Stormwater biogeochemistry in urban stream networks

Abstract:

Urbanization reduces infiltration and efficiently routes excess precipitation to receiving streams, resulting in elevated peak flows and lower residence times in river networks. Nutrient loads from urban areas and piped collection systems often bypass riparian zones leading to even greater nutrient export. Changing climate regimes are expected to change precipitation patterns resulting in higher peak flows leading to increased streambank erosion and channelization, and decreased residence times. Together, these reduce the capacity for biologically active headwater streams to attenuate nutrient loads. Stormwater control measures (SCMs, e.g., ponds & wetlands) have the potential to improve these conditions through reduction of scouring peak flows and changing water chemistry. Because performance monitoring typically ends at the outlet, the impact on ecosystem structure and function within the river network remains unknown. To address this gap, we monitored four small urban watersheds to quantify patterns of nutrient export during a range of storm sizes and measured denitrification as an indicator of ecosystem function. In all watersheds, SCMs were transformers of stream water chemistry. In the two urban sites, lower solute concentrations in SCM outflow reduced instream concentrations below the stream-SCM confluence. However, in the suburban watersheds, we were unable to empirically separate the effects of impervious surface runoff and mitigation because an increase in developed/impervious area coincided with addition of SCMs. We observed longitudinal increases in denitrification, which coincided with inputs of additional SCMs along the stream reach. Further, denitrification measurements showed that SCMs have greater potential to enhance rates at low levels of total imperviousness. Our results suggest that particularly in watersheds with low TI, SCMs may act synergistically to reduce runoff and pollutant loads within the watershed as well as enhance in-stream processes.