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.
Highlights
Nitrate (NO3) is essential for living matter by acting as a source of nitrogen (N) that forms the building blocks of molecules
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
Scatter plots for parameter values against the biases or Nash–Sutcliffe efficiency (NSE) scores from Monte Carlo (MC) simulations were produced
Summary
Nitrate (NO3) is essential for living matter by acting as a source of nitrogen (N) that forms the building blocks of molecules. Agricultural land is the major source of nitrate water pollution (Ferrier et al, 2004; Thorburn et al, 2003; Torrecilla et al, 2005). The EU Nitrate Directive, an integrated part of the EU Water Framework Directive (Directive 2000/60/EC), is implemented through a statutory instrument that sets rules (Action Programme rules) for best agricultural practices within Nitrate Vulnerable Zones (NVZs). These are designated areas where land drains and contributes to the excess nitrate found in contaminated groundwater and surface waters. Evidence of some improvements in groundwater nitrate (e.g. Smith et al, 2010) is beginning to emerge
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