Abstract
Stormwater runoff threatens the health of receiving waters through pollution, flooding, and erosion. Urbanization as well as climate change make the need for effective stormwater management techniques a paramount challenge in protecting our waterways and the ecosystems and human industries depending on them. Green infrastructure (GI) is a promising stormwater management technique that returns the hydrology of a developed area to its pre-developed hydrologic conditions through the use of infiltration, evapotranspiration, and onsite storage of runoff. However, there is still much to learn about how well individual GI systems mitigate stormwater runoff in situ and how one GI design compares to another. This dissertation addresses the knowledge gaps on GI through a field study on the performances of four types of GI systems treating road runoff from Lorton Road, a four-lane divided road in Fairfax County, Virginia. The studied GI systems are a bioretention (BR) system, bioswale (BS), compost-amended grass channel (CAGC), and a grass channel (GC). The three objectives of this dissertation are to (1) characterize the runoff volume reductions of the four systems and compare their performances, (2) evaluate the impact of deicing road salts on the capture and mobilization of trace metals in the GI, and (3) determine the extent of fecal contamination and mitigation by each GI system.
Runoff volume reduction of GI systems is an essential step in stormwater management because it reduces risks of flooding and erosion and also reduces the mass load of pollutants carried within runoff. During their first full year of operation, the Lorton GI systems were monitored for runoff volume reduction in every season for a total of 48 rain events with rain depths ranging from 2.8 mm to 97 mm. The GC achieved the largest relative volume reduction and average runoff reduction (78%, 81%), followed by the BR (71%, 73%), BS (56%, 60%), and CAGC (43%, 53%). However, the BR had the narrowest range of reductions, performing predictably well in a variety of conditions while the runoff volume reduction of the three swales (BS, CAGC, and GC) were all significantly inversely correlated with rainfall depth. In several instances, the CAGC and BS actually produced volume, an outcome attributed to particularly intense rainfall and wet soils. Overall, the BR and GC had comparable volume reductions, but selecting either system for a stormwater management plan depends on site-specific requirements such as available space and water treatment goals.
GI systems have been shown to capture trace metals from runoff, but this capture is often only a physical event and can be reversed in the proper conditions such as a sudden influx of road salt laden stormwater. The impact of deicing salts on the capture and mobilization of trace metals was evaluated through analyzing stormwater, soil, and groundwater at the Lorton field site. Flow-weighted composite samples of the stormwater entering and exiting each GI system were collected for 20 – 33 events, in all seasons from the spring of 2018 until the summer of 2020. Measurements of road salt (primarily NaCl) and trace metals (chromium, copper, nickel, and lead) showed no significant trends in the release of metals in the presence of elevated salt contents. In many instances regardless of the salt content, the GI systems were exporting trace metals. This export is attributed to low initial metal concentrations at or near irreducible concentrations. Soil sampling also revealed no significant release of metals in response to salt loading, though the sampling regime occurred over a mild winter season with relatively little road salt application. There was evidence that the mulch of the BR released metals, though the cause cannot explicitly be attributed to salt influx as it could also be a result of mulch decomposition. The groundwater at the BR received a surge of salt in the fall of 2019 in two of its wells with dampened responses in the other wells. The two wells with salt surges did not show evidence of mobilized metals, however. Between the stormwater, soil, and groundwater, there was little evidence of metal mobilization by the deicing salts and it is not believed to be a threat in a climate such as Lorton Road.
Fecal contamination is a leading stormwater pollutant and little is known on how well GI systems mitigate this pollutant. At the Lorton site, fecal contamination of the flow-weighted composite samples was measured using E. coli as the fecal indicator bacteria. In spite of the relatively high inflow concentration of E. coli from its contributing drainage area (CDA), the BR reduced E. coli concentration and mass loads significantly. The swales received much lower E. coli levels from their CDA, but significantly increased its concentration and had no significant impact on E. coli loads. Outflow E. coli concentrations of all four GI systems were regularly above recommended limits for recreational waters, indicating that GI systems could be a source of E. coli in stormwater runoff. Linear correlations found significant relationships between outflow E. coli and ambient temperature, dissolved organic carbon (DOC), and total dissolved nitrogen (TDN) (individually). The temperature relationship is attributed to potentially increased wildlife activity in warm weather as well as increased metabolic activity of the bacteria. The links between bacteria and DOC and TDN indicate a potentially significant role that bacteria in stormwater may have in the carbon and nitrogen cycles. Overall, the BR showed good mitigation of fecal contamination while the swales indicated that they might be attracting wildlife which increases E. coli occurrence. Improving designs to better reduce DOC and TDN as well as discouraging wildlife activity might help with decreasing fecal contamination in future designs.
This dissertation addresses important questions regarding the runoff volume and water quality improvements offered by GI systems. By investigating the pressing questions of volume reduction, interactions of salts and metals, and prevalence of fecal contamination, the findings of this dissertation can be applied to improve future GI system designs and performance expectations. In a changing climate and spreading urbanization, it is more important than ever to protect our water resources and GI systems are a promising tool to reach these goals.
