Abstract
PurposeWastewater effluent discharged into rivers from sewage treatment works (STWs) represents one of the most important point sources of soluble reactive phosphorus (SRP) pollution and is a major driver of freshwater eutrophication. In this study, we assess the ability of riverbed sediments to act as a self-regulating buffering system to reduce SRP dissolved in the water column downstream of STW outflows.Materials and methodsRiver water and riverbed sediment samples were collected from ten tributary outlets across the River Wensum catchment, Norfolk, UK, at monthly intervals between July and October 2016, such that 40 sediment and 40 water samples were collected in total. Of these locations, five were located downstream of STWs and five were on tributaries without STWs. Dissolved SRP concentrations were analysed, and the equilibrium phosphorus concentration (EPC0) of each sediment sample was measured to determine whether riverbed sediments were acting as net sources or sinks of SRP.Results and discussionThe mean SRP concentration downstream of STWs (382 μg P L−1) was double that of sites without a STW (185 μg P L−1), whilst the mean EPC0 for effluent impacted sites (105 μg P L−1) was 70% higher than that recorded at unaffected sites (62 μg P L−1). Regardless of STW influence, riverbed sediments across all ten sites almost always acted as net sinks for SRP from the overlying water column. This was particularly true at sites downstream of STWs which displayed enhanced potential to buffer the river against increases in SRP released in sewage effluent.ConclusionsDespite EPC0 values revealing riverbed sediments were consistently acting as sinks for SRP, elevated SRP concentrations downstream of STWs clearly demonstrate the sediments have insufficient SRP sorption capacity to completely buffer the river against effluent discharge. Consequently, SRP concentrations across the catchment continue to exceed recommended standards for good chemical status, thus emphasising the need for enhanced mitigation efforts at STWs to minimise riverine phosphorus loading.
Highlights
Since the early twentieth century, rapid population growth, intensification of agriculture and extensive urbanisation haveResponsible editor: Marcel van der Perk Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK led to widespread phosphorus (P) enrichment of aquatic environments (Cordell et al 2009) resulting in an array of detrimental economic (Pretty et al 2003) and ecological impacts (Hilton et al 2006; Jarive et al, 2018)
Despite EPC0 values revealing riverbed sediments were consistently acting as sinks for soluble reactive phosphorus (SRP), elevated SRP concentrations downstream of sewage treatment works (STWs) clearly demonstrate the sediments have insufficient SRP sorption capacity to completely buffer the river against effluent discharge
Through the analysis of water and riverbed sediment samples collected across the River Wensum catchment, this research demonstrates that STWs significantly impact upon the P dynamics of this lowland calcareous river system
Summary
J Soils Sediments (2018) 18:2107–2116 majority (70–90%) of the P is released as biologically available soluble reactive phosphorus (SRP), it carries increased risk of initiating deleterious changes in ecological communities (Demars et al 2005; Edwards and Withers 2008). Under the EU Urban Wastewater Directive (91/271/EC), STWs serving a population of 10,000– 100,000 people have water quality restrictions set at 2 mg P L−1 for effluent discharging into a surface watercourse, with this decreasing to 1 mg P L−1 for facilities serving > 100,000 people. Previous water quality monitoring studies have observed increases in dissolved P concentrations of > 600% downstream of STW outflows (House and Denison 2002; Demars and Harper 2005), with concentration orders of magnitude greater than the 40–120 μg P L−1 concentration recommended under the EU Water Framework Directive (2000/60/EC) to achieve ‘good’ ecological and chemical status
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