Scenario analysis for stream restoration actions aimed at reducing nutrient loads to the Baltic Sea

Wörman1, A., Riml1. J., Capell2, R. and Morén1, I.,

 

1 KTH Royal Institute of Technology, Stockholm, Sweden

2 Swedish Meteorological and Hydrological Institute, Norrköping, Sweden

 

Main targets of the Baltic Sea Action Plan are to reduce nutrient loading to the Sea by land-based remediation actions. This work shows a scenario application for restoration actions in streams with specific water quality targets aimed at reducing nitrogen export to the Baltic Sea. Denitrification that reduces the export of nutrients to recipient waters can occur both in the flowing water, in riparian zones and in the hyporheic zone of streams in which denitrification and adsorption is often significantly more intense. The exchange with such transient storage zones can be enhanced in a predictable way by introducing remediation structures, like check dams, sequences of riffle-and-pools, boulders and stream meanders, which enhances the water exchange with the hyporheic zone. A developed quantitative design method is used to forecast the effects of a multitude of remediation actions in Tullstorps Brook, Sweden, and generalized to 1001 watersheds distributed over the entire area of Sweden. This generalization was based on an analytical solution relating the nitrate reduction in surface water systems to hydrologic residence times and denitrification rates. The rate estimations were grounded in a vast literature survey of denitrification rates in streams, wetlands and laboratory environments, while estimates of hydrological residence times were determined from more basic estimates of stream network distances and flow velocities. Account was also taken to variation in the denitrification rate with temperature over the year and statistical variation in the result by means of Monte Carlo simulations.

We found that the predominant part of the nitrogen removal through denitrification occurs in first-order streams, such as drainage ditches through agricultural land. The founding is explained both by the high nitrogen load to those local streams and their long residence times. This study has shown that remediation actions in streams greatly can reduce the total export of nitrogen from Swedish catchments to the recipient Sea. If remediation actions are well designed and implemented to full potential in all local streams, the maximum possible reduction of the total mass export from local streams to the downstream Swedish surface water system is estimated to be 60 % ± 24 % including the current reduction and the enhancement based on remediation actions. The reduction of the nitrogen load at the river basin effluence depends on the behavior of downstream aquatic system in terms of further denitrification, atmospheric deposition and other relevant nitrogen processes as well as contribution from other nitrogen sources.