Abstract
PurposeEngineering and dredging strategies to manage sediment, along with river-scouring, can reveal older sediments. These present a unique opportunity to assess past sources of phosphorus (P) inputs into river sediments. We used the sediment concentrations of P, lead (Pb) and 206/207Pb isotopes to produce ‘first-order’ estimates of the source (diffuse agricultural or sewage treatment) of phosphorus.Materials and methodsSediment cores (n = 30) were collected from the length of the non-tidal River Nene, a lowland river in eastern England. Cores were analysed for sediment elemental concentrations and Pb isotopes. Principal component analysis and linear regression modelling were used to assess the relationships between P, Pb and Pb isotopes. Monte-Carlo simulations and boot-strapping were undertaken to estimate, with 95% confidence intervals, the source of P in these sediments.Results and discussionAnalysis of the relationships between PTotal, PbTotal and 206/207Pb isotope ratios suggested that sediments were deposited largely prior to the phasing out of tetra-ethyl Pb (PbBHT) from petrol. Regression models showed positive correlations between PTotal and PbTotal (R2 = 0.85). Principal component analysis suggested a strong sewage treatment signal for Pb and P enrichment. In the rural upper three water bodies, little sewage treatment work (STW)-derived P was found in the sediment, a consequence of limited STW input and greater sediment transport. In the more urbanised water bodies 4–6, ‘first-order estimates’ of STW P suggest that median concentrations were 30–40% of PTotal.ConclusionsThe strong relationships between Pb and P concentrations in river water provided the opportunity to use 206/207Pb isotope ratios to calculate ‘first-order’ estimates of the proportion of P released from STWs in the historical sediment. Understanding the sources of historical sediment P can be used to assess the success of current sediment management strategies and to base further mitigation measures. Results suggest that whilst much recent sediment P is removed, the legacy sediment remains to contribute P to the water body. Thus, options regarding the practical removal of these sediments and the extent to which this would improve water P status need to be assessed and balanced against options such as further decreasing soil P or STW P stripping.
Highlights
River sediment phosphorus has been identified as a major ‘legacy issue’ (Johnes et al 2007; Sharpley et al 2013; Environment Agency 2014), contributing as surface waters failing ‘Good Ecological Status’ in accordance with the EU Water Framework Directive (Directive EC 2000/ 60/EC)
Analysis of the relationships between PTotal, PbTotal and 206/207Pb isotope ratios suggested that sediments were deposited largely prior to the phasing out of tetra-ethyl Pb (PbBHT) from petrol
In the rural upper three water bodies, little sewage treatment work (STW)-derived P was found in the sediment, a consequence of limited STW input and greater sediment transport
Summary
River sediment phosphorus has been identified as a major ‘legacy issue’ (Johnes et al 2007; Sharpley et al 2013; Environment Agency 2014), contributing as surface waters failing ‘Good Ecological Status’ in accordance with the EU Water Framework Directive (Directive EC 2000/ 60/EC). Bowes et al (2008) modelled the relative contribution of point and diffuse sources for Total P (TP) in waters from three UK rivers finding that point sources accounted for 0–93% of yearly TP load, depending on flow and proximity to STWs. Sediments will act as a P source when labile phosphate is desorbed and released to the water column, for instance when soluble reactive P concentrations, decline to concentrations below the equilibrium phosphate concentration (EPC0) value (Froelich 1988). Sediments will act as a P source when labile phosphate is desorbed and released to the water column, for instance when soluble reactive P concentrations, decline to concentrations below the equilibrium phosphate concentration (EPC0) value (Froelich 1988) This may occur if P stripping in sewage treatment works (STWs) decreases soluble reactive P (SRP) in waters sufficiently. Seasonal changes in EPC0 concentrations have been found primarily through changes in sediment particle size (House and Denison 1997; House and Denison (2002)
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