Application of modelling in the assessment of control measures to reduce diffuse pollution

Dr Kevin Hiscock

School of Environmental SciencesUniversity of East Anglia

Norwich, UK

(E-mail k.hiscock@uea.ac.uk)

Acknowledgement: Han ZhangFunding: Chinese Scholarship Council & University of East Anglia

OUTLINE of PRESENTATION

• To demonstrate the objective use of groundwater flow and nitrate transport models in the assessment of diffuse pollution control measures

• To estimate the amount of land-use change needed to meet groundwater standards

• To assess the impact of land-use change on groundwater quantity (groundwater recharge and groundwater level)

• To compare arable land conversion to woodland with other agricultural management practices such as cover crops and fertiliser reduction

Background to nitrate pollutionBackground to nitrate pollution

Groundwater qualityGroundwater quality (nitrate)(nitrate)

Nitrate concentration in drinking water < 50mg/l

(Water Framework Directive 2000/60/EC)

Increasing woodland as a land-use change strategy Increasing woodland as a land-use change strategy for controlling diffuse pollutionfor controlling diffuse pollution

Current UK government policy (Current UK government policy (The England Forestry The England Forestry Strategy, Regional Forestry Frameworks, Community Forests Strategy, Regional Forestry Frameworks, Community Forests Partnerships, Farm Woodland Premium SchemePartnerships, Farm Woodland Premium Scheme) aims to ) aims to increase woodland cover (currently only 8.5% in increase woodland cover (currently only 8.5% in England)England)

NVZ - Nitrate Vulnerable Zones NVZ - Nitrate Vulnerable Zones

Pilot case studiesPilot case studies

Water4allWater4all Project Project Lincolnshire LimestoneLincolnshire Limestone Afforestation Afforestation Denmark, GermanyDenmark, Germany Give N reduction potential Give N reduction potential

Increase woodland, less recharge, water shortage? Increase woodland, less recharge, water shortage? Conversion from arable land to woodlandConversion from arable land to woodland

may reduce groundwater recharge and lead to fall of the may reduce groundwater recharge and lead to fall of the groundwater table as trees tend to consume more water than groundwater table as trees tend to consume more water than other vegetationother vegetation ( (Bosch & Hewlett 1982; Blackie 1993, Sahin & Bosch & Hewlett 1982; Blackie 1993, Sahin & Hall 1996; Calder 2003Hall 1996; Calder 2003))

Recharge volume: pine forest < oak woodland < Recharge volume: pine forest < oak woodland <

grasslandgrassland Trees and Drought Project of Lowland England Trees and Drought Project of Lowland England (TaDPoLE)(TaDPoLE)

may aggravate the problem of water shortages due to reduced may aggravate the problem of water shortages due to reduced recharge under forest cover recharge under forest cover ((Farley 2005; Farley 2005; Bending 1997; Calder Bending 1997; Calder 2000; Finch 20012000; Finch 2001))

the recharge of groundwater beneath broadleaved the recharge of groundwater beneath broadleaved woodland and grass sites in a Chalk aquifer study was woodland and grass sites in a Chalk aquifer study was almost the samealmost the same ((Roberts, 2006Roberts, 2006))

Tools and Methods

ArcGIS – ArcMap (Land-use type and area)

Recharge model – vegetation water requirement Visual MODFLOW v4.1 – groundwater flow model

Export coefficient model – nitrogen load and leaching

MT3DMS – nitrate transport model

MODPATH – define abstraction borehole capture zones

N Losses

Leachate concentration

Mass transport modelling

MT3DM

Scenario

predictions

GW Recharge Recharge Model

LCM

N Input

Export coefficient

MODFLOW

GW Flow modelling

Tree species conversion

Land-use change

Agricultural management

PATHLINE

Approach and modelling strategy

Location map of the study area

• West NottinghamshireWest Nottinghamshire, East Midlands

• Area: 50 km N-S Area: 50 km N-S ×× 30 km W-E30 km W-E • Sherwood Sandstone aquiferSherwood Sandstone aquifer

• Sandstone dips eastwards at Sandstone dips eastwards at about 1 in 50 about 1 in 50

• In the east, low-permeabilityIn the east, low-permeability mudstone overlies the sandstone mudstone overlies the sandstone

Dover

Greet

Ryton

IdlePoulter

Meden

Maun

Severn Trent Sandstone

Mudstone

Land-use map (LCM2000)

Arable agriculture covers approximately 75% of outcrop area and receives largequantities of nitrogen (e.g. manure and fertiliser applications to crops)

Nitrate concentration in abstraction boreholes (Unconfined aquifer, data source Severn-Trent Water, year 2007)

0

50

100

0 5 10 15 20 25 30 35 40

Borehole number

Nit

rate

co

nce

ntr

atio

n m

m/l

(mg

/l)

GW nitrate mg/l

3 - 10

11 - 35

36 - 50

51 - 70

71 - 230

SW nitrate mg/l

12 - 10

11 - 35

36 - 50

51 - 70

N loss kg N/yr1634 - 6000

6001 - 12000

12001 - 15000

15001 - 28000

Diffuse N losses and observed NO3- concentrations

Groundwater Flow Modelling

Criteria for model calibration groundwater levels in 27 observation boreholes

(Environmental Agency)

River flows at 10 river gauging stations (National River Flow Archive)

Particle-tracking MODPATH: using calibrated GW model to

generate capture zone of abstraction borehole

85

89

93

97

101

105

109

113

Jan-8

6

Jan-8

8

Dec-8

9

Dec-9

1

Dec-9

3

Dec-9

5

Dec-9

7

Dec-9

9

Dec-0

1

Dec-0

3

Dec-0

5

Gro

un

dw

ate

r H

ea

d (

m)

Observation Head

Calculated Head

20

24

28

32

36

40

Jan-

86

Jan-

88

Dec

-89

Dec

-91

Dec

-93

Dec

-95

Dec

-97

Dec

-99

Dec

-01

Dec

-03

Dec

-05

Gro

un

dw

ate

r H

ea

d (

m) Observation Head

Calculated Head

30

34

38

42

46

50

54

Jan-8

6

Jan-8

8

Dec-8

9

Dec-9

1

Dec-9

3

Dec-9

5

Dec-9

7

Dec-9

9

Dec-0

1

Dec-0

3

Dec-0

5

Gro

un

dw

ate

r H

ea

d (

m)

Observation Head

Calculated Head

34

38

42

46

50

54

Jan-

86

Jan-

88

Dec

-89

Dec

-91

Dec

-93

Dec

-95

Dec

-97

Dec

-99

Dec

-01

Dec

-03

Dec

-05

Gro

undw

ater

Hea

d (m

)

Observation Head

Calculated Head

Modelled GW level vs. Observed GW level in 1986-Modelled GW level vs. Observed GW level in 1986-20062006

Well 1729DUCHESS PLANTATION

Well 1740 KIGHILL

Well 1793CLIPSTONE FOREST

Well 1722CROSSLEY HILL

Observed vs. simulated river flow

Station 280116 (Ryton)

0

100000

200000

300000

400000

500000

600000

Jan

-86

Jan

-87

Jan

-88

Jan

-89

Jan

-90

Jan

-91

Jan

-92

Jan

-93

Jan

-94

Jan

-95

Jan

-96

Jan

-97

Jan

-98

Jan

-99

Jan

-00

Jan

-01

Jan

-02

Jan

-03

Jan

-04

Jan

-05

Jan

-06

Dis

cha

rge

(m

3 /d)

measured

modelled

Station 28118 (Meden)

0

40000

80000

120000

160000

200000

240000

Jan-9

4

Jan-9

5

Jan-9

6

Jan-9

7

Jan-9

8

Jan-9

9

Jan-0

0

Jan-0

1

Jan-0

2

Jan-0

3

Jan-0

4

Jan-0

5

Jan-0

6

Dis

cha

rge

(m

3/d

)

modelled

measured

GW pathline calculation to determine capture zones Budby

Budby Forest

Nitrate Transport Model

Nitrate concentration recharge Nitrogen losses- Export coefficient modelling GW recharge Nitrate concentration

Criteria for nitrate transport model nitrate concentrations at 24 groundwater

monitoring points (EA) / groundwater abstraction boreholes (STW)

Nitrate concentration = Nitrogen losses / GW recharge

Example: Estimation of N loss for a cell (2 km x 2 km)

Source (462000, 384000)

Area (ha)

Input N (kg/ha)

Export Coef. loading(N/ha)

N loss (kg/yr)

Source Area (ha)/ head

Input N (kg/ha or head)

Export Coef. loading(N/ha)

N loss (kg/yr)

Wheat 55.60 177.1 0.23 2264.8 Perm- grass 39.5 115.5 0.25 1141.7

Winter Barley 22.51 143.3 0.2 645.1 Grazing grass 0.2 123.7 0.05 0.9

Spring Barley 25.53 110.5 0.4 1128.4 Woodland 10.3 10 102.9

Oats 1.29 112.6 0.3 43.6 Set-aside 40.0 30.00 1199.8

Other cereals 11.11 118.7 0.3 395.6 All other land 13.0 5.00 65.0

Potatoes 13.65 168.9 0.39 899.1 Labour 7.5 2.14 15.9

Sugar beet 29.72 105.4 0.17 532.5 Cattle 56.2 70.2 0.16 635.4

Field beans 0.00 32.8 0.48 0.0 Sheep 248.5 10.1 0.17 426.6

Peas 5.57 49.1 0.48 131.3 Pigs 0.0 18.8 0.10 0.0

Oilseed Rape 14.72 175.0 0.42 1081.9 Poultry 14.0 0.6 0.07 0.5

Other arable 14.81 101.5 0.3 451.0 Rough grass 25.2 5.00 126.0

Horticulture 21.22 76.8 0.35 570.4 Broadleaves 35.5 6.00 213.0

Bare fallow 0.15 5 0.8 Conifer 15.5 12.00 186.0

Tem-grass 10.33 176.2 0.25 455.0 Non-agri 5.1 5.00 25.7

Total cell N loss kg N/yr

12738.9

Modelled nitrate distribution and N losses in 2006

0

20

30

40

50

60 mg/l

nitrogen loading 1634 - 5474

5475 - 10183

10184 - 14128

14129 - 20000

20001 - 27482

N kg/yr/cell

0

1

2

3

4

5

6

7

8

9

10

Jan-

86

Jan-

88

Dec

-89

Dec

-91

Dec

-93

Dec

-95

Dec

-97

Dec

-99

Dec

-01

Dec

-03

Dec

-05

Nitr

ate

conc

ernt

ratio

n [m

g/l]

Observed value

Modelled value

Modelled and observed groundwater nitrate concentrations

0

10

20

30

40

50

60

70

80

90

100

Jan-

86

Jan-

88

Dec

-89

Dec

-91

Dec

-93

Dec

-95

Dec

-97

Dec

-99

Dec

-01

Dec

-03

Dec

-05

Nitr

ate

conc

ernt

ratio

n [m

g/l]

Observed value

Modelled value

0

10

20

30

40

50

60

70

80

90

100

Jan-

86

Jan-

88

Dec

-89

Dec

-91

Dec

-93

Dec

-95

Dec

-97

Dec

-99

Dec

-01

Dec

-03

Dec

-05

Nitr

ate

conc

ernt

ratio

n [m

g/l]

Observed value

Modelled value

0

10

20

30

40

50

60

70

80

90

100

Jan-

86

Jan-

88

Dec

-89

Dec

-91

Dec

-93

Dec

-95

Dec

-97

Dec

-99

Dec

-01

Dec

-03

Dec

-05

Nitr

ate

conc

ernt

ratio

n [m

g/l]

Observed value

Modelled value

BOUGHTON 2

BURTON JOYCE (confined) PAPPLEWICK

AMEN CORNER 2

Modelled and observed nitrate concentrations (Forest areas)

BUDBY FOREST 1

0

5

10

15

20

25

30

35

40

45

50

Jan-

86

Jan-

88

Dec

-89

Dec

-91

Dec

-93

Dec

-95

Dec

-97

Dec

-99

Dec

-01

Dec

-03

Dec

-05

Nitr

ate

conc

ernt

ratio

n [m

g/l]

Observed value

Modelled value

CLIPSTONE FOREST 3

Model application to scenario prediction

Tree species conversion Conifer Broadleaf Broadleaf Conifer

Land-use Change Arable lands Woodland Arable lands Unfertilised grassland

Land management practices Cover crops in winter Fertiliser reduction

Prediction 1: Effect of tree species conversion on GW

Site 1 Site 2 Site 3

Broadleaf replaces conifer

14.4%5.8 cm

26.3%35.4 cm

8.5%5.3 cm

Conifer replaces broadleaf

-11.7%- 6.5cm

-3.7 %- 2.9cm

-8.3 %- 4.9cm

Changes of GW recharge (%) and water level (cm)

1

2

3

Prediction 2: Effect of land-use change on GW

Arable

27.7%

Grass

15.2%

Conifer

20.2%

Other

17.1%

Broadleaf

19.8%

Arable

?

Grass

15.2%

Conifer

?

Other

17.1%

Broadleaf

?

11935 kg N/yr N Losses 9250 kg N/yr

Soil Leachate NO3-

GW NO3-

99.75 mg/l 89.14 mg/l

62.21 mg/l 50 mg/l

y = 0.521x + 3.4051

R2 = 0.8665

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0

Soil NO3 (mg/l)

Gro

undw

ater

NO

3 (m

g/l)

Estimation of proportion of woodland (Far Baulker)

Estimation of area of land-use change required to meet 50 mg/l groundwater nitrate (Far Baulker)

Baseline Increase Woodland Increase Grassland

Arable 50.66% 29.90% 32.25%

Grass 15.20% 15.20% 33.60%

Broadleaf 8.75% 19.13% 8.75%

Conifer 8.30% 18.67% 8.30%

Other 17.10% 17.10% 17.10%

Total wood 17.05% 37.80%

Change +20.75% +18.40%

Effect of land-use change on GW nitrate concentration(Far Baulker)

0

50

100

Jan/86

Jan/88

Dec/89

Dec/91

Dec/93

Dec/95

Dec/97

Dec/99

Dec/01

Dec/03

Dec/05

Dec/07

Dec/09

Dec/11

Dec/13

Dec/15

Dec/17

Dec/19

Dec/21

Dec/23

Dec/25

Time

Nit

rate

co

nc

en

tra

tio

n m

g/l

Observation

baseline

40% w oodland

30% grassland

0

50

100

Jan-86

Jan-88

Dec-89

Dec-91

Dec-93

Dec-95

Dec-97

Dec-99

Dec-01

Dec-03

Dec-05

Dec-07

Dec-09

Dec-11

Dec-13

Dec-15

Dec-17

Dec-19

Dec-21

Dec-23

Dec-25

Time

Nitr

ate

co

nce

ntr

atio

n m

g/l

Observation

average monthly baseline

100% cover crop

Cover CropsTo be applied on spring cultivated cereals, set-aside, horticultural cropsNot to be applied on winter cereals, sugar beet and potato crops

Prediction 3: Effect of Agricultural Practices on GW

Effect of fertiliser reduction on GW nitrate concentration

0

50

100

Jan-86

Jan-88

Dec-89

Dec-91

Dec-93

Dec-95

Dec-97

Dec-99

Dec-01

Dec-03

Dec-05

Dec-07

Dec-09

Dec-11

Dec-13

Dec-15

Dec-17

Dec-19

Dec-21

Dec-23

Dec-25

Time

Nit

rate

co

nc

en

tra

tio

n m

g/l

Observation

baseline

32% fertiliser reduction

40% fertiliser reduction

64% fertiliser reduction

• Groundwater flow and nitrate transport models can represent

historical data appropriately

• Borehole capture zones generated by a calibrated groundwater

model can be used to propose land-used change scenarios

• Calibrated models can be used to assess the effect of land-use

change on both groundwater quantity and quality

• Tree species conversion in specified model cells did not greatly

affect groundwater level, although recharge reduction occurs as a

result of conifer replacing broadleaf forest

• Land-use change has more effectiveness in controlling nitrate

pollution than agricultural management practices used in isolation

Conclusions