Impact of Point-Source Pollution on Phosphorus and Nitrogen Cycling in Stream-Bed Sediments

Diffusive equilibration in thin films (DET) was used to study the cycling of phosphorus and nitrogen at the sediment-water interface in-situ and with minimal disturbance to redox conditions. Soluble reactive phosphate (SRP), nitrate, nitrite, ammonium, sulphate, iron and manganese profiles were measured in a rural stream, 12m upstream, adjacent to and 8m downstream of a septic tank discharge. Sewage fungus adjacent to the discharge resulted in anoxic conditions directly above the sediment. SRP and ammonium increased with depth through the fungus layer to environmentally-significant concentrations (440 µM and 1800 µM, respectively) due to release at the sediment surface. This compared to only 0.8 µM of SRP and 2.0 µM of ammonium in the water column upstream of the discharge. Concomitant removal of ammonium, nitrite and nitrate within 0.5 cm below the fungus-water interface provided evidence for anaerobic ammonium oxidation (anammox). ‘Hotspots’ of porewater SRP (up to 350 µM) at the downstream site demonstrated potential in-stream storage of the elevated P concentrations from the effluent. These results provide direct in-situ evidence of phosphorus and nitrogen release from river-bed sediments under anoxic conditions created by sewage-fungus, and highlight the wider importance of redox conditions and rural point sources on in-stream nutrient cycling.