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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
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
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
Phosphorus in surface water
28 juni 2017
Observation 2:
Low dissolved P concentrations
Particulate P• mineral P?• organic P?
Data WSHA
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
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
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
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
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
Suspended Particulate Matter samples‘Fifty Shades of Brown’
28 juni 2017
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Hunze Haarlemmermeer
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
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
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
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
Experimental results
28 juni 2017
Time interval vs. SPM concentration Critical bed shear stress (14 experiments)
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
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
Van der Grift et al., 2014; HESS.
Hupsel brook field experiment(Rozemeijer et al., 2010)
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)
Fosfaat filtration steps
28 juni 2017