title: The changing trend in nitrate concentrations in major aquifers due to historical nitrate loading from agricultural land across England and Wales from 1925 to 2150.
abstract: Nitrate is necessary for agricultural productivity, but can cause considerable problems if released into aquatic systems. Agricultural land is the major source of nitrates in UK groundwater. Due to the long time-lag in the groundwater system, it could take decades for leached nitrate from the soil to discharge into freshwaters. However, this nitrate time-lag has rarely been considered in environmental water management. Against this background, this paper presents an approach to modelling groundwater nitrate at the national scale, to simulate the impacts of historical nitrate loading from agricultural land on the evolution of groundwater nitrate concentrations. An additional process-based component was constructed for the saturated zone of significant aquifers in England and Wales. This uses a simple flow model which requires modelled recharge values, together with published aquifer properties and thickness data. A spatially distributed and temporally variable nitrate input function was also introduced. The sensitivity of parameters was analysed using Monte Carlo simulations. The model was calibrated using national nitrate monitoring data. Time series of annual average nitrate concentrations along with annual spatially distributed nitrate concentration maps from 1925 to 2150 were generated for 28 selected aquifer zones. The results show that 16 aquifer zones have an increasing trend in nitrate concentration, while average nitrate concentrations in the remaining 12 are declining. The results are also indicative of the trend in the flux of groundwater nitrate entering rivers through baseflow. The model thus enables the magnitude and timescale of groundwater nitrate response to be factored into source apportionment tools and to be taken into account alongside current planning of land-management options for reducing nitrate losses.
authors.affiliation: British Geological Survey, Keyworth, Nottingham NG12 5GG, UK. Electronic address: lei.wang@bgs.ac.uk.
authors.forename: L
authors.initials: L
authors.lastname: Wang
authors.affiliation: British Geological Survey, Maclean Building, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: M E
authors.initials: ME
authors.lastname: Stuart
authors.affiliation: British Geological Survey, Maclean Building, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: M A
authors.initials: MA
authors.lastname: Lewis
authors.affiliation: British Geological Survey, Keyworth, Nottingham NG12 5GG, UK.
authors.forename: R S
authors.initials: RS
authors.lastname: Ward
authors.affiliation: ADAS UK Ltd., Pendeford House, Pendeford Business Park, Wobaston Road, Wolverhampton WV9 5AP, UK.
authors.forename: D
authors.initials: D
authors.lastname: Skirvin
authors.affiliation: Centre for Ecology and Hydrology, Maclean Building, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: P S
authors.initials: PS
authors.lastname: Naden
authors.affiliation: Sustainable Soils and Grassland Systems Department, Rothamsted Research, North Wyke, Okehampton EX20 2SB, UK.
authors.forename: A L
authors.initials: AL
authors.lastname: Collins
authors.affiliation: British Geological Survey, Maclean Building, Wallingford, Oxfordshire OX10 8BB, UK.
authors.forename: M J
authors.initials: MJ
authors.lastname: Ascott
keywords: Groundwater quality; Nitrate; Saturated zone; Time-lag; Unsaturated zone