| Abstract |
This report forms the first deliverable of a project jointly funded by BGS and the Environment Agency to consider the potential for incorporating the outputs from the BGS unsaturated zone travel time work in assessing the risks to water from nitrate. This is to help to inform the nitrate vulnerable zones (NVZs) designation process. In England, the Environment Agency advises Defra on identifying areas for designation as NVZs. Over time, the designation process has developed and become more complex since the first round of designations in 1996. The designation process for groundwater initially used only public supply monitoring data and the associated source catchment area. In December 2000, the European Court of Justice held that the UK had failed to designate sufficient NVZs for the protection of all waters, not just for drinking water sources. This resulted in the development of revised methodologies for the designation of NVZs which separately address surface waters, groundwater and waters at risk of eutrophication. This was implemented in 2002. Further reviews have been carried out in 2008 and 2012 and as a result, modifications and improvements to methods have been made at each designation round. For groundwater the Environment Agency developed a numerical risk assessment procedure that uses a range of risk factors including both nitrate concentration data and nitrate-loading data to assess the risk of nitrate pollution. The loading data is based on farm census returns made to Defra and combined using the NEAP-N methodology developed by ADAS (Lord and Anthony, 2000). The overall risk assessment considers both current observed concentrations and predicted future concentrations as well as current loadings. However, this approach has a number of disadvantages including a lack of a specific term for the time of travel to the water table and emergence of pollutant both into groundwater and to groundwater discharge points that support surface water features. Instead, these issues are considered at the conceptual level in workshops with local EA hydrogeologists. A key question for Defra and the Agency is how long it will take for nitrate concentrations to peak and then stabilise at an acceptable, lower level, in response to existing and future land management control measures. This is most important for soils, for aquifers, for lakes and for groundwater-fed wetland systems that respond less quickly to changes in loading. Groundwater and lake catchment numerical models can provide first-order estimates of likely response times, but can be difficult and costly to set-up for many different situations and are difficult to apply consistently at the national scale. A previous review of nitrate vulnerable zones suggests a range of further needs: to understand the recent developments in nitrate pollution simulation and particularly the potential to understand/characterise past nitrate loading from changing land management practices and correlate these with observed nitrate concentrations over time; to evaluate the retention of nitrate in catchments, particularly in the unsaturated zone of soils and aquifers; to simulate the recent and future anticipated decreases in nitrate loading by sectors within the UK; to understand the likely time taken for nitrate concentrations to peak and then stabilise at an acceptable, lower level, in response to existing and future control measures. Without evidence of how long it may take systems to recover it is difficult to evaluate the effectiveness of existing measures or decide whether additional measures are necessary. The aim of this project is to investigate the use of new models to inform decision-making on nitrate pollution in groundwater and the potential for incorporating unsaturated zone processes in future NVZ designations. The work described here forms the first task of this project and aims to review NVZ methodology and recent designation experience. |
