Biohydrochemical Enhancements for Streamwater Treatment: Engineered Hyporheic Zones to Increase Hyporheic Exchange, Control Residence Times, and Improve Water Quality

摘要

Nonpoint source pollution is the number one cause of water quality impairments to US rivers and lakes, and stormwater is the fastest growing category of nonpoint source pollution. In nature, nonpoint source pollutants can be treated by streambed sediments of impaired streams in a process analogous to biological sand filtration. This streambed biofilter is called the hyporheic zone (HZ), and it has been gaining attention in stream restoration due to its unique role in improving water quality. In particular, the HZ can attenuate pathogens (indicators), nutrients, and metals (the top three pollutant classes that lead to stormwater quality regulatory action) from the entire upstream watershed, thereby capturing nonpoint source pollution better than distributed BMPs. However, exchange between polluted surface waters and their HZs are often limited and inefficient. Prior to our project, the past two decades of research on the HZ had not been translated into effective Best Management Practices (BMPs) for stormwater managers. This knowledge gap prevented stormwater and stream restoration projects from properly engineering HZs to increase hyporheic exchange and optimize (nonpoint source) pollutant removal. In particular, an HZ BMP needs to 1) drive hyporheic exchange flows, 2) control hyporheic residence times, and 3) be customizable for removal of specific contaminants of concern. Currently, low-head dams are used to drive hyporheic exchange, but standard designs do not control residence times and are not customizable, so they have minimal water quality benefits. The objectives of this PhD research were to develop and test a novel engineered HZ BMP to improve streamwater quality. Specifically, we utilized manipulations of streambed media to create a modular BMP called Biohydrochemical Enhancements for Streamwater Treatment (BEST). BEST modules are comprised of subsurface modifications to streambed permeability to drive hyporheic exchange, paired with reactive geomedia (e.g., woodchips) to enhance biogeochemical conditions needed for pollutant removal. BEST were explored through three studies. The first featured a numerical model evaluating multiple BEST modular designs on hyporheic exchange flows and contaminant attenuation. The most promising BEST design from the numerical model was then installed in a constructed stream flume alongside an all-sand control channel. The second study featured conservative and reactive tracer experiments to compare the impact of BEST on hyporheic transient storage and attenuation of a model compound, resazurin, which undergoes first-order microbially mediated degradation under aerobic conditions. The third study used the same flumes to compare BEST to the control for the attenuation of urban stormwater contaminants: nitrogen and atrazine. The cumulative results of these studies indicate that BEST can provide substantial improvements to streamwater quality over reaches of hundreds of meters in small streams or constructed urban stormwater channels (e.g., flow rates < 10 L/s). Numerical models highlight the importance of impermeable "book ends" in BEST modules to maximize hyporheic exchange and control residence times. Flume studies of this design showed that BEST increased the effective HZ exchange volume by 50% compared to the control, which led to 45-95% increases in the reach-scale attenuation rates of multiple stormwater contaminants. In other words, stormwater channels that incorporate BEST modules could reach water quality targets in 45-95% less reach length compared to an all-sand streambed (e.g., sand filter). The BEST design tested in these experiments was well suited to fast, aerobic reactions (e.g., nitrification), but future designs will be tailored for anaerobic reactions to broaden the range of pollutants that can be treated (e.g., nitrogen via denitrification). Overall, the results suggest that BEST could be an adaptable and complementary stormwater and stream restoration BMP to increase attenuation of nonpoint source pollutants within small, impaired streams.