OR/16/036 Introduction

Stuart, M E, Wang, L, Ascott, M, Ward, R S, Lewis, M A, and Hart, A J. 2016. Modelling the groundwater nitrate legacy. British Geological Survey Internal Report, OR/16/036.

Background

As described by Stuart et al. (2016) in the project Phase 1 briefing report, the increase of nitrate in groundwater was first identified as an issue for the Chalk of the Eastbourne area in the 1970s. Awareness of the extent of high and rising nitrate in groundwater nationally and across the European Union gradually increased, and it became clear that concentrations in public supply sources often exceeded the World Health Organisation (WHO) drinking water values used at this time. By the late 1970s the importance of storage of nitrate in unsaturated zone porewater had also become recognised. Pioneering work showed that at sites with good cropping records a relationship between historical land use and porewater nitrate concentration could be determined and that retention in the unsaturated zone can retard the migration of nitrate for years or decades.

In response to the growing European-wide problem the European Commission implemented the Nitrates Directive (91/676/EEC). This sets out a series of requirements on Member States to assess and control the potential for pollution of waters with nitrogenous compounds generated from agricultural sources. One of these requirements is that Member States carry out an assessment of all waters every four years. In England the Environment Agency advises Defra on this matter and proposes areas subject to potential pollution from nitrate for designation as nitrate vulnerable zones (NVZs) in compliance with the Directive.

More widely groundwater is protected by the European Water Framework Directive (2000/60/EC). As part of this groundwater bodies have to achieve a series of environmental objectives which include preventing or limiting (in the case of nitrate) inputs of pollutants, achieving good status and reversing upward trends in pollutant concentrations. Good status has to be achieved by the end of 2015 although an extension (up to 2027) is allowed, provided that an acceptable justification can be provided. This includes delays in achieving the objective due to natural conditions. Whereas the Nitrates Directive focuses on delivering measures to address agricultural sources of nitrate, the WFD requires measures for all sources of nitrate pollution. The WFD also has different dates for achieving objectives, standards/thresholds and reporting cycles (6 years compared to 4 years for the nitrates directive). However the measures implemented under the Nitrates Directive contribute significantly to achieving WFD objectives.

For delineation of groundwater NVZs 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 assesses both current observed and predicted future concentrations as well as current N loadings. However, this approach has a number of disadvantages, including the lack of consideration of the time of travel to the water table and the potential emergence of pollutant both into groundwater and to groundwater discharge points that support surface water features. A key question for Defra and the Environment 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, aquifers, lakes and groundwater-fed wetland, systems that respond less quickly to changes in loading. Groundwater and lake catchment 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.
• Evaluate the retention of nitrate in catchments, particularly in unsaturated zone of soils and aquifers.
• Examine the recent and future anticipated decreases in nitrate loading by sectors within the UK.
• 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.

Project objectives

The aim of the project is to investigate the potential use of new numerical models to inform decision-making on nitrate pollution in groundwater and the potential for giving consideration to incorporating such models of unsaturated zone processes in the NVZ process. The background to the nitrate legacy in groundwater and to the approaches to NVZ designation was described in Stuart et al. (2016).

The work described here formed the main part of the project and aimed to evaluate the potential role for the application of modelling the unsaturated zone in the NVZ process, in particular using the BGS Nitrate Time Bomb (NTB) model (Wang et al. 2012). This report describes three areas addressed by the project. These were:

• The development of the BGS NTB model using improved water level and geological classification datasets and new approaches to nitrate transport in low permeability deposits and to estimating nitrate velocity in groundwater systems using groundwater recharge and aquifer properties. This work is described in Development of the BGS unsaturated zone model.
• A series of case studies comparing the application of the BGS approach to other nitrate modelling. These were selected to provide comparison at different scales. The Thames case study is at the river basin scale in the Chalk and is referenced to the study of Howden (2010 & 2011). The South Downs study considers the approach to modelling of a catchment in the Chalk. The reported study contains an approach to the unsaturated zone which is very similar to the BGS model. It also highlights the value of linking work carried out under the WFD with NVZ designation. The third study is at multi-borehole scale in the Permo-Triassic sandstone and uses an approach to the saturated zone similar to that reported by Wang et al. (2013) for the Eden Valley. These studies are described in Thames case study, South Downs case study and Permo-triassic sandstone case study.
• A contextual section (see Contextual review) evaluates the areas where the NTB model could be of value in a future NVZ designation process and implementation of the WFD. This includes an evaluation of the benefits and limitations of the BGS model in terms of its structure, effective use of data and, in particular, evidence of improved explanatory or predictive skill. This section also aims to detail some potential approaches to integration of the NTB model with the Environment Agency’s NVZ designation methodology and its potential application to support certain aspects of WFD implementation.