through it along with the water?

Rain garden

Roadside rain garden in Seattle, Lisa Stiffler.

The fate of stormwater pollution

Scientists have answered the question of where runoff pollutants wind up through two types of experiments. They have sampled the runoff flowing into and out of actual rain gardens, and they’ve done laboratory experiments where polluted water is run through a column of soil analogous to a rain garden. (**I’ve listed many of the useful references that I’ve been able to find on stormwater pollution at the end of the post.)

The scientists found that the gardens do a great job catching metal pollutants and oil and grease — in some cases trapping more than 90 percent of the pollutants — keeping them out of streams and lakes where they harm wildlife and contaminate water for swimming, fishing, and other human uses. Rain gardens, often called bioretention systems, swales, or bioswales in the scientific literature, have a mixed record in terms of capturing bacteria based on tests done in the field.

Once the pollutants are trapped in the rain gardens, what happens to them next?

The journal Stormwater published in June a list of possible fates for pollution in stormwater systems. Here’s my adaptation of that list, edited to apply more specifically to rain gardens:

  • Volatilization. Pollutants, particularly some of those associated with petroleum or oil and grease, evaporate.
  • Sedimentation. In the case of standing water, heavier particles settle into the soil below.
  • Adsorption. Certain dissolved pollutants stick to particles floating in the stormwater or settled into the soil.
  • Absorption. Stormwater and pollutants soak deeper into the soil. Pollutants may accumulate in the soil, percolate through it with the water, or dissipate through microbial action, adsorption, or volatilization.
  • Microbial action. Bacteria and other microorganisms break down pollutants in the water or soil, often into forms that are less environmentally harmful.
  • Plant resistance and uptake. Decaying plant material increases adsorption and provides a good habitat for microbes that gobble pollution. Plants may also suck up pollutants from the soil through their roots, though not in large amounts.
  • Filtration. Particles are captured by a filter, if one is present.

This is what can happen to the pollutants, but the question is what actually does happen? Let’s look at petroleum chemicals, metals, and bacteria in turn.

Car exhaust

Car exhaust, Flickr user eutrophication&hypoxia

Petrol pollution

One of the most prevalent categories of runoff pollution is oil and grease from leaking cars and spills at gas pumps, vehicle exhaust, and burning wood and fossil fuels. The contaminants include petroleum hydrocarbons, and a category of environmentally hazardous chemicals called polycyclic aromatic hydrocarbons, or PAHs.

Scientists with the University of Minnesota recently performed experiments in the field and the lab to track PAHs in stormwater systems. They collected more than 70 soil samples from more than 50 rain gardens and bioretention infrastructures in the Twin Cities. The rain gardens were capturing water from various types of land use, including parking lots, roofs, and streets. The researchers found much higher levels of petroleum hydrocarbons in the rain garden versus the non-rain garden soils, but the levels were essentially safe in both: “all soil concentrations were about one thousand times less than regulatory action levels,” the scientists reported.

And what was even more interesting was the fact that the rain garden soil contamination was much less than they expected based on the volume of petroleum hydrocarbons being flushed into the gardens with the runoff. Where were the pollutants going?

To answer that question, the scientists did experiments with naphthalene, one of the PAHs found in rain gardens. They ran naphthalene-contaminated water through simulated rain gardens and discovered that the pollutant was adsorbed by the soil, biodegraded, or taken up by plants. The bottom line: “biodegradation typically destroys the contaminant, rather than simply retaining or transforming the contaminant.”

Other studies similarly have found that bacteria in the soil of rain gardens like to dine on  hydrocarbons. Research published in 2006 in the journal Water Environment Research found that 90 percent of petroleum pollutants were biodegraded by microbes in two to eight days.

And right here in the Northwest, the city of Portland’s Bureau of Environmental Services has been doing its own testing of rain gardens and other green infrastructure to track pollutants. The bureau found PAHs at all of the rain garden facilities it tested, “but typically at concentrations well below human health guidelines.” Interestingly, when they measured PAHs in non-stormwater soils next to the rain gardens, they found similar amounts of PAHs, suggesting that the pollutants are simply prevalent in many places in the urban environment.

Also keep in mind that many of the stormwater facilities tested by Portland were treating runoff coming from parking lots and roads that are being used by more cars and trucks than your average residential site, so the pollutant loads could be higher than a front-yard rain garden would likely have.


Spraying pesticides, Flickr user Bryan Gosline.

Heavy metals

Heavy metals found in stormwater include copper, cadmium, lead, mercury, and zinc and come from a wide range of natural and human sources such as vehicle brake pads, aviation fuel, pesticides, and weathered paint. When runoff contaminated with metals streams into a rain garden, multiple studies show that the pollutants are largely adsorbed by particles in the soil and mulch. A small fraction of the metals are taken up by plants.

A 2007 study published in Chemosphere using faux stormwater contaminated with copper, lead, cadmium, and zinc in a lab simulation found that between 88 to 97 percent of the metals were captured in the soil media and up to 3 percent was trapped by plants.

But even if all the metals are being held in the rain garden, it’s still not a large volume of toxics. A 2003 study concluded that it would take about 20 years for rain garden soils soaking up runoff to reach EPA limits for the amount of heavy metals allowed in recycled sewage waste used as compost. Recent research from the University of Minnesota concluded that it would take 76 years or more for rain garden soils to reach saturation, depending on the heavy metal.

But does that mean a dog can safely dig in a rain garden, or a child can tromp through it without concern for their health (the fate of the plants put aside)?

Another way to parse the potential risk is to look at the metals in samples collected from local rain gardens and stormwater ponds. Again, the city of Portland has data from actual rain gardens and swales around the city. King and Kitsap counties in the Puget Sound region have data from the sediment scooped out of stormwater ponds. The ponds are not green infrastructure, are collecting runoff from roads that could have higher traffic than residential areas, and are not vegetated so they’re less likely to be providing some of the natural pollution treatment that occurs in rain gardens. However, the ponds can still provide meaningful information, though their pollution levels could likely be higher than what you’d expect from a rain garden in front of someone’s house that’s capturing roof and residential street runoff.

Here are charts comparing the average concentrations of some heavy metals found in Northwest stormwater ponds and rain gardens. I’ve compared the amounts to Washington’s cleanup standards under the Model Toxics Control Act. These are the cleanup standards for the soil at sites that can then be used for residential or other uses. I’ve also included the amount of pollution that is allowed in compost that can be used in Washington under the WSDA International Organic Program. Both provide benchmarks for what is considered safe to humans.

CHART Metals in Sediment from NW Stormwater Ponds or Rain Gardens, Compared to WA Safety Stds for Soil Clean-up and Compost

As you can see, the amount of metal contamination in the stormwater ponds and rain gardens is well below safety standards used in Washington State.

Here are the data again in a table:

TABLE Metals in Sediment from NW Stormwater Ponds or Rain Gardens, Compared to WA Safety Stds for Soil Clean-up and Compost

Bacteria and viruses

Microorganisms in stormwater are eliminated numerous ways. An army of tiny creatures living in the water and soil including zooplankton, protozoa, nanoflagellates, microflagellates, amoeba, and bacteria will prey upon the offending microorganisms, which include viruses and other bacteria and protozoa. Sunlight can kill or inactivate some of the microorganisms.

No poop

No Dog Poop to drain, Flickr user Sweet One

Despite the numerous paths to destruction, microorganisms in stormwater are tricky. Unlike pollutants such as oil and grease and metals, green stormwater infrastructure doesn’t have such a stellar track record for capturing and removing bacteria. It appears to be more readily destroyed when it’s in the water, but more likely to survive in the soil. When researchers measure the amount of bacteria entering and exiting a rain garden out in the field, sometimes the water leaves the rain garden with even higher levels of bacteria than entered it, and in other cases 90 percent of the bacteria are removed. Some experts have suggested that the waste from birds, pets, and other wildlife recontaminate the water that’s leaving the rain garden.

Of course there are bacteria and other microorganisms everywhere, all the time, and most of them don’t hurt people. So the real concern is whether the microbes washed into rain gardens pose an actual risk to humans. A lengthy 2007 study from the Water Environment Research Foundation (WERF) determined that bacteria and viruses that can make people sick have been detected in stormwater samples (Table 2-4). But when they tried to make a link between stormwater pathogens and actual cases of illness, the scientists concluded “the literature does not support widely applicable and defensible relations between pathogens and indicators in stormwater…” (page 2-13).

A Puget Sound area stormwater expert I spoke with explained the risk like this. If the water in question was coming from leaking septic systems or sewers, that waste would include human pathogens and bacteria and would be much more likely to cause illness in people. However, the bacteria found in stormwater most often comes from birds and wildlife, so the risk to humans is much lower.

Tallying the toxics

Rain gardens and similar environmentally friendly stormwater infrastructure are being embraced worldwide because they do their job so well. They sponge up polluted runoff, keeping the foul chemicals out of the places that are home to beloved wildlife and where people like to play and fish.

The worry is that these same, very efficient rain gardens that are cropping up in our parking strips and front yards are doing their job so well that they could become residential toxic sites. But in fact are they? Not according to the research that’s available. Here’s the score on pollutants in rain gardens, in summary:

Petroleum pollutants/PAHs: Studies from the field and laboratory find that rain gardens do a great job of capturing petroleum pollution, and that the chemicals are largely eliminated when they’re destroyed by bacteria in the soil.

Heavy metals: Soil and mulch in rain gardens contain particles that will adsorb and hold metals including copper, cadmium, lead, and zinc. A small fraction of the metals are sucked into plant roots and vegetation.

While metals are not degraded in rain gardens, they’re present at very low levels. When Northwest counties test for metals in the sediment that’s scooped from the bottom of stormwater ponds or rain gardens that drain parking lots and other city surfaces — material that would likely have higher levels of metals than your average residential rain garden — they found that the contamination levels were still below soil and compost standards meant to protect human health.

Bacteria and viruses: While some research has found bacteria and viruses that can cause disease in humans in stormwater, sunlight as well as other microorganisms in the runoff and soil of rain gardens can destroy the pathogens. Also, most of the microorganisms present come from animal waste and are less likely to cause illness in people.

The bottom line is that the soil in rain gardens is safe for kids and pets. That said, people are advised to wash their hands after working or playing in any soil, which can contain naturally occurring metals, fecal waste from the neighbor’s dog, or any number of compounds one wouldn’t want to ingest. And remember that while rain gardens are attractive landscape features, the plants and soil are also doing an important job, so they need to be treated with some care.


*In recent years, Washington regulators have tried to identify the source and volume of pollution that fouls the Salish Sea, which stretches from southern British Columbia down through Puget Sound. They’ve released multiple reports on the issue, including “Control of Toxic Chemicals in Puget Sound” and “Toxics in Surface Runoff to Puget Sound: Phase 3 Data and Load Estimates.” A great source from the first document is Table 30, which zeroes in on the specific sources of the pollutants, e.g. the top source of lead is “ammunition and hunting shot use, loss of fishing sinkers, loss of wheel weights.”

**References for pollution removal, fate, and treatment include:


Berlin World Congress Session Brief: Risk Assessment, Pest Management and Phytoremediation


Gertie Arts, Alterra Wageningen University and Research Centre, Silvia Mohr, German Federal Environment Agency and Udo Hommen, Fraunhofer Institute for Molecular Biology and Applied Ecology

During the 6th SETAC World Congress in Berlin, the session “Plants and Chemicals in the Environment” was organized by the SETAC Aquatic Macrophyte Ecotoxicology Group (AMEG). Plants are key components of ecosystems. By performing photosynthesis, they produce O2 and organic material. Plants therefore form the basis of many aquatic and terrestrial food webs. The session focused on how plants interact with chemicals. The current risk assessments for chemicals consider risks for aquatic primary producers based on standard tests with algae and free-floating Lemna species. Contrary to this, sediment-rooted aquatic macrophytes with lower growth rates are not tested for standard risk assessments. Besides the fact that chemicals can affect plants, plants can also accumulate and biodegrade chemicals and thus contribute to lowered exposure concentrations. This ability of plants is potentially useful for phytoremediation and mitigation purposes. Both aspects were covered by this session. Six platform presentations focused on risk assessment and phytoremediation and also addressed terrestrial and marine plants besides aquatic ones. The poster corner covered ecotoxicology, test development and phytoremediation.

Session Highlights
Effects of chemicals on plants may be detected in the laboratory; however, field trials are very important for validation of approaches to extrapolate effects observed in the laboratory to the population and ecosystem level in the field. In a terrestrial field study, the buttercup Ranunculus acris was an important indicator plant for detecting herbicidal drift effects. The ability of plants to flower was the most sensitive endpoint for this species. Another study showed that the effects of a herbicide mixture in multispecies tests with several aquatic macrophytes were predictable from effects observed in single-species tests. Species sensitivity distributions (SSDs) for aquatic macrophytes showed that for the compounds considered, the standard test species in risk assessment—including the Eurasian water milfoil Myriophyllum spicatum—are protective for the effects on other aquatic macrophyte species. By combining algae and aquatic macrophyte data, the method is applicable even with low availability of macrophyte tests. As another higher-tier tool for aquatic macrophyte risk assessment, the first steps in modeling are currently being undertaken. A toxicokinetic/toxicodynamic model was presented for calculating the effects of pesticides on Myriophyllum spicatum considering the internal concentration in the plants. The study showed that the modeled data were in good agreement with laboratory test results. The model will be further refined. A new and important topic was the finding of intracellular uptake of hydrophobic substances in aquatic macrophytes. This research showed that macrophytes indeed bioconcentrate hydrophobic substances and thus can be used for phytoremediation. A last presentation discussed the application of marine plants in constructed wetlands to remediate nutrient-rich waste water from aquaculture facilities.

The poster corner addressed some topics in addition to the main platform session. The potential allelochemical influence of Myriophyllum was discussed. Several posters presented test designs for species that have recently been proposed as new regulatory test species, i.e. Myriophyllum species and the reed mannagrass, Glyceria maxima. Also several posters addressed phytoremediation by plants. Aquatic macrophytes not only adsorb chemicals but they also influence their chemical environment, for example, by changing the pH and O2 conditions and producing dissolved organic carbon. This might have effects on the fate of chemicals. Also the rhizosphere might be important for both aquatic and terrestrial plants to contribute to adsorption and degradation and therefore to phytoremediation.

Take-home Message
In current risk assessment schemes, aquatic macrophytes are being addressed more and more. New regulatory species have been proposed and the development of laboratory tests and guidelines is in process. Higher-tier risk assessment for terrestrial and aquatic plants receives more attention and effects on the higher levels of populations and ecosystems are studied and discussed. However, guidance on how to perform such tests is still needed. Phytoremediation is a promising field in which aquatic and terrestrial plants might help to decrease environmental concentrations of toxicants.

Author's contact information:gertie.arts@wur.nl, silvia.mohr@uba.de, udo.Hommen@ime.fraunhofer.de


Window Farming?


Window Farms



For a great overview of what phytoremediation is see:




International Journal of Phytoremediation



Contact me for information on a specific article.



Phytoremediation Research within the Department of Defense






Will Detroit use vacant lots to grow weeds for biofuel?


By Tyler Falk | April 26, 2011, 5:09 PM PDT

With Detroit’s population at a 100-year low, and the city planning to consolidate neighborhoods, there’s no shortage of vacant lots or ideas about what to do with them. Add another idea to the list: growing weeds for biofuel.

Jim Padilla Jr., the owner of The Power Alternative, a biofuel refinery based in Michigan sees potential in growing a weed called pennycress on Detroit’s vacant land to produce biodiesel,Midwest Energy News reports. But it’s not just the benefit of biodiesel that makes the idea attractive, there could be other benefits.

Padilla said the crops could be grown on vacant land in downtown Detroit and would serve a dual purpose — producing high-quality biodiesel and remediating land contaminated with heavy metals.

Pennycress naturally absorbs heavy metals as it grows, Padilla says, through a process known as phytoremediation. Because Detroit was once home to several lead smelters, much of the vacant land is contaminated. By growing pennycress for biodiesel, over time the sites would be cleaned up.

It sure beats the alternative.

“The alternative is to dig and haul (the contaminated soil) and move it somewhere else,” he said. “That cost is about $250,000 per acre. You can spend it there or you can phytoremediate, create jobs, clean it up, make biomass for power, and produce biodiesel.”

To explore the possibilities, Padilla has partnered with local organizations, the University of Detroit-Mercy and Michigan State University, which was just awarded $2.9 million for biofuels research by the U.S. Department if Agriculture.

It’s a worthy idea, to be sure. Any project that’s a net-gain for the struggling city should be considered. But is there really enough vacant land to make the project worthwhile? Or would the land be better served to continue growing the city’s local agriculture scene? What about using the land for other renewable energy projects?

Do you think that using the vacant lots to produce biofuel is the best use of the land?

Photo: Andrew Jameson/Wikimedia Commons




Europe's Largest and Newest Green Living Wall is in London

by Bonnie Alter, London  on 04.18.11


lobby view photo
Photo: B. Alter

It's been called Europe's largest and newest green wall. Installed in a newly renovatedhotel in January makes it new and as for large, it stretches from the second to eleventh floor on the outside of the building.

Looking up from the reception area in the hotel, through the glass, it is a lovely and healthy looking green wall.




mint hotel photo
Photo: building.co.uk

Created by a green wall specialist, a total of 180,000 evergreen plants have been inserted into real soil. Forty different species have been selected based on the orientation. Thrifts and red Campion has been used for the south facing element of the wall while shade loving plants including ferns have been used on the north side. Plants include Liriope Muscari, Vinca Minor, Heuchera and bulbs such as snowdrops for seasonal colour.

frosts wall photo
Photo: frosts landscape

The wall is made up of 4,100 planting modules which each contain 45 planted cells which are 70mm deep. The plants were grown off-site for 6 months before the modules were fixed to the building. An automatic irrigation system is built into the wall and supplies a combination of water and liquid fertiliser to keep the plants healthy.Apparently, "if anything goes wrong with the irrigation system a text message is automatically sent to the landscape company so they can take corrective action before any damage is done."

There is also a green roof at the top of the hotel which is a combination of Sedum and Wildflower plugs. The specialists have donated a number of Bee Walls that will be situated on the roof.

Although it is not as exciting or innovative as some, it is a welcome addition to the growing numbers of green walls in the country.





ANDREA: Plant-based Air Purifier 


ANDREA: Plant-based Air Purifier (White)




Interesting idea, poor execution. Probably not worth the price. 


Phytoremediation with native plants





International Journal of Phytoremediation




United Nations Environment Programme



Division of Technology, Industry and Econo

Phytotechnologies A Technical Approach in Environmental Management

Introduction - An Ecosystem's Perspective > C. Phytotechnology

Sunset over riverbank  

The term phytotechnologydescribes the application of science and engineering to study problems and provide solutions involving plants. Although the term is not widely used, it is useful in promoting a broader understanding of the importance of plants and their beneficial role within both societal and natural systems. Underlying this concept is the use of plants as living technologiesto help address environmental challenges. Phytotechnology applications employ ecological engineering principles and are considered to be ecotechnologies. Hence phytotechnologies are based on the science of ecology and consider the ecosystem as an integral component of human and societal interventions involving the natural environment. A related term is biotechnology, which refers to the application of science and engineering to study problems and provide solutions involving living beings. The term biotechnology can also refer to the manipulation of the genetic structure of cells to produce modified organisms with an augmented capacity to perform certain functions. Table 3 summarizes these definitions. Table 3: Defining Phytotechnology

Sponsored by: Interstate Technology and Regulatory Council

eco = living systems, ecological TECHNOLOGY = the application of science and engineering to study problems and provide solutions ecotechnology = the application of science and engineering to study problems and provide solutions involving ecological systems
PHYTO = plant, flora, vegetation PHYTOTECHNOLOGY= the application of science and engineering to study problems and provide solutions involving plants
bio = life, of living beings, biological biotechnology = the application of science and engineering to study problems and provide solutions involving living beings

Just as there are many different applications of biotechnology, there are also many different applications of phytotechnology. Some of these applications are well established in sectors such as medicine, agriculture and forestry to name a few. There are also many important environmentally related applications. As shown in Table 4, the environmentally beneficial applications of phytotechnologies can generally be divided into five categories: augmenting the adaptive capacity of natural systems to moderate the impacts of human activities; preventing pollutant releases and environmental degradation; controlling pollutant releases and environmental processes to minimize environmental degradation; remediation and restoration of degraded ecosystems; and incorporating indicators of ecosystem health into monitoring and assessment strategies. The integrated ecosystems management component of this focuses on the use of phytotechnologies to augment the capacity of natural systems to absorb impacts. The prevention component involves the use of phytotechnologies to avoid the production and release of environmentally hazardous substances and/or the modification of human activities to minimize damage to the environment; this can include product substitution or the redesign of production processes. The control component addresses chronic releases of pollutants and the application of phytotechnologies to control and render these substances harmless before they enter the environment. The remediation and restoration component embodies phytotechnologies and methods designed to recuperate and improve ecosystems that have declined due to naturally induced or anthropogenic effects. The monitoring and assessment component involves the use of phytotechnologies to monitor and assess the condition of the environment, including releases of pollutants and other natural or anthropogenic materials of a harmful nature. Table 4: Environmentally Beneficial Applications of Phytotechnologies Environmentally Beneficial Applications of Phytotechnologies Some specific examples of phytotechnology applications include:

The use of plants to reduce or solve pollution problems that otherwise would be more harmful to other ecosystems. An example is the use of wetlands for wastewater treatment.
The replication of ecosystems and plant communities to reduce or solve a pollution problem. Examples are constructed ecosystems such as ponds and wetlands for treatment of wastewater or diffuse pollution sources.
The use of plants to facilitate the recovery of ecosystems after significant disturbances. Examples are coal mine reclamation and the restoration of lakes and rivers.
The increased use of plants as sinks for carbon dioxide to mitigate the impacts of climate change. Examples of this are reforestation and afforestation.
The use of plants to augment the natural capacity of urban areas to mitigate pollution impacts and moderate energy extremes. An example is the use of rooftop vegetation, or “ greenroofs”. More information and examples about the use and applications of phytotechnology is presented in Section 3.




Phytotechnologies is a set of technologies using plants to remediate or contain contaminants in soil, groundwater, surface water, or sediments. These technologies have become attractive alternatives to conventional cleanup technologies due to relatively low capital costs and the inherently aesthetic nature of planted sites.










Phytoremediation Online Decision Tree Document




Introduction to Phytoremediation







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