By Carly Greyell
Living in King County, rain is a natural part of life. But you might not realize that when rain collects on impervious surfaces, like roadways and roofs, it can pick up a variety of pollutants. Every day activities, like driving a car, walking the dog, and fertilizing your lawn, can contribute pollutants like heavy metals, oil, bacteria, solids, and nutrients. This polluted rain water is referred to as stormwater and King County and other local jurisdictions are working hard to reduce the amount of pollutants that stormwater adds to our local waterbodies, including lakes, rivers, and Puget Sound.
One way to treat stormwater is through bioretention where stormwater is filtered through a soil mixture that includes compost and sand. In 2012, the City of Shoreline installed a number of bioretention planter boxes (designed like concrete-lined rain gardens), along Aurora Avenue North, which is part of Highway 99. The bioretention planter boxes treat some of the stormwater that drains to Echo Lake. King County recently designed a study to see how effective the planter boxes remained three years after construction, and whether they could reduce stormwater pollutants that contribute to human health risks such as PCBs.
The King County Environment Lab (KCEL) Field Sciences Unit braved the rain to collect samples during eight storms from December 2015 to February 2017. Samples of both the untreated stormwater entering the bioretention planter boxes and the treated stormwater after it had filtered through the soil mixture were collected for chemical analysis. Pollutant concentrations between the treated and untreated stormwater were then compared to determine how effective the planter boxes were at removing pollutants from the stormwater.
The results were mostly positive: the bioretention planter boxes significantly reduced concentrations of most pollutants, including chemicals associated with oils and exhaust, total copper, total zinc, solids, and even PCBs. Average reductions for these pollutants ranged from 81 percent to 99 percent at each site. Levels of dissolved heavy metals were not always reduced, but concentrations in both the treated and untreated stormwater were very low. These findings are consistent with results from studies that evaluated new bioretention, suggesting the planter boxes are continuing to perform as expected, three to five years after installation.
Unfortunately, nutrients in the stormwater were not consistently reduced, and in many cases the stormwater treated by the planter boxes actually had higher concentrations than the untreated stormwater. This is particularly problematic for phosphorus, which, when elevated, can lead to increased algal blooms in lakes. This problem has been recognized with bioretention across the region, and local researchers are currently evaluating alternative bioretention soil mixtures for stormwater treatment in areas at risk for algal blooms.
Despite the good news in treatment, the maintenance requirements for these planter boxes were higher than expected. Water enters these installations from the busy roadway through cuts in the curb, which were easily blocked with dirt, leaves, and other debris. Without regular debris clearing, stormwater was blocked from entering the planter boxes, and bypassed treatment.
Thanks to the results of this and similar studies, stormwater engineers and managers are learning how to improve stormwater treatment techniques.
Polychlorinated biphenyls, or PCBs, are a group of chemicals developed in the 1930s that had a range of uses, including additives for construction materials, such as paints and caulks. However, PCBs were linked with negative health effects such as cancer and hormone disruption, and in 1979, U.S. production was banned. Unfortunately, most PCBs do not break down easily, and older buildings and industrial sites remain important sources of PCBs.
In several local waterbodies, including Lake Washington, fish consumption advisories warn that certain fish species contain PCBs at levels that are unsafe to eat. Researchers are learning that one of the main ways PCBs enter these water bodies is through stormwater. However, PCBs are not currently regulated under stormwater permits. Studies, like the one described here, are important so that we can learn how best to reduce PCBs in stormwater, thus preventing human health issues.
The Washington State Department of Ecology helps local municipalities like King County and the City of Shoreline manage their stormwater permits. Since 2013, most Washington state stormwater permitees have pooled resources to fund regional stormwater monitoring and studies that evaluate how well permit requirements serve to improve stormwater treatment. Washington state permitees selected the study described here for funding through this program.
Carly Greyell is an ecotoxicologist in the King County Science and Technical Support Section. She has been supporting many of the Lower Duwamish source control projects and involved in ongoing toxics monitoring and projects assessing the effectiveness of stormwater treatment.