Application of modelling in the assessment of control measures to reduce diffuse pollution Dr Kevin Hiscock School of Environmental Sciences University of East Anglia Norwich, UK ( Acknowledgement: Han Zhang Funding: 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 pollution Groundwater quality (nitrate) Nitrate concentration in drinking water < 50mg/l (Water Framework Directive 2000/60/EC)

Increasing woodland as a land-use change strategy for controlling diffuse pollution  Current UK government policy ( The England Forestry Strategy, Regional Forestry Frameworks, Community Forests Partnerships, Farm Woodland Premium Scheme ) aims to increase woodland cover (currently only 8.5% in England)  NVZ - Nitrate Vulnerable Zones  Pilot case studies  Water4all Project Lincolnshire Limestone Lincolnshire Limestone  Afforestation Denmark, Germany Denmark, Germany Give N reduction potential Give N reduction potential

Increase woodland, less recharge, water shortage? Conversion from arable land to woodland Conversion from arable land to woodland may reduce groundwater recharge and lead to fall of the groundwater table as trees tend to consume more water than other vegetation ( Bosch & Hewlett 1982; Blackie 1993, Sahin & Hall 1996; Calder 2003 ) may reduce groundwater recharge and lead to fall of the groundwater table as trees tend to consume more water than other vegetation ( Bosch & Hewlett 1982; Blackie 1993, Sahin & Hall 1996; Calder 2003 ) Recharge volume: pine forest < oak woodland < grassland Trees and Drought Project of Lowland England (TaDPoLE) Recharge volume: pine forest < oak woodland < grassland Trees and Drought Project of Lowland England (TaDPoLE) may aggravate the problem of water shortages due to reduced recharge under forest cover ( Farley 2005; Bending 1997; Calder 2000; Finch 2001 ) may aggravate the problem of water shortages due to reduced recharge under forest cover ( Farley 2005; Bending 1997; Calder 2000; Finch 2001 ) the recharge of groundwater beneath broadleaved woodland and grass sites in a Chalk aquifer study was almost the same ( Roberts, 2006 ) the recharge of groundwater beneath broadleaved woodland and grass sites in a Chalk aquifer study was almost the same ( Roberts, 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 Nottinghamshire West Nottinghamshire, East Midlands Area: 50 km N-S ×30 km W-E Area: 50 km N-S × 30 km W-E Sherwood Sandstone aquifer Sherwood Sandstone aquifer Sandstone dips eastwards at about 1 in 50 Sandstone dips eastwards at about 1 in 50 In the east, low-permeability In the east, low-permeability mudstone overlies the sandstone mudstone overlies the sandstone Dover Greet Ryton Idle Poulter Meden Maun Severn Trent Sandstone Mudstone

Land-use map (LCM2000) Arable agriculture covers approximately 75% of outcrop area and receives large quantities of nitrogen (e.g. manure and fertiliser applications to crops)

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

Diffuse N losses and observed NO 3 - 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

Modelled GW level vs. Observed GW level in Well 1729 DUCHESS PLANTATION Well 1740 KIGHILL Well 1793 CLIPSTONE FOREST Well 1722 CROSSLEY HILL

Observed vs. simulated river flow

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)

Modelled nitrate distribution and N losses in mg/l N kg/yr/cell

Modelled and observed groundwater nitrate concentrations BOUGHTON 2 BURTON JOYCE (confined) PAPPLEWICK AMEN CORNER 2

Modelled and observed nitrate concentrations (Forest areas) BUDBY FOREST 1 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 1Site 2Site 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 ? kg N/yr N Losses 9250 kg N/yr Soil Leachate NO 3 - GW NO mg/l89.14 mg/l mg/l50 mg/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) BaselineIncrease WoodlandIncrease Grassland Arable50.66%29.90%32.25% Grass15.20% 33.60% Broadleaf8.75%19.13%8.75% Conifer8.30%18.67%8.30% Other17.10% Total wood17.05%37.80% Change+20.75%+18.40%

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

Cover Crops To be applied on spring cultivated cereals, set-aside, horticultural crops Not 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

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