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