assessing the potential for using constructed wetlands as mitigation options for phosphorus and...

Assessing the Potential for using Constructed

Wetlands as Mitigation Options for Phosphorus and

Sediment within UK Agriculture

Clare Deasy* & John QuintonLancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK

*email: [email protected]

Chris StoateGame & Wildlife Conservation Trust Allerton Project, Loddington House, Main Street, Loddington,

Leicestershire, LE7 9XE, UK

Alison BaileyDepartment of Agriculture, University of Reading, Reading RG6 6AR, UK

Research funded by UK Department for

Environment, Food and Rural Affairs

contract WQ0127

UK Context

• Many available mitigation measures for diffuse pollution:

– 44 in Diffuse Pollution User Manual used in UK (Cuttle et al. 2007)

But

• Evidence for effectiveness lacking:

– Largely based on expert elicitation

� Will measures really work, if so what will the effect be?

– Evidence available often from outside UK

� Are measures suitable for UK agriculture?

• Economic & social aspects neglected

• What other impacts might these measures have?

– Pollution swapping

– Biodiversity & wildlife impact

– Flood impact

UK Context

UK Agricultural Land

Total area: 18 million ha

Crops : 5 million ha

Includes:

•Cereals (3.3)

•Oil seed rape (0.6)

•Sugar beet (0.1)

•Peas & beans (0.2)

•Potatoes (0.1)

•Vegetables (0.1)

Grass: 7 million ha

(dairy, cattle, sheep)

Mitigation Options for Phosphorus & Sediment

MOPS1 (2005-2008)

•Tested effectiveness of practical in-field measures for mitigating sediment & phosphorus loss from land under winter-sown cereals

MOPS2 (2008-2013)

•Builds on the findings of MOPS1

•Aims to fill further gaps in our understanding of agricultural diffuse pollution & diffuse pollution mitigation:

– Mitigation of runoff & pollution losses from spring-sown crops(e.g. potatoes) - ADAS

– Use of edge-of-field options (constructed wetlands) - Lancaster University

Deasy et al. (2009), JEQ 38:2121-2130

Constructed Wetland Conceptual Model

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 improved streamwater quality =

effective mitigation

Pollution Swapping Potential

Sus

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olid

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NO

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3 ov

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nd fl

ow

NH

4 ov

erla

nd

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em

issi

ons TP

Par

ticul

ate

P o

verla

nd fl

ow

Dis

solv

ed P

ove

rland

flow

P le

achi

ng

TO

C

DO

C

CO

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issi

ons

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4 em

issi

ons

H2S

em

issi

ons

Pes

ticid

es

path

ogen

s

-150

-100

-50

0

50

100

150

200in

crea

seStevens & Quinton, 2009, Critical Reviews in Environmental

Science and Technology, 39(6): 478-520

?? ??

Constructed Wetland Research Aims

• Review use of ponds & wetlands for controlling diffuse

pollution

• Establish effectiveness of constructed wetlands for control of

diffuse pollution through creating & monitoring 10 UK

experimental sites

• Consider potential for pollution swapping

• Assess cost effectiveness and socio-economic factors involved

• Make recommendations to Defra on constructed wetland use

& design within UK agricultural catchments

Constructed Wetland Research Sites

• To date, 6 constructed wetlands

have been built & instrumented, &

are operational

• Research sites are 3 farms in the

Midlands & North West on different

soil types:

– Loddington, Leicestershire:

clay soils

– Crosby Ravensworth, Cumbria:

silt soils

– Plumpton, Cumbria:

sand soils

Crosby Ravensworth,

Cumbria

Plumpton, Cumbria

Loddington, Leicestershire

3 Constructed

wetland designs:

Shallow single pond

1 x 50 cm vegetated

filter

Shallow paired ponds

2 x 50 cm vegetated

filters

Deep & shallow

paired ponds

1.5 m sediment trap +

50 cm vegetated filter

MOPS2 Constructed Wetlands

1 2

4 5

3

6

8 97 10

DitchSurface Drain

Drain StreamSurface

Surface DrainDitch TBC

LargeSmall Medium

Medium SmallLarge

Small LargeMedium TBC

Shallow Single Shallow Paired Deep & Shallow PairedDesign

3 Constructed

wetland designs:

Shallow single pond

1 x 50 cm vegetated

filter


2 x 50 cm vegetated

filters

Deep & shallow

paired ponds



3 Soil types:

Clay

Silt

Sand

May affect sediment

& nutrient

characteristics


1 2

4 5

3

6

8 97 10

DitchSurface Drain

Drain StreamSurface


LargeSmall Medium

Medium SmallLarge



3 Constructed

wetland designs:

Shallow single pond

1 x 50 cm vegetated

filter


2 x 50 cm vegetated

filters

Deep & shallow

paired ponds



3 Soil types:

Clay

Silt

Sand

May affect sediment

& nutrient

characteristics

3 constructed

wetland sizes:

Small – 0.025%

catchment area

Medium – 0.05%

catchment area

Large – 0.1%

catchment area


1 2

4 5

3

6

8 97 10

DitchSurface Drain

Drain StreamSurface


LargeSmall Medium

Medium SmallLarge



3 Constructed

wetland designs:

Shallow single pond

1 x 50 cm vegetated

filter


2 x 50 cm vegetated

filters

Deep & shallow

paired ponds



3 Soil types:

Clay

Silt

Sand

May affect sediment

& nutrient

characteristics

3 constructed

wetland sizes:

Small – 0.025%

catchment area

Medium – 0.05%

catchment area

Large – 0.1%

catchment area


1 2

4 5

3

6

8 97 10

DitchSurface Drain

Drain StreamSurface


LargeSmall Medium

Medium SmallLarge



3 Flow types:

Surface runoff

Drainflow

Ditch & Streamflow

May affect sediment

& nutrient

characteristics

Monitoring Methods

Pollution Swapping Effects

• Groundwater:

– Monitoring of groundwater levels and water quality through piezometernetwork at ‘at risk’sites (underway)

• Greenhouse gases:

– Measurement of greenhouse gas emissions from constructed wetlands (2011+)

� Groundwater risk from leaching

� Air quality risk from greenhouse gas emissions

Socio-Economic Impacts

• Spreadsheets to establishfarm-scale costs (underway)

• Farmer questionaires (2012)

• Farmer focus groups (2012)

� Cost effectiveness

� Farmer attitudes

� Likely farmer uptake

Other Impacts

• Assessment of flood peakretention

• Ecological assessments

� Flood benefits

� Wildlife & biodiversity benefits

Monitoring Methods

Mitigation Effectiveness

• Monitoring of water quality at inlets & outlets

– Continuous monitoring of water level (discharge) & turbidity (sediment)

– Event/baseflow sampling

• Bed sediment sampling

• Tracer experiments

� Wetland sediment & nutrient budgets

� Sedimentation rates

� Water & sediment residence times

� Wetland effectiveness

also

� Fertiliser value of stored sediment

Storm Event Data

• Data for storm samples

analysed for suspended

sediment and total phosphorus,

nitrogen and carbon can be

compared to turbidity values

• Good turbidity-sediment

relationships mean continuous

turbidity monitoring can be

used to infer sediment transfer

through wetland

Storm Event Data










through wetland

• Nutrient-sediment

relationships are also strong

Storm Event Data










through wetland

• Nutrient-sediment

relationships are also strong

• In some wetlands, high nutrient

levels lead to algal growth and

poor turbidity data

Typical Storm Event Response

• Measured discharge and

turbidity peak at wetland inlets

and outlets in response to

rainfall

• In this case, discharge peak is

higher for wetland outlet than

inlet due to influx of

groundwater in unlined wetland

• Turbidity peak at wetland inlet

occurs before and is greater than

outlet peak due to transfer times

and sedimentation

• Storm samples taken for

phosphorus also indicate

decrease in concentrations

through wetland

Trapped Sediment Load

• Sediment plumes present at inlet

flumes indicate coarse sediment

is trapped in both deep and

shallow ponds

• Depths of deposited sediment

will be measured in all ponds this

summer

• Sediment cores will be used to

determine particle size, total

phosphorus, total nitrogen and

total carbon

• Potential value of dredged

sediment as fertiliser will be

assessed

Sediment trapped at wetland inlet

• Brackenburgh Wetland:

– Deep & shallow paired ponds

– Fed by drain

– 0.1% catchment area (30 ha)

– Sandy soils

Inlet & Outlet Concentrations

• Loddington Wetland:

– Shallow paired ponds

– Fed by ditch (offline)

– 0.1% catchment area (10 ha)

– Clay soils

Initial Results

1 2

4 5

3

6

8 97 10

Source c. 20 kg

Sink c. 700 kg

Sink c. 30 kg

?Sink

coarse sediment

Sink coarse sediment


• Estimated total depending on size, farm and soil type: €1000-4000

• Capital costs include :

– Excavation, building of banks and drainage diversion

(2-3 days work @ €250-500 day)

– Drainage if required (variable cost)

– Fencing if required (approx €5 m)

– Seed mixes for site stability & biodiversity (€20-80)

– Instream work if required:

• UK Land Drainage Consents Application (€40)

• Crayfish survey (€60)

N.B. Figures do not include research costs such as installation of flumes

MOPS Constructed Wetland Costs

Summary

• Between 2008 and 2013, MOPS will construct and monitor 10

constructed wetlands to determine their mitigation effectiveness for UK

agriculture

• Project is trialling different constructed wetland designs, sizes, soil types

and flow types

• 6 wetlands now complete and instrumented, remaining 4 wetlands to

be built this summer

• Event data indicate turbidity a good indicator of sediment and total

nutrient transfer through wetlands

• Initial indications suggest some sites may be sediment sources to

stream in first year after construction, while at other sites deposition of

sediment is evident

• Wetland effectiveness expected to increase as wetlands mature

Acknowledgements

The Mitigation of Phosphorus & Sediment projects

MOPS1 & MOPS2 are collaborative research

projects funded by Defra, undertaken by Lancaster

University, ADAS, the Game & Wildlife

Conservation Trust & the University of Reading

Building a Constructed WetlandWith thanks to: the Allerton

Project, Loddington; Mike & Ruth

Tuer, Crake Trees, Crosby

Ravensworth; The Brackenburgh

Estate, Penrith & Smiths Gore

assessing the potential for using constructed wetlands as mitigation options for phosphorus and...

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