OR/16/036 Conclusions and further work

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.

Summary and conclusions

Model development and benchmarking

A series of significant developments to the BGS NTB model have been made as part of this project to address a number of over simplifications used by the first version of the model. The improvements have included incorporation of a spatially and temporally distributed nitrate input function, unsaturated zone thickness derived from OS river data, travel time attribution and application of larger scale geological mapping and process modelling of recharge through the unsaturated zone. They now allow this national model to be applied at sub national scale. The model has the capability to be modified to take account of particular aquifer characteristics.

This has a number of benefits in that it can be applied at the different scales required for effective management and protection of groundwater (and associated receptors) under the EU Nitrates Directive and Water Framework Directive — point, groundwater body and catchment. An important and unique additional benefit is that it is nationally consistent and so its application is not geographically limited unlike other models. It therefore lends itself to providing evidence to support national reporting requirements and to undertake scenario modelling in terms of management option appraisal and environmental (including climate) change. The model improvements have also enabled the first estimate to be made of the mass of nitrate stored within the unsaturated zone. This indicates that previous assumptions have been inaccurate and significantly underestimated the mass of nitrate in the sub-surface.

The BGS approach was evaluated in three case studies using other modelling approaches applied at different scales. These were a basin-scale model of the Chalk (Howden 2010 and 2011), a multi-borehole scale model in the Permo-Triassic sandstone and a catchment in the Chalk of the South Downs (AMEC, 2014a, b).

• For the Thames the BGS model gave comparable results to the original study back to 1925 provided that the same nitrate input function was used. Both models failed to predict nitrate concentrations in the Thames after the mid-1980s and has raised some very interesting questions about the future behaviour of nitrate in the River Thames and the processes operating.
• For the Permo-Triassic site a similar approach was used to the BGS model in the Eden Valley. This replicated the existing model developed for the EA for both in terms of trend assessment and in the lack of dilution available for blending purposes.
• For the Chalk of the South Downs a model which treated the unsaturated zone in a similar way to the BGS model had already been constructed by AMEC for nitrate catchment management. This model provided a good fit to observed concentrations and confirmed the importance of estimating unsaturated zone delays. The assessment of modelled travel time from different areas of the catchment clearly illustrated the arable areas which would give a relatively rapid respond to changes in nitrate management.

The case studies demonstrate that the BGS NTB model can be benchmarked against other nitrate modelling approaches and gives acceptable results at a range of scales. The NTB has not as yet been parameterised to allow bypass flow or denitrification to be represented.

Potential linkage with Environment Agency NVZ designation and WFD

To illustrate the potential linking of the BGS Nitrate Time Bomb (NTB) model with the Environment Agency’s NVZ designation methodology, a GIS approach was used to identify areas of England where unsaturated zone lags may be significant and where there is uncertainty in the NVZ designation. A national overview of areas of designation uncertainty identified large areas of England in particular the chalk outcrop of southern and eastern England. National and regional scale assessments of where the risk model indicates there are mismatches between monitoring and loading data were compared to unsaturated zone travel times.

The results of this analysis suggest a number of areas where the BGS model might be a very useful additional component in any future NVZ delineation method. This could either be through formal integration into the designation modelling methodology or to provide supporting evidence.

There are also a number of ways the model may potentially help with implementation of the WFD. Nitrate is the most significant and widespread cause of failure to achieve environmental objectives for groundwater, especially the good status and trend reversal objectives. The improved BGS NTB model has the potential to provide more robust evidence to support assessment of the monitoring data used to demonstrate compliance with the objectives and the measure being implemented.

The model can be used to assess future nitrate trends at points and across groundwater bodies to determine if and/or when threshold values are likely to be exceeded or when trends reversed. Further the model could be used to:

• Evaluate the impact of programmes of measures (under different) scenarios as part of an options appraisal.
• Consider the longer term impacts of a variety of environmental change factors.
• Be applied to other potential pollutants which are of concern under the WFD.

Further work

There are a number of areas where further development could make the NTB model of greater value:

• Development of different scenario tests, such as nitrate loading changes due to different land use/management measures, and under climate change scenarios.
• Introducing detailed nitrate fate and transport processes in the groundwater system into the NTB model when applying to catchment-scale studies.
• Assessing the potential impact of karst behaviour and bypass flow on nitrate movement.
• Incorporating both nitrate and water processes in the NTB model to contribute to the development of a new model for NVZ designation.

NTB model outputs have the potential to be used for a range of applications.

• Providing outputs to the EA for the next round of NVZ designation and integration into the risk scoring methodology.
• Using outputs of USZ modelling to inform AMP6 water company catchment management work.
• Making some outputs of the work publicly available through a web GIS, e.g. through BGS, the Environment Agency or Defra. This could include a high level depth to water/USZ travel time map.