Evaluation of constructed wetlands for mitigation ofdiffuse pollution from agriculture: uncertainties

Mary Ockenden, Clare Deasy, John Quinton and Ben SurridgeLancaster Environment Centre, Lancaster University, UK

Chris Stoate and John SzczurThe Allerton Project, Game & Wildlife Conservation Trust, Loddington, Leicestershire, UK

Nerilde FavarettoUFPR, Curitiba, Parana, Brazil

• Field Wetlands – conceptual model

• Experimental design – choice of studysites

• Some successes

• Some challenges

• Summary

Outline

Field Wetlands as Mitigation Options

An unlined constructed wetland for diffuse pollution mitigation

Trapped sediment & P, N, C

Groundwater interactions

Atmospheric interactions

Mitigated runoff outPolluted runoff in

CO2, NO2, CH4

NO3

Sediment, P, N, C Sediment, P, N, C

Potential for reduced groundwater quality =pollution swapping

Potential for reduced air quality= pollution swapping

Potential for improvedstreamwater quality =

effective mitigation

Economicviability?

Floodretention &

wildlifehabitat

Evidence available fromoutside UK

V. limited data

V. limited data

Field Wetland Conceptual Model:

10 MOPS2 Field Wetlands

1 2

4 5

3

6

9 108

7

DitchSurface Drain

Drain Stream Surface

Surface DrainDitch

LargeSmall Medium

Medium Large Large

Small LargeMedium

Shallow Single Shallow Paired Deep & Shallow Paired

Surface

Small

Sediment trapped

Whinton Hill, Cumbria2009/10: 26 tonnes2010/11: 22 tonnes

Crake Trees, Cumbria2009/10: 4 tonnes2010/11: Sediment volumeusing Total Station

Loddington,Leicestershire2009/10: 1 tonne2010/11: Sedimentdepth from sedimenttraps

Soluble nutrient reduction

A

C

B

D

E

ABCDE

ABCDE

Phosphorus

Ammonium

Whinton Hill,Cumbria

Flow direction

Flow direction

Crake Trees,Cumbria

Surface runoff1 – 6 April 2011

Suspendedsediment and TPconcentrationsare higher atinlet than outlet

Suspended sediment and total phosphorus reduction

0

1

2Ra

inm

m

0

1000

2000

Tu

rbid

ity(N

TU

)

inlet

middle

outlet

0

1000

2000

Su

sp

en

de

d

so

lids

mg

l-1 inlet

middle

outlet

31 Mar 02 Apr 04 Apr 06 Apr0

2

4

6

TP

mg

l-1

inlet

middle

outlet

Some challenges

Depth – discharge relationship ismore uncertain when:

•Depth too high (flooded) ortoo low (below level of detection)

Need: more frequent velocity/flow measurements

Turbidity – suspended sedimentrelationship breaks down when:

•Turbidity is high due to algae orsensors get blocked at heavily pollutedsites

Need: different sampling regime for‘dirty’ sites

Pollution swapping?

Whinton Hill, Cumbria:

PO4-P (mg l-1)

Phosphate concentration 3 mbelow ground surface, July 2010

Multilevel piezometers installed in arrayaround and through wetland

Results suggest leakage of PO4 (and NH4)to groundwater

Summary

This data has provided evidence for the success of field wetlands formitigation of diffuse pollution from agriculture•Sediment trapped•Nutrient concentrations reduced between inlet/outlet

The sites in this project are typical of real life agricultural situations. Thishighlights the difficulty of maintaining continuous field data – and a need toadapt to each different situation. Particularly challenging areas have been:•Flow data for load estimations•Turbidity – suspended sediment relationship

If fluxes of nutrients via pollution swapping pathways are significant, needto consider the priorities for catchment management – different for eachcatchment•Groundwater monitoring•Atmospheric interaction

Acknowledgements

The Mitigation of Phosphorus & Sediment projectsMOPS1 & MOPS2 are collaborative research projectsfunded by Defra, undertaken by Lancaster University,ADAS, the Game & Wildlife Conservation Trust & theUniversity of Reading

Building a Constructed WetlandWith thanks to:

Hall Farm, Loddington, Leicestershire;

Crake Trees Manor Farm, Crosby Ravensworth, Cumbria;

Brackenburgh Home Farms, Calthwaite, Cumbria; and

Seborwens Farm, University of Cumbria, Newton Rigg, Cumbria

3 Wetland sizes – generally smaller than used elsewhere but appropriate for UK agriculture:•Large – 0.1% catchment area (100 m2 for 10 ha)•Medium – 0.05% catchment area (50 m2 for 10 ha)•Small – 0.025% catchment area (25 m2 for 10 ha)

3 Wetland types – simplified designs for easy and cheap construction:•A: Shallow single pond - 1 x 50 cm deep vegetated filter•B: Shallow paired ponds - 2 x 50 cm deep vegetated filters•C: Deep & shallow paired ponds - 1.5 m deep sediment trap + 50 cm deep vegetated filter

3 Soils – may affect sediment & nutrient characteristics:•Clay•Silt•Sand

3 Flow sources – may affect sediment & nutrient characteristics:•Surface runoff•Drainflow•Ditch & Streamflow

UK ‘Field Wetland’ Designs