In August 2015, during the course of giving her standard presentation on the evils of RTS on a webinar sponsored by the US Fish & Wildlife Service (FWS) National Conservation Training Center, Dr. Barbara Mahler of the US Geological Survey was challenged by a webinar attendee who claimed that the results of a study of an RTS-sealed parking lot shows that soil filtration dramatically reduces PAH concentrations and effectively eliminates toxicity. A few months later, a paper reporting the results of the study was published. PCTC commissioned Exponent® to conduct a post-publication peer review (PPPR) of the paper (McIntyre et al., 2016). Through the review, PCTC learned more about bioretention studies at Washington State University’s Puyallap Research Center, as well as about the effect of biofiltration/retention by a system that mimics natural soils on runoff collected from an RTS-sealed parking lot. The full PPPR report is available here.
The data in McIntyre et al. (2016) indicate that the toxicity of the first flush from an RTS-sealed lot is similar to the ongoing contributions of toxicity of highway runoff and of unsealed parking lots. McIntyre et al. collected the first flush sample only 2 hours after RTS application. PCTC suggests that RTS should not be applied if rain is anticipated within 24 hours after application. It is not clear whether or how the study might have differed if the first simulated rain event had been delayed to allow the sealant additional time to cure. The next sampling (simulated rain) event was 7 days later, followed by two more sampling events on day 13 and day 207.
With no bioretention system in place, PAH concentrations in simulated runoff decreased by about 90% between the first (2 hour) and subsequent sampling events. PAH concentrations in runoff collected on days 7, 13, and 207 were about the same as each other. Use of a bioretention system resulted in reducing more than 99% of the PAHs in samples collected on the first day and all subsequent days.
Toxicity was measured using juvenile Coho salmon and zebrafish embryos. Zebrafish embryos were included because they are regarded as especially sensitive to exposure to PAHs in laboratory experiments. For both the juvenile salmon and zebrafish embryos, the effect of soil bioretention was dramatic, increasing survival to 100% for both species at all four sampled time points. Runoff that was not filtered through soil resulted in 0% zebrafish embryo survival for the 2-hour time point, but 100% survival for the 3 subsequent time points. Similarly, 0% of the salmon survived in 2-hour runoff without soil filtration, but the survival rates increased for subsequent time points. It is notable that, for the Coho salmon, PAH concentrations and toxicity effects were not correlated for the later three time points, suggesting that survival rates were influenced by some other factor.
Exponent’s PPPR conclusion is that the work reported by McIntyre et al. in this paper confirms what that research team has shown using several types of paved surfaces: soil retention effectively reduces the impact of runoff from highways and parking lots on aquatic life.
The study affirms real world experience. In the real environment, studies that have tried to find detrimental effects on ecosystems related to routine RTS applications have been unsuccessful. Research by the Washington State-Puyallup team suggests that soil acts as a natural buffer.