Influence of hydraulic residence time on pesticides retention in constructed wetlands

Céline Gaullier (LIEC), Sylvie Dousset (LIEC), Nicole Baran (BRGM), David Billet (LIEC)

GFP Nancy – 15-18 of may 2017

� Surface water pollution by pesticidesRunoff, leaching, drainage , spray drift� Limited by grass strip (between crop and river)

� ineffective in case of drained plot

� ‘Free water surface CW’ : mitigation of contaminant concentration transported to the river (30 years)

� Efficiency of CW depends on: • Hydraulic parameters

Flow rate/Water level � Hydraulic residence time (HRT)• Morphology : ratio length/width• Molecules properties : solubility, Kow, Koc• ………

Introduction

From National Engineering HandBook (2002)

� Contradictory results in the

literature

(CW)

Introduction

� Lorraine : drained surface > 16 % cultivated surface (clay +++)� vulnerability of surface water

Frequence of drained surface in Lorraine Vulnerability of pesticides transfert in surface water

LowMediumHigh

Very LowLowMediumHighVery high

Introduction

� 9 CWs set up since 2010/2011

Introduction

� 9 CWs set up since 2010/2011� 2 studied sites during the thesis

3 ponds – 215 m²HRT : 1 h to 16 h

Manoncourt sur Seille (54)

Ville sur Illon (88)

Ditch – 100 m²HRT : 30 min to 3 h

Introduction

Aim: determine hydraulic influence on the efficiency of the CWs

Field80 molecules / 15 metabolites

Pilot-scale4 molecules

� Weekly monitoring: dissolved and particulate� Daily monitoring: dissolved� Br- tracing

� Field = very complex system

� Pilot = under controlled conditions (flow rate, water level, volume etc.)

Stream

Pilot-scale experiment – Questions ?

� Does the HRT influence the effectiveness? � 2 HRT studied

� Do pesticide properties influence his mitigation?

� Does the presence of dead zones/short-circuiting impact the effectiveness?

� What is the fate of pesticides during stagnation?

BSC CYP IPU DMT

Pilot-scale experiment

� Pilot-scale: 1/20° et 1/40° (2.5 m of lenght)� Flow rate/water level representative of the field (1-2 L/s and 8 L/s)

3 ponds Ditch

Manoncourt sur Seille (54)Ville sur Illon (88)

Experimental design

� Example of drainage dynamic in the field

� 4 pesticides + 1 anion (tracer of hydrodynamic) injected at the inlet(charge)

� Alternating of charge (pesticide/anion) and discharge (water) phases

� Automatic sampling in the outlet

StagnationHigh flow

(12 h)Low flow

(36 h)Stagnation

(50 d)

Low HRT High HRT

Results – Concentration at the outlet (3 ponds)

� C/C0 low HRT (high flow) > C/C0 high HRT (low flow)

� [IPU] > [BSC] during charge phase

Results – Effectiveness of the molecules

For the 2 pilots � Low HRT: mitigation of BSC > CYP > IPU and DMT

� in agreement with their Kd (batch)

� High HRT: similar mitigation of the 4 molecules

� Mitigation at high HRT > mitigation at low HRT

� HRT influence the molecule’s mitigation

Low HRT High HRT

Results – Effectiveness of the pilotsLow HRT High HRT

Flow rate (mL/min): 55 7 8 1.8Water level (cm): 3 1.5 1.5 1Volume (L): 9.4 1.7 4.7 1.2

For the 2 HRT� Mitigation ditch > 3 ponds

– Water level, flow rate, and volume ditch < 3 ponds

� Hydraulic parameters influence the pilot effectiveness

Results – The most influencing parameters

� Similar mitigation between ditch and 3 ponds (no significantly different)– flow rate and water level similar– volume ≠ (area and shape ≠)– HRT ≠

� Water level and flow rate are most influencing than HRT

Low HRT High HRT

Flow rate (mL/min): 55 7 8 1.8Water level (cm): 3 1.5 1.5 1Volume (L): 9.4 1.7 4.7 1.2

Results – The most influencing parameters

� The better the mitigation, the lower the flow rate and water level

Low HRT High HRT

Flow rate (mL/min): 55 7 8 1.8Water level (cm): 3 1.5 1.5 1Volume (L): 9.4 1.7 4.7 1.2

Results – Fate of pesticides during stagnation (3 ponds)

1st order kinetict1/2 = 2,3 dt1/2 = 1,4 d

� Fast dissipation from the aqueous phase

� Fate of pesticides ?� Sorption to sediment ? (T50)� Extraction QUECHERS� no quantified (concentration < LOQ)

� Degradation in the water? � Analysed of metabolites from IPU &

DMT

� Metabolites quantified

IPU (2 metabolites): 5% of initial pesticide amountDMT (8 metabolites): 37% of initial pesticide amount

Conclusion

� At pilot-scale the effectiveness is influenced by:• Flow rate and water level � the lower, the better the mitigation• Property of molecules � in case of low HRT (high flow rate), the better

the K d, the better the mitigation• Fast aqueous dissipation � could explained the better mitigation with

high HRT

� Field results: intense monitoring over the 3 years of the thesis– Relation between hydraulic parameters and effectiveness?– Behavior of the molecules ? Different or similar according to

hydraulic?

⇒ Influence of hydraulic will be identical on the effectiveness for the dissolved and the particulate?

Thanks for your attention