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