One of life’s best little pleasures is the soft melody of rain falling on a tin roof. Backdropped by a blue grey sky, the drops steadily pitter and patter on the roof’s surface, cascading down to splash on the ground. The droplets may then join together and trickle across an impermeable surface, be it a driveway, parking lot, or loading dock. Just as erosion eats away at hillsides by carrying particles of dirt, this water will pick up the remnants of normal wear of pavement and vehicles. It may find its way into an asset created for treating stormwater, or it may go unfiltered directly into the streams and groundwater.
Through every part of this journey to the ocean, the runoff can pick up heavy metals. And let me tell you, that melody is far more degenerate than the rain on your tin roof. Be it Cd, Cu, Pb, Ni, Cr, or Zn (that’s cadmium, copper, lead, chromium, and zinc), these elements at high concentrations threaten the survival of aquatic organisms, poison people, and corrupt our infrastructure. Ergo, if they are not regularly monitored and mitigated through stormwater maintenance techniques, their presence can pose as a daunting liability for you and your business.
Sources and Effects of Heavy Metals Runoff
Throughout this process of water making its way to the ocean, there are opportunities aplenty for heavy metals runoff:
- Cadmium can find its way onto roads and parking lots from tire wear, brake pads, and insecticides on landscaping. Cadmium will build up in invertebrates and can cause kidney toxicity in mammals.
- Chromium pollution comes mostly from moving engine parts, brake lining wear, and the corrosion of welded metal plating. It gives animals respiratory problems, immune system weakness, birth defects, tumors, and infertility, and can clog the gills of fish.
- Copper can come from bearing and bushing wear, moving vehicle engine parts, the wear of brake lining, and similar pesticides to those using cadmium. It doesn’t break down in the environment, so it will accumulate in plants and animals. Acute toxic effects may include mortality of organisms; chronic toxicity can result in reductions in survival, reproduction, and growth.
- Iron will rust and corrode off of auto bodies. It can also come from moving engine parts and steel roadway structures. Iron contamination will give rivers an orange hue; it can build up in internal organs, eventually killing animals. Like many other examples of heavy metals runoff, the buildup of iron in our food sources can have negative implications for we humans via bio-magnification.
- Lead, which is poisonous to most life forms, is sourced from leaded gasoline and tire wear. Lead will build up in plant roots and then enter the food chain. It causes neurological issues in animals.
- Nickel is brought to the watershed through asphalt paving. Similar to lead, likewise comes from gasoline, as well as diesel and lubricating oil. It can diminish the growth of plants and algae, and can cause various kinds of cancer in animals.
- Zinc lastly comes from grease, motor oil, and tire wear. Zinc will increase acidity in water and accumulate in soils. Its effects are most prominent in cattle and plants, and significantly reduces crop yields.
With all those different sources and effects for this microscopic menace, addressing it can be daunting for a business owner. Where do you begin?
Maintenance: Nipping it in the Bud
A common theme in all of those pollutants is that they deposit onto pavement and asphalt. When your customers drive into, park on, and leave your parking lot, tiny amounts of the vehicles are left behind due to the normal wear of vehicles, as well as abnormal events like oil leaks. A simple way to mitigate this as a part of an integrated stormwater pollution prevention plan (SWPPP) is street sweeping.
Sweeping at regular intervals will collect the majority of sediment on a parking lot before it enters your storm infrastructure. There will always be some sediment that makes it way into your management assets, but the slower that builds up, the more infrequently these assets need to be serviced, and the more money you save.
Regulators don’t delude themselves into thinking that every pollutant can be removed and that every invasive species will be eliminated and that every pipe will be 100% clear at all times. There are maximum acceptable concentrations where if you’re under the threshold, you’re passing your inspection while resting easy that your negative impact on the environment is relatively low. This is kind of in that same category as the certain number of maggots allowed in canned mushrooms (it’s 20 maggots “of any size” and 75 mites per 100 grams...this Thanksgiving, maybe opt for the fresh farmers market varieties).
Commonly, our clients will have us bring sweeper trucks at once-a-week intervals, which collects the majority of heavy metals deposited on the parking lot. That said, there may be unexpected sources like scrap metal or rusty building infrastructure that are carrying metals directly into your stormwater system. Likewise, getting most of the pollutants from sweeping is just one step in making sure our clients are compliant with their relevant stormwater codes.
Bioretention Treatment of Heavy Metals Runoff
As we saw above, much of the damage to the environment comes from uptake in plants and animals. In that regard, we see the bullrush, grasses, and other plants common to bioretention systems as the “front lines,” guarding their downstream brethren from harm. The roots and the soils they weave through act as sedimentation and filtration, trapping particulate forms of heavy metals from progressing downstream.
One major challenge of heavy metals is that they often dissolve into an aqueous form once in water. This means you’ll get individual atoms of iron or manganese that cannot be simply filtered out. So, we must rely on chemical reactions. The elements will be drawn into the roots of these plants and taken out of the water cycle. Adsorption into organic matter and clay is a common treatment method as well, which is why ponds with compost have an improved heavy metal removal lifespan.
You may have bioretention cells like swales on your property already. For them to function properly, they need regular maintenance. The vegetation should be kept to the right height for its species and should be removed from the area immediately around the inlets (not surprisingly, plants grow the thickest in this area, as it’s where the nutrients first enter the pond through). Once these plants have taken up so much metal that they’re saturated and cannot absorb any more, we replace them with fresh plants.
Another type of treatment involves underground vaults with stormwater control measures (SCMs) designed to treat whatever comes through. This often includes filter media that absorbs dissolved metals while capturing particulate metals. As with the plants, this media needs to be inspected and periodically replaced to most effectively manage your rusty runoff.
Between sweeping, monitoring, maintaining, and repairing your assets, Restoration + Recovery will simplify and optimize your pollution prevention processes so that you can focus on your customers. Reach out to us for an inspection to determine your pollutant profile and stormwater infrastructure health.
1. Shanstrom, N. (2014, June 2). Long-Term Use of Bioretention for Heavy Metals Removal. Retrieved from https://www.deeproot.com/blog/blog-entries/long-term-use-of-bioretention...
2. Frenzel, C. (2009, April 7). Stormwater and Heavy Metals. Retrieved from http://advisorycouncil.org/2009/04/07/stormwater-and-heavy-metals/
3. U. S. Food and Drug Administration/Center for Drug Evaluation and Research. (2018, October 26). The Food Defect Action Levels: Levels of natural or unavoidable defects in foods that present no health hazards for humans. Retrieved from https://www.fda.gov/Food/GuidanceRegulation/GuidanceDocumentsRegulatoryI...
4. Copper production & environmental impact. (2018). Retrieved from http://www.greenspec.co.uk/building-design/copper-production-environment...