Event-scale suspended sediment fluxes in a dryland environment valley-bottom wetland: Implications for downstream sediment and phosphorous fluxes

Abstract

Suspended fine sediment in rivers and small streams contributes to fluvial pollution loads because of its association with anthropogenically introduced heavy metals, pesticides / herbicides, and nutrients, including phosphorous (P). By acting as sinks for suspended sediment, wetlands located along drainage lines have considerable potential in reducing downstream sediment and nutrient fluxes. The aim of this research was to document downstream sediment fluxes within a single valley-bottom wetland to establish the effectiveness of this common wetland type in retaining P attached to sediment. The study was accomplished by conducting a topographic survey (dGPS and 0.5 m Lidar-derived DEM) with surface and sub-surface sediment sampling to establish long-term (decades to centuries) processes of erosion and deposition within the wetland, as well as consider storage of P. This was paired with event-based suspended sediment sampling at eight positions within the wetland over a wet season, as well as time-averaged suspended sediment sampling at a seasonal-scale at the wetland inlet and outlet. Sediment samples were characterised in terms of organic content, total phosphorous and particle size, which were determined using loss-on-ignition, ICP-AES following aqua regia extraction, and a combination of dry sieving and settling, respectively.Over the wet season, average suspended sediment concentrations increased to a maximum value of 0.26 g.L−1 and then declined, potentially because of the exhaustion of locally available sediment. The total P concentration of incoming sediment was 2174 mg/kg compared to 800 mg/kg at the wetland outlet. However, when considering the seasonal flux for the single year sampled, it was found that the wetland was a net downstream contributor of sediment (11 g of sample were retained at the inlet, relative to 19 g at the outlet). Despite this, the wetland retained ∼37 % of the total P supplied to it from upstream sources, with the remainder passing through the wetland as a result of sediment reworking in reforming channels. Thus, while wetlands may be effective at retaining some sediment and nutrients, they are not an alternative to more formal water treatment works.