Optimizing placement of constructed wetlands at landscape scale in order to reduce phosphorus losses
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ENVIRONMENTAL EFFECTS OF A GREEN BIO-ECONOMY
Optimizing placement of constructed wetlands at landscape scale in order to reduce phosphorus losses Faruk Djodjic
, Pia Geranmayeh, Hampus Markensten
Received: 12 December 2019 / Revised: 22 April 2020 / Accepted: 15 May 2020
Abstract Constructed wetlands (CWs) are one of the main countermeasures to reduce diffuse phosphorus (P) losses, but there is still a lack of systematic guidance accounting for spatially variable effects of hydraulic and P load on P retention. We present a three-step modelling approach for determining suitable placement of CWs in four different size groups (0.1–1.0 ha), based on incoming hydraulic and P load. The modelled hypothetical CW area was only 17% of that previously estimated and area of efficient CWs is even lower. The mean area-specific P retention increased with CW size. However, the spatial variation in retention was large for all size groups and largest (6–155 kg ha-1 year-1) for the smallest CWs due to highly variable incoming P loads, showing the possible benefits of targeted placement of CWs. The presented modelling approach has also flexibility to include and account for possible future changes in land cover and management. Keywords Constructed wetlands Modelling Optimised placement Phosphorus retention
INTRODUCTION Following successful reductions in nutrient loads from point sources such as wastewater treatment plants, agriculture is now considered to be the main non-point source of eutrophying nutrients in many parts of the world (Carpenter et al. 1998; Sharpley et al. 2015). In Sweden, agriculture is estimated to be the largest anthropogenic source of both nitrogen (N, 23 300 t) and phosphorus (P, 460 t) (Ejhed et al. 2016). Construction or restoration of wetlands is an important countermeasure to reduce nutrient delivery to aquatic ecosystems (Fisher and Acreman 2004; O’Geen et al. 2010). Since 2010, approximately 4500 hectares of
new wetlands have been constructed or restored in Sweden, with on average of more than 500 ha of new wetlands constructed annually, at a cost of approximately 30 million SEK per year (Swedish Environment Protection Agency 2019). As intensification of mitigation efforts to reduce nutrient losses from agriculture is required, further increases in the number and area of wetlands can be expected. For instance, the Swedish government will invest * 200 million SEK in the construction/restoration of wetlands in the period 2018–2021. The P removal efficiency of CWs receiving runoff from non-point sources varies considerably, between 1 and 88% (Braskerud et al. 2005; Kynka¨a¨nniemi 2014), with a mean value of 33% and a median value of 26% (Kynka¨a¨nniemi 2014). A systematic review of European, Asian, and American CWs showed that the median removal efficiency of total P (TP) was 44% for CWs treating agricultural runoff. However, CWs with precipitation-driven flow had lower removal efficiency, 21% TP (Land et al. 2016). The P removal efficiency varies depending on wetland design, location, annual var
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