Lei Wang, Marianne Stuart, Sean Burke, Rob Ward

　　British Geological Survey, Keyworth, Nottingham, UK, NG12 5GG

　　Email: lei.wang@bgs.ac.uk

　　Food and freshwater are two life-supporting ecosystem services that ensure our survival, well-being and economic development. Maintaining both food and water security is an international challenge. China has doubled its grain harvests by consuming 36% of the global synthetic fertilisers - tripling since 1977. Many of China’s 750 million farmers believe that higher fertiliser application leads to higher yield. While man has benefited from the application of chemical nitrogen fertilisers to gain increased agricultural productivity, any excess nitrate applied can be leached from the soil into freshwater. Nitrate-contaminated water can cause long-term environmental damage and threaten both economic and ecosystem health. Although legislation has been introduced to reduce nitrate water pollution, this remains an international problem. Agricultural land is the major source of nitrate water pollution. For example, in England, over 70% of nitrate in groundwater and surface water has been shown to be derived from agricultural land. The degradation of freshwater quality due to nitrate remains a problem in the UK. In many areas, nitrate concentrations are more than 50 mg NO₃ L^-1 with a rising trend in both rivers and aquifers.

　　Through recent research, it has become increasingly clear that it could take decades for leached nitrate from the soil to discharge into freshwaters. However, current environmental management strategies in many countries, including the UK, rarely consider the nitrate time lag in the groundwater system.

　　We developed a nitrate time bomb (NTB) model to modelling nitrate processes in the groundwater system. Whilst NTB contains simplified conceptual models, it can represent the major nitrate and hydrogeological processes in the groundwater system at both national and catchment scales, such as spatio-temporal nitrate loading, low-permeability superficial deposits, dual-porosity unsaturated zones and nitrate dilution in aquifers. The NTB model has been successfully used to simulate annual nitrate concentrations from 1925 to 2150 in the major aquifers in the UK and four local aquifer zones in the Eden Catchment, England. Monte Carlo simulations were undertaken to analyse parameter sensitivity and calibrate the model using observed datasets. These results can help decision makers to understand how the historical nitrate loading from agricultural land affects the evolution of groundwater quality due to the nitrate legacy in the groundwater system. This NTB approach is particularly valuable to evaluate the long-term impact and timescale of land management scenarios and programmes of measures introduced to help deliver water quality compliance. The Environment Agency began to use the NTB model at both the policy and local level in managing nitrate water pollution in England and Wales for better implementation of the EU Water Framework Directive and EU Nitrates Directive. This model requires relatively modest parameterisation and is readily transferable to other areas.