The salty truth behind gassy ponds: stratification and greenhouse gas emissions in urban stormwater ponds in southern Ontario

Abstract

Urban stormwater management ponds are established end-of-pipe components of green stormwater management infrastructure in North America. The main aim of this infrastructure is to protect our cities and homes from storm flooding and to clean the water before introducing it to protected ecosystems. While created for stormwater management, stormwater ponds provide key ecosystem services in urban areas, such as storing carbon, preserving biodiversity, and enhancing the connection to nature for the surrounding communities. Regardless of these services, stormwater ponds also accumulate large loads of organic matter, nutrients, and contaminants from their catchment and undergo a range of aerobic to anaerobic microbial processes that create and release significant amounts of greenhouse gases (GHGs) such as methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O). Pond design and catchment inputs may affect the processes and the GHG exchanges from the ponds. Therefore, this study aims to understand the processes that drive GHG emissions under the different design and contaminant conditions of depth and road salt concentration in four ponds over a study period from June - November and April - May the following year with no sampling over the winter months. The high salt ponds had greater stratification induced by road salt density gradient impact then their low salt counterparts, which was reflected in water specific conductivity and dissolved oxygen (DO), the stratification indexes calculated from the water column density gradients of each sample location, and the redox water quality sampling profiles. The CH4 emissions of the high salt ponds were also higher than their low salt counterparts, with the shallow ponds having higher emissions then the deep ponds, while the CO2 and N2O emissions were not driven by depth or road salt. A redundancy analysis showed that there were six significant predictors for the GHG emissions: specific conductivity, temperature, pH, dissolved CH₄ concentration, DO, and pond depth. CO₂ emissions were associated with deeper, cooler, low-conductivity conditions, whereas CH₄ diffusive emission and N₂O emissions were associated with high-conductivity, low-oxygen waters and ebullitive CH₄ emission aligned with warm, higher pH environments. These findings highlight the complex and dynamic role of stormwater management infrastructure in urban GHG emissions and the importance of understanding the processes that govern them.