Restoring instream wood as a Nature-based Solution to nutrient pollution

Nutrient pollution is among the biggest water quality challenges facing environmental engineers, causing ecosystem health decline and requiring significant treatment before water is safe for human use. The remediation of nutrient pollution can occur along the river corridor where nutrients can be retained and transformed to environmentally benign products (e.g. N₂) by microbially mediated reactions. Catchment mismanagement, especially the removal of wood from channels, has reduced the capacity of rivers to fulfil this function, limiting reactions by two primary mechanisms: transport and reaction kinetics. These limitations could be abated by restoring rivers with instream wood, thereby providing a Nature-based Solution to this persistent water quality challenge.

Little research has directly investigated the potential of wood introductions to increase nutrient transformations in the river corridor. Here, we present results on two linked experiments which investigated the transport and reaction kinetics of nutrients at different scales.

The transport limitation refers to the total flux of water into the hyporheic zone, a hotspot of biogoechemistry in the river sediment, and its residence time therein. Instream wood causes an obstacle for river flow which could induce hyporheic exchange of suitable properties to allow favourable nutrient transformations. To investigate this, we installed wood features in a lowland stream in the UK and conducted a series of smart tracer injections to estimate (metabolically active) transient storage using a powerful before-after-control-intervention experimental design.

The reaction kinetic limitation refers especially to the availability and quality of organic matter, which could be improved by wood decomposition. An incubation experiment investigates the potential of instream wood as a source of organic matter and the effect of this on nutrient transformation rates and greenhouse gas production.

Preliminary analysis suggests that wood is effective at increasing nutrient transformations in both mechanisms. In the incubation experiment, nutrient transformation rates were higher in systems with wood compared to those without. In the field experiment, more metabolically active transient storage was observed in the sub-reach where restoration features had been installed. Our results provide evidence for the effectiveness of wood in river restoration and the efficacy of Nature-based Solutions for water quality challenges.