Green stormwater infrastructure is designed to capture runoff and infiltrate/treat it before it goes into surface waters. Most runoff in urban areas comes from impervious surfaces: roadways, buildings or parking lots. Each surface brings its own pollutants like copper, chromium, lead, nickel or zinc, and others from roads and parking lots, and calcium or lead from buildings, just to name a few.
As it turns out, green stormwater infrastructure (GSI) soils do accumulate iodine (148% greater) and calcium (65% greater) at higher levels than the surrounding non-green infrastructure soils, but they do not accumulate lead, mercury or cadmium at higher rates than surrounding soils and, in some cases, GSI soils had lower levels of those toxic elements than surrounding soils. Rubidium, tin and zinc are lower in GSI soils, even when surrounding soils have high concentrations, which may be due to media additives in blending the GSI soils or interaction between the media and vegetation.
The study focused on GSI practices designed primarily to infiltrate stormwater at the site. The team, primarily based in Pennsylvania, looked at a number of variables, including the type of ownership of the site, distance to roadways, project age and soil geology among others at 59 GSI sites in Philadelphia. The GSI sample set included samples from 51 tree or storage trenches, 12 planters, six curb bumpouts, 41 rain gardens, 48 swales (grassed waterways, infiltration berms or basins), four wetlands and 57 gully repairs. The sites are owned by the Philadelphia Water Department or Philadelphia Parks & Recreation and surrounded by a variety of land uses, including schools, farms, recreation, residential, commercial and transportation. Non-GSI soils were sampled on land adjacent to the stormwater feature 8 to 10 ft. away from the GSI soils.
The study found that all samples had heavy metals levels higher than the levels recommended for residential and ecological uses. All samples were also higher in heavy metals compared to non-urban soil background levels. Soils in GSI did not accumulate “elements of concern” – like lead, cadmium or mercury – any differently than the non-GSI control sites and, in some cases, the levels were lower in GSI soils. The study didn’t take a look at mobility of heavy metals or exposure risk to the public or to maintenance workers.
The bottom line: The study didn’t find that elements of concern for human health were any different in GSI soils than in non-GSI soils. The authors conclude that urban soils in cities like Philadelphia in general may be of greater concern for contamination and that all soils in the study, whether they were in GSI or not, had greater levels of heavy metals than are recommended for ecological and residential use simply because they were urban.
Kondo, Michelle C.; Sharma, Raghav; Plante, Alain F.; Yang, Yunwen; Burstyn, Igor. 2015. Elemental concentrations in urban green stormwater infrastructure soils. Journal of Environment Quality. doi: 10.2134/jeq2014.10.0421. [12 p.].
NewTerrain February 1, 2016