Particle deposition, resuspension and phosphorus accumulation in small constructed wetlands

Pia Geranmayeh,Barbro Ulén,Karin S Tonderski,Karin M Johannesson

doi:10.1007/s13280-017-0992-9

Abstract

To improve understanding of phosphorus (P) retention processes in small constructed wetlands (CWs), we analysed variations in sediment deposition and accumulation in four CWs on clay soils in east-central Sweden. Sediment deposition (in traps) generally exceeded the total suspended solids (TSS) load suggesting that resuspension and wetland base erosion were important. This was confirmed by quantification of particle accumulation (on plates) (1–23 kg m−2 year−1), which amounted to only 13–23% of trap deposition. Spatial mean P concentrations in accumulated sediment on plates (0.09–0.15%) were generally similar to temporal mean P concentrations of particles in water (0.11–0.15%). Deposition/accumulation was minor in one wetland with high hydraulic load (400 m year−1), suggesting that such small wetlands are not efficient as particle sinks. Economic support for CWs are given, but design and landscape position are here demonstrated to be important for effective P retention.

Highlights

Wetlands are often constructed to retain particles and associated phosphorus (P) transported from agricultural areas
This study examined seasonal and spatial variations in sediment deposition in traps and annual variations in sediment accumulation on plates in four small wetlands constructed on agricultural clay soils in order to reduce the particle and P load to nearby streams and lakes
Estimated high loads of total suspended solids (TSS) determined using the flow-proportional technique were confirmed with independent measurements using online sensors in Ber wetland in later years (Supplementary Material)

Summary

INTRODUCTION

Wetlands are often constructed to retain particles and associated phosphorus (P) transported from agricultural areas. Braskerud (2003) studied particle retention in two wetlands and found that the area-specific retention of total suspended solids (TSS) increased with HL Braskerud attributed this to increased transport from the catchment of more settled coarser soil particles at higher HL. The small particles are probably more prone to resuspension than silt-sized particles or larger aggregates This means that a relatively low HL, lower than that in the study by Braskerud (2003), would probably be needed to retain clay particles with associated P. This study examined seasonal and spatial variations in sediment deposition in traps and annual variations in sediment accumulation on plates in four small wetlands constructed on agricultural clay soils in order to reduce the particle and P load to nearby streams and lakes. The objectives here were to (i) analyse the relationship between seasonal sediment deposition, estimated TSS load and HL; (ii) estimate sediment deposition and accumulation in different parts of the wetlands; (iii) evaluate the importance of resuspension and internal erosion for annual sediment accumulation and (iv) examine the relationships between P concentrations in TSS and in accumulated sediment

MATERIALS AND METHODS

RESULTS

DISCUSSION

CONCLUSIONS