Environmental trade-offs Offs when using Regenerative Stormwater Conveyance for Managing Nutrients in Urban Ecosystems
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Hydrologic modifications in urban ecosystems impact water quality via soil compaction, impervious surface cover, drainage, and channelization. Urban stream restoration strategies that are designed to regain lost ecosystem function such as nutrient uptake may have unintended consequences and environmental trade-offs. Regenerative Stormwater Conveyance (RSC) is a restoration approach based on engineering stream channels to incorporate shallow pools, riffles, and grade controls to increase storm water retention and control erosion, and adding a carbon layer (e.g. wood chips) to the stream bed to enhance microbial processes like denitrification. We examined RSC effects on nutrients (P and N) and trace metals (Fe and Mn) at two sites 5 and 7 years post-construction, using combined field and laboratory measurements. Field measurements showed that RSCs usually had lower dissolved oxygen (DO) and pH relative to nearby untreated stream reaches, but did not have consistently different P, N, Fe, Mn, or dissolved organic carbon (DOC) suggesting that engineering ecosystem function such as nutrient uptake is affected and ultimately limited by watershed variability. In lab simulation experiments, we observed removal of N and P when sediment was amended with wood chips and leaf litter, and that these DOC sources had significantly different effects on nutrient and metals release suggesting that organic matter additions to streams drive nutrient transformation and that trade-offs exist between P and N management in restored streams depending on quantity and quality of DOC source and anoxic conditions in the hyporheic zone. An additional trade-off occurs in some RSCs where elevated groundwater levels expand the zone of anoxia but, in turn, drown and kill riparian zone trees, potentially reducing DOC inputs. Consideration of trade-offs and unintended consequences are critical for choosing and implementing BMPs in urban ecosystems that will be most effective at removing nutrients and metals.
We published the following paper of our results: Duan S, P Mayer, S Kaushal, B Wessel, T Johnson. 2019. Regenerative stormwater conveyance (RSC) for reducing nutrients in urban stormwater runoff depends upon carbon quantity and quality. Science of the Total Environment 652:134-146.