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28 juni 2017
Biogeochemical and hydrological controls on phosphorustransport in lowland catchments
Bas van der Grift
Martin WassenJasper Griffioen Paul SchotLeonard Osté
With help from:Joachim RozemeijerYpe van der VeldeThilo BehrendsDeirdre Clark Arjen KratzEmma van PoptaWilbert BerendrechtMiguel de Lucas PardoHans Peter BroersErwin Meijers
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28 juni 2017
What happens in between?Controls on P transport in catchment
P loads from soil to surface water
Phosphorus transport in catchments
Data WSHA
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Phosphorus in surface water
28 juni 2017
Observation 1:
variable in time
Data WSHA
P-total in unfiltered samples:common practice in surface water monitoring
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Phosphorus in surface water
28 juni 2017
Observation 2:
Low dissolved P concentrations
Particulate P• mineral P?• organic P?
Data WSHA
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Groundwater-fed lowland catchments
28 juni 2017
Data national monitoringnetwork groundwater quality2009
Reactive subsurface• anoxic groundwater• Fe(II)-bearing• discharges to surface water
high load of dissolved Fe(II) to surface water
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Aeration of Fe-bearing groundwater
28 juni 2017
just after sampling 1 day later
Fe2+(aq) + 2.5H2O + 0.25O2 Fe(OH)3(s) + 2H+
Fe precipitates have a high affinity for binding PO4
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Fe(II) oxidation experiments
28 juni 2017
PO4 added after Fe(II) oxidation PO4 present during Fe(II)
Effect of formation Fe(III) precipitates on PO4 immobilisation
Geochemical fundamentally different
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Experiments with initial dissolved P/Fe2+ ratio of 0.18
PO4 present: precipitation ofFe2.44PO4(OH)4.32P/Fe = 0.41
PO4 depleted: precipitation ofFe(OH)3
P/Fe initial = 0.18
Formation of Fe hydroxyphosphates is very efficient process to immobilise dissolved phosphate
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Effect of seepage of Fe-bearing groundwatermonitoring of phosphorus fractionation in surface water
Sampling and chemical analysis of surface water and suspended particulate matter (SPM)• six catchments• 49 locations – from field ditch to catchment outlet• 96 samples – from winter to summer• Sampled SPM by filtration
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Suspended Particulate Matter samples‘Fifty Shades of Brown’
28 juni 2017
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Hunze Haarlemmermeer
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Sequential chemical extraction
Fractionate:• Dissolved P (ortho-P)• Loosely bound or
exchangeable P • Organic P• Calcium-bound P• Iron-bound P• Residual P (apatite P)
28 juni 2017
high
low
bioavailability
Particulate P
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Results from field survey in six catchment
28 juni 2017
Fe-P = 0.62 x TPFe-P = 0.80 x PP
Fractions of particulate P Fractions TP in surface water
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Conceptual model of abiotic in-stream processes influencing phosphate transport
28 juni 2017
Removal of dissolved phosphate from solution by formation of Fe-hydroxyphosphate
Transport of particulate phosphate as a function of hydrodynamics
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Erodibility of soft sediments from drainage ditches and channels in polders
Gust Erosion Microcosm System
Van der Grift et al., 2017; In Prep.
Samples from two areas
• What forces are needed to erode bed sediment?• Where and how often are these forces reached in a polder?
Undisturbed cylindrical sediment cores
Erosion Chamber
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Experimental results
28 juni 2017
Time interval vs. SPM concentration Critical bed shear stress (14 experiments)
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Maximum bed shear stress during the period from March 2004 to April 2005
28 juni 2017
350 km watercourses (not including field ditches) 30 km where maximum bed shear stress > 0.1 Pa (erosion possible)
Sobek-Delwaq model for polder Quarles van Ufford
• Geographic setting of a polder tempers peak flow • Low maximum hydrodynamic forces that generally may not
be strong enough to cause erosion of bed sediment• Polder function as peak runoff control system
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Main conclusions
• Discharge of Fe-bearing groundwater in combination the limited water flow in artificial and strongly managed watercourses reduce the environmental risk caused by P loads from diffuse agricultural sources.
• Transport and ecological impact of P in surface waters are strongly controlled by the exfiltration of Fe-bearing groundwater
• Flow-induced bed shear stress in watercourses in lowland catchment seldom exceeds the critical shear stress for erosion, which limits resuspension of particulate P
• Sedimentation of particulate Fe-bound P results in a steady accumulation of a pool of ‘legacy P’ in bed sediments
28 juni 2017
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Van der Grift et al., 2014; HESS.
Hupsel brook field experiment(Rozemeijer et al., 2010)
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Measured and modelled PO4 concentrations upon aeration of groundwater
28 juni 2017
Maximum P/Fe ratio of Fe hydroxyphosphate ≈ 0.6
• Experimental data from Griffioen (2006)• Modelled PO4 immoblisation by precipitation of Fe hydroxyphosphate (Fe2.5PO4(OH)4.5)
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Fosfaat filtration steps
28 juni 2017