Do constructed wetlands work?

Our research with Lancaster University on the potential of field corner constructed wetlands to reduce sediment and nutrient movement to water has just been published*. Three designs were tested across three soil types, including sandy and silty soils in Cumbria, and our clay soils at Loddington.  Although rainfall inevitably had a major influence on sediment accumulation, wit more rain in Cumbria than at Loddington during the study period, the influence of soil type was considerable.  Sandy soils accumulated 0.8 tonnes per hectare of catchment per year, silty soils 0.4 tonnes, and Loddington clay, 0.03 tonnes.  Phosphorus concentration was highest in sediment with low particle size but this had little influence on the effect of soil type on overall accumulation of phosphorus.

Sediment accumulation behind a WFF project ditch dam

So does this mean field edge wetlands have limited potential in catchments with clay soils such as those of the Water Friendly Farming project?  It does seem that wetlands in sandy and silty soils are likely to capture sediment more rapidly, but we know from previous work at Loddington that simple ditch dams can be very effective at both accumulating sediment and providing biodiversity benefits.  Similar structures created in the Water Friendly Farming project have also already proved to be effective at some sites.  The current study involved only three wetlands, and as we report in the paper, siting of such wetlands is critical to their performance.  In terms of practical application, flexibility in design is also critical so that the structure can be adapted to site specific constraints and opportunities to optimise sediment and nutrient capture.  Finally, wetlands that fail to capture fine particulate matter associated with clay soils, and those on sandy or silty soils that fill up quickly, both indicate that appropriate soil management in the field is the most fundamental approach to reducing soil and nutrient loss to water.

*Ockenden, M., Deasy, C., Quinton, J., Surrindge, B. & Stoate, C. 2014. Keeping agricultural soil out of rivers: Evidence of sediment and nutrient accumulation within field wetlands in the UK.  Journal of Environmental Management 135: 54-62.