Phosphorus accumulation in constructed wetlands : A study of 10 wetlands constructed on agricultural clay soils in Södermanland

Sammanfattning: Eutrophication is a major problem causing algal blooms and impaired water quality, especially regarding coastal areas and seas. It is a result of an excessive supply of nutrients such as phosphorus (P), where agriculture accounts for the largest share of the anthropogenic nutrient load. Furthermore, arable fields with clay soils have been shown to have among the highest P-losses. In this study, 10 constructed wetlands located in agricultural areas with high clay content in the topsoil were examined. The aims were to investigate how the P accumulation is related to wetland design and catchment factors and to see if there is an optimal hydraulic load (HL) and wetland size for P accumulation. The study is based on P analyses of sediment cores sampled in March 2021, calculations of catchment factors in ArcMap, water flow calculations, and statistical analyses. Sedimentation of particles with associated P is considered to be the primary process for P retention in wetlands. Accordingly, the results showed that the P accumulation was positively correlated to particle retention. The total P accumulation varied between 8 and 96 kg ha-1 yr-1 and the total particle accumulation was 7-130 t ha-1 yr-1. The wetland design factors, including wetland area, water depth, and length-to-width ratio (L:W), had no significant influence on the P accumulation. However, water depths greater than 1.5 m inhibited the P retention which suggested that the water depth should be limited to approximately 1 m to avoid too large particle settling distances.  The proportion of agricultural land within each catchment did not show any correlation to the P accumulation. The clay content in the topsoil was however positively correlated to both particle and P accumulation. This suggested that the sedimentation process was promoted even though the inflowing particles potentially consisted of a large proportion of clay particles, which have a low sedimentation velocity. The erosion risk of the catchment area had no significant influence on the P accumulation, meaning that a high expected particle load did not entail an increased particle and associated P accumulation. High HL values of 450 and 850 m yr-1 were shown to counteract the P retention. This corresponded to wetlands smaller than 0.1% of the catchment area. A possible turning point of HL where the P accumulation is inhibited could also be distinguished, ranging between HL 200 and 300 m yr-1. An optimal HL for increased P accumulation could not be determined due to such few observations. However, it could be concluded that an HL up to approximately 210 m yr-1 had a positive influence on the particle and P accumulation and that wetlands should be larger than 0.1% of the catchment area to efficiently retain P.