Nitrate transport in Chalk catchments: monitoring, modelling and policy implications
Introduction
Nitrate is one of the most problematic and widespread of potential groundwater contaminants. It is (indirectly) toxic to humans, as post-ingestion reduction to nitrite causes a form of oxygen starvation that in extreme cases leads to death (Canter, 1997). There is also evidence linking nitrate ingestion with increased risk of gastric cancer (Sandor et al., 2001). Livestock, crops and industrial processes can be seriously affected by excessive levels of nitrate in groundwater (Canter, 1997), while elevated nitrate levels in surface water systems have a detrimental impact on river ecology (Hayes and Greene, 1984). Due to these hazards, conservative legislation exists regarding allowable nitrate levels in groundwater and water supplies. Satisfying this legislation is becoming increasingly difficult due to the rising upward trend in nitrate concentrations observed in both surface waters and groundwater over the last decades. Nitrate levels in Cretaceous Chalk aquifers within southern and eastern England are of particular concern, as these aquifers provide 20% of all national water supplies, and up to 60% of the groundwater supply (Downing, 1998). In some systems, concentrations are now hugely above inferred values in pristine conditions; typical baseline nitrate concentrations in UK Chalk groundwaters are thought to be between 2 and 4 mg NO3 l−1, with an absolute maximum of 5 mg NO3 l−1 (Buss et al., 2005) but concentrations in excess of 50 mg NO3 l−1 (the maximum legal limit) have been recorded in Chalk groundwaters since the early 1970s (Foster and Crease, 1974).
It is generally accepted that these increases are in main due to intensification of agricultural practices (Foster and Crease, 1974, Limbrick, 2003, Wade et al., 2004). While such a link implies that, with appropriate farm management, a reversal of this trend is possible, there have been increasing concerns regarding the short-to-medium term prognosis of such a reversal in the Chalk. Growing consensus that the Chalk unsaturated zone highly retards a variety of chemicals (Foster, 1993, Mathias et al., 2006, Gooddy et al., 2006, Jackson et al., 2007) suggests that much of the historical agricultural loading is still en route to the groundwater within this unsaturated zone. It is likely that this retardation is currently masking the extent of the water quality problem, with negative impacts of present-day practices partially buffered by the less intensive land management earlier within the 20th century.
This paper discusses the above issues, and explains why conventional water quality models fail to represent adequately the important unsaturated zone processes and the complexity of the groundwater response. Recent work has extended an established catchment-scale nitrogen model (INCA-N, Wade et al., 2002) to provide an appropriate representation of these processes for the Chalk to evaluate nutrient management options (INCA-Chalk, Jackson et al., 2007). Land use management scenario predictions from this model suggest that the time-scales demanded by the EU Water Framework Directive are not achievable for many Chalk systems, and provide information on what deadlines and management strategies may be appropriate. However, considerable prediction uncertainties remain due to sparse data, spatial heterogeneity observed in subsurface profiles, and unresolved hypotheses of process response. An evaluation of available data in two Chalk catchments (the Pang and Lambourn, UK) is used to interrogate performance of individual components of the INCA-Chalk model, and suggest where further effort might best be directed to improve understanding and better inform policy.
Section snippets
Nitrate legislation in the UK
The main directives controlling nitrogen levels in water bodies and drinking supplies in the UK are the European Union Drinking Water Directive (98/83/EEC), Groundwater Directive (80/68/EEC), and the Nitrates Directive. In the UK, the first two apply nationwide, while the Nitrates directive applies only to designated Nitrate Vulnerable Zones (NVZs). These are integrated through the EU Water Framework Directive (WFD) which came into force in December 2000 to expand the scope of water protection
Nitrate level predictions in Chalk catchments
Catchment-scale nitrogen models can be broadly classified into metric, conceptual and physics-based models (Wheater et al., 1993). Models in all categories have utility for aspects of nitrogen management (Quinn, 2004, Lacroix et al., 2006); however discussion here is restricted to consideration of their predictive capacity. Metric models are essentially statistical relationships between existing input and output datasets with rudimentary, if any, physical basis; extrapolation of predictions to
Subsurface data on nitrates in the Chalk
Data from the Pang and Lambourn catchments were obtained and an analysis of solute profile data, land use data, fertiliser application data, groundwater level data and climate data performed. Correlations between features in solute profiles and other data sets were investigated with a view to establishing the source of solute peaks and other characteristics. Recent porewater chemistry profiles collected as part of the LOCAR research initiative described in Wheater and Peach (2004) and Wheater
Conclusions
More than half of UK groundwater supplies are abstracted from the Chalk aquifers of eastern and southern England, many of which are becoming nitrate polluted as a consequence of UK agricultural practices. While changes in land use management are arguably the most effective long-term means of controlling this, there are well-founded concerns that the unsaturated zone in lowland Chalk will prevent control of nitrogen levels being achieved within the time-scales demanded by incoming European and
Acknowledgements
The authors thank Dan Butterfield from Reading University for data preparation and coding of the INCA-Chalk model, Tabitha Sudworth and the rest of the Data Centre team at CEH Wallingford for providing LOCAR data, and Brian Adams at the British Geological Survey (BGS) for providing additional Pang and Lambourn solute profile data. Thanks also to Peter King of Yattendon Estates and Charles Ledgerwood of Westbrook Farm for supplying valuable land use and nitrogen application data. This work was
Bethanna Jackson is a Research Associate in the Department of Civil and Environmental Engineering, Imperial College London. Her research interest is mathematical modelling of surface and subsurface flow and transport problems, and her most recent work is on the impact of land management changes on flooding and water quality.
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Cited by (0)
Bethanna Jackson is a Research Associate in the Department of Civil and Environmental Engineering, Imperial College London. Her research interest is mathematical modelling of surface and subsurface flow and transport problems, and her most recent work is on the impact of land management changes on flooding and water quality.
Claire A. Browne is currently a hydrologist at Halcrow Group Ltd., Edinburgh. Her research as an Imperial College Masters student (2006) involved a study of subsurface nitrate transport in chalk and a performance evaluation of the subsurface flow and transport model, INCA-Chalk (Integrated Nitrogen in Chalk Catchments).
Adrian Butler is Reader in Subsurface Hydrology in the Department of Civil and Environmental Engineering, Imperial College London. His research interests include flow and transport in permeable catchments, including the unsaturated zone, and their impact on groundwater quality and environmental systems.
Denis Peach is Chief Scientist of the British Geological Survey, having previously led the BGS Groundwater Programme for 10 years. He is a hydrogeologist most recently focussed on developing interdisciplinary catchment research (as in the UK LOCAR programme). His specific interests are developing better conceptual understanding of the saturated and unsaturated flow regimes in the Chalk aquifer to allow better modelling and prediction.
Andrew J. Wade is Reader in Hydrology in the Aquatic Environments Research Centre at the University of Reading. His research focuses on understanding the factors and processes controlling pollutant transport and the ecological response in catchments. Specifically he is interested in the development of mathematical models to describe the likely changes in water quality and freshwater ecology to environmental change.
Howard Wheater is Professor of Hydrology in the Department of Civil and Environmental Engineering, Imperial College London and a Fellow of the Royal Academy of Engineering, UK. His research interests focus on hydrological processes and modelling, with applications to flood, water resource and waste and pollution management.