Estimation of the natural background of phosphate in a lowland river using tidal marsh sediment cores

[摘要] Elevated phosphate ( PO 4 ) concentrations can harm theecological status in water by eutrophication. In the majority of surfacewaters in lowland regions such as Flanders (Belgium), the local PO 4 levels exceed the limits defined by environmental policy and fail todecrease, despite decreasing total phosphorus (P) emissions. In order tounderpin the definition of current limits, this study was set up toidentify the pre-industrial background PO 4 concentration in surfacewater of the Scheldt River, a tidal river in Flanders. We used thesedimentary records preserved in tidal marsh sediment cores as an archivefor reconstructing historical changes in surface water PO 4 . Forsediment samples at sequential depths below the sediment surface, we datedthe time of sediment deposition and analysed the extractable sediment P. Theresulting time series of sediment P was linked to the time series ofmeasured surface water- PO 4 concentrations (data 1967–present). Bycombining those datasets, the sorption characteristics of the sediment couldbe described using a Langmuir-type sorption model. The calibrated sorptionmodel allowed us to estimate a pre-industrial background surface water PO 4 levels, based on deeper sediment P that stabilized atconcentrations smaller than the modern. In three out of the four cores, thesediment P peaked around 1980, coinciding with the surface water PO 4 .The estimated pre-industrial ( ∼1800 ) background PO 4 concentration in the Scheldt River water was 62 [57; 66 (95 % CI)]  µg PO 4 -P L −1 . That concentration exceeds the previouslyestimated natural background values in Flanders (15–35  µg TP L −1 ) and is about half of the prevailing limit in the Scheldt River(120  µg PO 4 -P L −1 ). In the 1930s, river waterconcentrations were estimated at 140 [128; 148]  µg PO 4 -P L −1 , already exceeding the current limit. The method developed hereproved useful for reconstructing historical background PO 4 concentrations of a lowland tidal river. A similar approach can apply toother lowland tidal rivers to provide a scientific basis for localcatchment-specific PO 4 backgrounds.

[学科分类] 大气科学