Sediment chemistry of urban stormwater ponds and controls on denitrification

Abstract

. 2018. Sediment chemistry of urban stormwater ponds and controls on denitrification. Ecosphere 9(6):e02318. 10. 1002/ecs2.2318 Abstract. Stormwater ponds and retention basins are ubiquitous features throughout urban landscapes.These ponds are potentially important control points for nitrogen (N) removal from surface water bodies via denitrification. However, there are possible trade-offs to this water quality benefit if high N and contaminant concentrations in stormwater pond sediments decrease the complete reduction of nitrous oxide (N 2 O), a potent greenhouse gas, to dinitrogen (N 2 ) during denitrification. This may occur through decreasing the abundance or efficiency of denitrifiers capable of producing the N 2 O reductase enzyme. We predicted that ponds draining increasingly urbanized landscapes would have higher N and metal concentrations in their sediments, and thereby greater N 2 O yields. We measured potential denitrification rates, N 2 O reductase (nosZ) gene frequencies, as well as sediment and porewater chemistry in 64 ponds distributed across eight U.S. cities. We found almost no correlation between the proportion of urban land cover surrounding ponds and the nutrient and contaminant concentrations in the stormwater pond sediments within or across all cities. Regression analysis revealed that the proportion of potential N 2 and N 2 O production that could be explained was under different environmental controls. Our survey raises many new questions about why N fluxes and transformations vary so widely both within and across urban environments, but also allays the concern that elevated metal concentrations in urban stormwater ponds will increase N 2 O emissions. Urban stormwater ponds are unlikely to be a problematic source of N 2 O to the atmosphere, no matter their denitrification potential.

Greenhouse gas emissions from urban ponds are driven by nutrient status and hydrology

“…Because our ponds were all within an urban environment, we hypothesize that anthropogenic influences local to each pond (e.g., point sources of nutrients) override any effect of pond physicality (i.e., area) or function. This hypothesis is supported by sediment data from 64 urban stormwater ponds in the United States, where neither nutrient nor pollutant concentrations varied according to the proportion of urban land cover (Blaszczak et al 2018). The authors suggested that factors such as pond age, the presence of legacy pollutants, retention time, groundwater exchange, and management regime (e.g., use of fertilizers nearby and use of algaecide) could instead control the biogeochemistry of individual ponds.…”

It Is Not Easy Being Green: Recognizing Unintended Consequences of Green Stormwater Infrastructure

“…Performance expectations are based on the incoming pollutant load, permanent pool volume, and retention of sediment [136]. Often, stormwater ponds are also expected to remove nitrogen through sedimentation and denitrification [75,[137][138][139]. In addition to removing pollutants, stormwater ponds also provide ecosystem services (such as carbon sequestration and biodiversity) and cultural services (such as recreation and education, when properly designed and located) [14,140].…”

“…Overgrown vegetation surrounding ponds impedes access for maintenance and shelters the water surface from wind mixing and oxygenation [141,142]. Stormwater ponds have varied suitability as ecological habitats [138,140,143,144] and may struggle to serve as both a valued water feature and a pollutant-capturer [124,145].…”

“…On the other hand, stormwater treatment ponds will receive untreated stormwater rich in contaminants to protect downstream water bodies of greater ecological, economic, and sentimental value. With respect to reducing pollutants downstream, and reducing maintenance costs, watershed managers should look upstream toward reducing sources of pollutants, such as chloride and suspended solids, that impair the pollutant removal function of GSI; one strategy would be to reduce the catchment area to pond area ratio, which is often ten to a hundred or more [138]. Once priorities are established, specific steps can be taken to meet performance goals.…”

“…For this reason, low-maintenance stormwater ponds may be possible in upstream portions of watersheds draining small surface areas. Stormwater ponds are often designed to receive stormwater runoff from large drainage areas [138], however, leading to rapid sediment and pollutant accumulation. Any ponds located further downstream in a watershed and receiving runoff from large areas would require much more intensive maintenance and pretreatment.…”