Greenhouse gas flux from stormwater ponds in southeastern Virginia (USA)

Stormwater ponds are ubiquitous features of developed landscapes of the eastern United States. Their design specifically controls the pace of water runoff from impervious cover of surrounding watersheds. Ponds accumulate organic matter that typically decomposes anaerobically in bottom sediments, and thus may be significant sources of greenhouse gases to the atmosphere (e.g., carbon dioxide (CO2), methane (CH4)). We sampled fifteen stormwater retention ponds in southeastern Virginia (USA) during summer 2018 to determine the diffusive emission of greenhouse gases with respect to measured environmental parameters. The equivalent CO2 (CO2e) flux from ponds ranged from 8.3 to 80 mmol m-2 h-1, with CH4 contributing 94%, CO2 6% and nitrous oxide less than 1% of the CO2e flux, on average. From linear mixed-effects modelling, diffusive flux of CO2 was inversely associated with pH. Maximum depth best explained diffusive flux of CH4, with surface area of secondary importance, i.e. CH4 flux was higher in smaller and more shallow ponds. With 300 stormwater ponds in the county where we conducted this study, we estimate that, during a 100-day warm season, these ponds emit 2.3 × 109 ± 1.5 × 109 SD g C as CO2e. As small, human-constructed ponds are becoming common features of urbanizing landscapes globally, results from this study suggest that, collectively, small ponds can contribute substantially to climate forcing. Better pond designs that reduce sediment methanogenesis, however, can mitigate the hypothesized potential disservice of GHG emissions from unvegetated stormwater retention ponds.