Design of stream remediation measures for nutrient retention and attenuation in the hyporheic zone

Ida Morén, Anders Wörman and Joakim Riml

 

KTH Royal Institute of Technology, Stockholm, Sweden

 

In agricultural areas, streams can serve as an important nutrient sinks, specifically through the interaction between surface water and ground water, so called hyporheic exchange. Biogeochemical processes are particular intense in the hyporheic zone and enhancing hyporheic flows through modification of stream geomorphology may therefore be an effective tool for retention and attenuation of excess nutrients. However, there is a lack of specific design guidelines for in-stream remediation measures. In this study an analytic model was developed that relates the stream-morphology to the distribution of hydraulic head gradients along the streambed and, further, to quantitative water quality objectives. This was done by linking a 2D spectral model for hyporheic exchange to a longitudinal solute transport model, accounting for first-order decay and equilibrium partitioning between dissolved and particulate phases. The spectral approach provides a way to predict and evaluate hyporheic exchange based on stream geomorphology and hydraulics, on all relevant scales. To evaluate the model, Rhodamine WT tracer tests were performed in Tullstorps Brook, which is a partly restored stream in Sweden. The model can be used for guidance when designing in stream remediation features for improved water quality. We show that in every stream reach there is a Damkhöler number, defined as the product between the hyporheic residence time and the denitrification rate along the hyporheic streamlines, which optimize the nitrate mass removal. However, one Damkhöler number can be associated with several different stream-morphology designs. Furthermore, we show how various stream morphologies utilize a different degree of the total hydraulic head fall over a stream reach and specifically compare the self-cleaning capacity of features that create a stepwise decent in the surface water profile with a degraded agricultural stream. The results shows that steps have the potential to increase the self-cleaning capacity of a degraded agricultural stream, but only if the hyporheic zone is not constrained to a shallow depth by upwelling groundwater or an impermeable geological layer.