Anthropogenic Impacts, Stormwater Runoff Dynamics and Opportunities for Upscaling Multifunctional Urban Green Spaces Using High-Frequency Sensing

Hernandez Gonzalez, Liliana Maria

doi:https://doi.org/10.21985/n2-m54z-0g27

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Anthropogenic Impacts, Stormwater Runoff Dynamics and Opportunities for Upscaling Multifunctional Urban Green Spaces Using High-Frequency Sensing

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Unrestrained urban growth combined with the increase in the intensity and frequency of precipitation events, due to climate change, has become a major challenge for urban green spaces. Urban sprawl increases ecosystem degradation and the vulnerability of cities to floods as a result of excess stormwater runoff and non-point source pollution. To study these challenges and potential solutions in urban green spaces, we designed and deployed a network of in situ sensors measuring surface and groundwater level, soil moisture, precipitation, temperature, electrical conductivity, and discharge at 30-min intervals. In a native prairie-wetland complex, Gensburg Markham Prairie (GMP), anthropogenic inputs are a concern given observed ecosystem degradation around the periphery of the prairie. To understand soil quality and the potential for habitat degradation in GMP, we evaluated concentrations of metals and phosphorus in soil profiles, identified possible sources of contamination, and assessed the likelihood that contamination would impact the prairie ecosystem. We observed enrichment of Pb, Cu, Zn, and P because of deposition from the urban environment, but the enrichment levels are low enough that they are not expected to negatively impact the ecosystem of GMP. The detailed soil chemistry data constitute a spatial contamination map that can be used to assess potential long-term impacts on the ecosystem, such as reduction of plant growth and species diversity, and inform site management and biodiversity conservation efforts. Inland freshwater wetlands throughout the northern U.S. are experiencing an increase in salinity due to road salt runoff during the winter months. Salinization affects soil texture, contaminant transport, microbial activity, and plant growth in wetlands. Therefore, there is a pressing need to understand the dynamics of road salt intrusion in these ecosystems. Two years of continuous data in GMP show periodic spikes in electrical conductivity (EC) during winter months, generally by an order-of-magnitude, due to intrusion of road salt applied on nearby roads. Traditional water quality monitoring methods, such as monthly sampling, would likely have missed these abrupt changes in EC levels caused by rapid snowmelt runoff events. Spectral analysis of EC time-series in ditches suggests that salinity dynamics are not correlated at super-annual timescales, indicating that the dynamics are event-driven, and the introduced solutes are rapidly exported from the site. This study supports the development of improved de-icing strategies by local agencies and informs site-specific management of wetland ecosystems under anthropogenic stressors. A GIS-based spatial analysis tool, named Multifunctional Urban Green Stormwater Infrastructure (MUGSI), was developed for identifying and prioritizing locations for green infrastructure investments at the parcel level. The tool was applied in two watersheds – the Lower Des Plaines and the Little Calumet – and the City of Chicago in Cook County, IL – as part of a 5-year pilot program to assess the feasibility of a regional stormwater credit trading program in Cook County, termed StormStore. A stormwater credit trading program allows developers to meet a portion of their stormwater detention requirements on their sites by purchasing credits for stormwater retention or detention from others in the same watershed who have voluntarily retrofitted their properties for increased stormwater storage. We collected multiple datasets from the region to determine the suitability and benefits of candidate sites and developed 13 metrics to represent the benefits of green infrastructure. The metrics assess the multifunctionality of green infrastructure for stormwater management, species diversity, green space connectivity, air quality, water quality, urban heat island amelioration, environmental justice, green cover increase, and public green space accessibility. The results of the local GI assessment show that priority sites for investment are primarily located near industrial corridors, major highways, minority neighborhoods, and along the banks of the Lower Des Plaines and Little Calumet Rivers. The MUGSI tool advances the capability of urban planners, property developers, stormwater agencies, and conservation groups to prioritize sites for green infrastructure in Cook County and other urban areas based on their multifunctionality, feasibility, and overall benefit. The global increase of urban impervious land cover poses a significant threat to the aquatic ecosystems. Green roofs (GR) are one approach to mitigate the negative impacts of urbanization on aquatic ecosystems, such as thermal surges and pollution from urban runoff. We evaluated the ecohydrological behavior of two fully established GR under differing management regimes at the Chicago Botanical Gardens from July to September 2019. Drainage outflow from a managed GR (perennial native and non-native plants) and an unmanaged GR (perennial natural prairie vegetation) was monitored over a storm season (summer and early fall). Thermal dynamics dissolved organic matter (DOM) composition and nitrate concentration of runoff were assessed. The managed GR runoff had a lower DOC concentration and less humic-like DOM signal (SUVA254) than the unmanaged GR. In contrast, lower concentrations of nitrate and more recalcitrant DOM (less protein-like compounds relative to humic-like compounds) were released from the unmanaged GR. The unmanaged GR also displayed a greater capacity to reduce thermal surges associated with storm events. Our study provides new information on the links between GR management and runoff water quality. Further, the impacts of GR management on the mitigation of thermal surges and DOM composition can help to improve future GR design, as these ecohydrological responses have been largely overlooked to date. Our findings are scalable and can support future urban planning, particularly for scenarios where green infrastructures are used to mitigate the impacts of climate change on urban ecosystems.

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