Do Roadside Stormwater Control Measures (SCMs) Offer a Possible Carbon Sink

BOUCHARD, NATALIE ROSE. Do Roadside Stormwater Control Measures (SCMs) Offer a Possible Carbon Sink? (Under the direction of Dr. William F. Hunt (Chair), III and Dr. Deanna L. Osmond (Minor Committee Member). As legislation continues to focus on greenhouse gas (GHG) and carbon dioxide (CO2) emissions and reductions, the terrestrial biome offers an attractive possibility to sequester carbon. Currently, the terrestrial pool is regarded as a CO2 sink, but scientists wonder to what extent this trend will continue. Urbanization changes the existing landscape, and little study has focused on the carbon (C) dynamics of specific urban landuses. In this research the roadway environment, specifically the grassed right-of-way (ROW), was studied for carbon sequestration potential. Transportation corridors are present world-wide, and the vegetated filter strip and swale (VFS/VS), a common stormwater control measure, often constitutes the grassed right-of-way adjacent to roadways. Carbon pools within roadway VFS/VS soils of North Carolina were specifically examined in this study. Work was conducted in two North Carolina physiographic regions: the Piedmont (characterized by clay-influenced soils) and the Coastal Plain (mostly sandy soils). Approximately 700 soil samples were collected in VFS/VSs and wetland swales alongside major highways and analyzed for percent total soil C (% total C). Mean soil C densities were 2.55 kg C m -2 in the Piedmont and 4.14 kg C m -2 in Coastal Plain roadway VFS/VSs. Because grasslands were reported to have similar carbon density values to those observed in this study, they could be a surrogate landuse for roadway VFS/VSs if no specific roadside data were available. Using a 37-year soil chronosequence, the carbon sequestration rate within Piedmont VFS/VSs was calculated at 0.053 kg C m -2 yr -1 . Using segmented linear regression models (SAS © 9.2), the sequestration rate was calculated to be 0.155 kg C m -2 yr -1 for 15 years (PROC NLMIXED) and 0.099 kg C m -2 yr -1 for 21.5 years (PROC NLIN). The roadside grass sequestration rate assumed by the Federal Highway Administration Carbon Sequestration Pilot Program (0.17 kg C m -2 yr -1 ) overestimates carbon accumulation by a factor of 3 in the linear model, and by a factor of 1.1 to 1.7 with the segmented linear models. Carbon density did not differ between dry or wetland swales, although % total C was significantly greater in wetland swales. Because C density and % total C in swales were not well defined by age, it appears more appropriate to assess wetland swales and dry swales using a range of carbon values, rather than a rate of carbon sequestration. Dry swale C densities ranged between 0.63 to 7.73 kg C m -2 (mean = 3.05 kg C m -2 ); the mean C density of wetland swales (5.56 kg C m -2 ) was greater than that dry swales, with a wider range of values (1.02 to 22.78 kg C m -2 ). If promoting C sequestration is ever a factor in ROW management, wetland swales would be more desirable than dry swales. While the VFS/VS sequestration rate is comparable to other grassed systems, the estimated 200 – 300 tons C per km road lane expelled during roadway construction (Cass and Mukherjee 2011) is marginally offset through terrestrial sequestration in roadside VFS/VSs. Per every km of roadway constructed, Piedmont VFS/VSs would offset between 4% and 7% of C emitted during construction, depending on which model was used. Do Roadside Stormwater Control Measures (SCMs) Offer a Possible Carbon Sink? by Natalie Rose Bouchard A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Master of Science Biological and Agricultural Engineering Raleigh, North Carolina