Research and analysis

Nitrate source apportionment for groundwater-dependent terrestrial ecosystems

Published 18 February 2020

1. Chief Scientist’s Group research summary

This project examined the sources of nitrogen contributing to nutrient enrichment at two nationally designated groundwater-dependent terrestrial ecosystems (GWDTEs) and explored land use and management actions to reduce the amount of nitrogen reaching the sites. The purpose of the study was to:

1. identify and trial a ‘transferable approach’ to nitrate source apportionment at wetland sites
2. identify potential mitigation measures at each site that would ensure the long-term sustainability of the wetland plant communities for which the sites are designated.

Too much nitrate can have an adverse effect on wetland plant communities, particularly those wetlands where the ecology is specifically adapted to a low nutrient setting. These wetlands tend to be associated with a high species diversity per unit area, and in general support more rare species than wetlands in higher nutrient settings.

Groundwater thresholds designed to protect wetland ecology range from 4 to 26 mg NO3 l-1 depending on the type of wetland. This is substantially less than the World Health Organisation’s drinking water standard of 50 mg NO3 l-1 which is used to define polluted groundwater under the Nitrate Directive. Consequently, measures to protect wetlands typically have to go beyond good practice and can have significant implementation costs associated with them.

Understanding the sources of nitrate and the amount they are contributing to the problem is critical to identifying potential remedial actions.

In this project, different source apportionment approaches were trialled at 2 wetland sites, Wybunbury Moss Site of Special Scientific Interest (SSSI) in Cheshire and Newbald Becksies SSSI in east Yorkshire.

The project identified:

• essential aspects of a wetland conceptual model relevant to a source apportionment study
• tools and techniques that may be of use in defining a site’s catchment and apportioning multiple sources of nitrate

The focus here is on modelling approaches whilst a partner project SC130039 has looked at field data collection techniques (Farr & Hall 2014; Farr et al. 2018). The projects’ findings will enable the Environment Agency and its partners to better determine the relative contributions of different nitrate sources to a wetland catchment and to identify the most appropriate programmes of measures to improve wetlands that are in an unfavourable condition due to high nitrate levels.

1.1 An approach to wetland source apportionment

A suggested approach to nitrate source apportionment was developed drawing on experience gained with the 2 trial sites and wider knowledge.

Nitrate source apportionment at a wetland site broadly comprises the following steps:

1. Develop or review the catchment conceptual model and catchment definition
2. Identify the main sources and pathways of nitrate in the catchment
3. Collate the catchment data
4. Deploy an appropriate methodology to predict nitrate leaching from sources in the catchment
5. Validate and interpret the results.

Having a robust conceptual model is crucial to developing a reliable estimate of source apportionment. In particular, it is necessary to understand the sources of water contributing to the site, and the size and location of the catchment area. This will define the sources and pathways of nitrate that could have an impact on the site. Without this information it is impossible to reliably identify or apportion nitrate sources, or to identify measures that will manage nitrogen sources reaching the site effectively.

A number of tools are available to help estimate nitrate leaching from the variety of sources that may be present. For many point sources such as septic tanks or sewage discharges to ground, literature estimates of typical nitrate loadings are available.

For diffuse sources such as leaching from agricultural land, however, it is often necessary to use models.

This project used the Nitrogen & Phosphorous loading tool developed by AMEC to estimate point source contributions and both the Environment Agency’s Nitrate Leaching Tool and the ADAS Farmscoper model to estimate nitrate leaching from agricultural land. Other nitrate leaching tools are also available. Ultimately the choice of tool is often driven by the availability of data, particularly for larger catchments. Whenever possible, model results should be compared with observed site-specific data.

Actions should generally be targeted at those areas within the catchment that contribute the highest nitrate load to the wetland site. However, there may also be benefits in targeting areas that have rapid pathways to the site so as to achieve shorter term improvements.

1.2 Newbald Becksies site study

Newbald Becksies SSSI is a small spring-fed wetland, which is currently in unfavourable condition due to nutrient enrichment. A range of information from site observations, geophysics surveys and the Environment Agency’s regional groundwater model was used to improve the delineation of the catchment. This determined that the contributing catchment area is likely to be primarily to the south and that the extent varies over time, depending on the hydrological conditions.

Nitrate source apportionment calculations, based on an assumed (but realistic) crop rotation, suggested that the dominant source of nitrate in the catchment is from arable land, particularly from oilseed rape and vining peas. Leaching rates from grassland and rough grazing land are predicted to be much lower.

Reductions in nitrate inputs to Newbald Becksies are most likely to be achieved through targeted measures to limit leaching from arable land in the area immediately to the south and east of the site and, in the medium to longer term, from the area further to the south of the site. However, the reductions in nitrate leaching that are likely to be achievable through commercially viable mitigation measures associated with arable land were not found to be compatible with the target water quality for the site of 9mg NO3 l-1. Only extensive grazing land or forestry are compatible with the low desired concentrations.

1.3 Wybunbury Moss site study

Wybunbury Moss SSSI is a National Nature Reserve and part of the West Midlands Mosses Special Area of Conservation. It is a ‘quaking bog’ meaning it has a floating peat raft, surrounded by fen and mixed woodland. The site is currently in unfavourable condition due to concerns about nutrient enrichment. It has a small catchment area, which is predominantly to the north and west of the site. Land use in the catchment is predominantly extensively grazed grass, with only 2 arable fields identified, and some point sources.

The results of the modelling work suggest that:

• the point sources and non-agricultural sources are not significant contributors of nitrogen
• agricultural land to the north of the site is contributing to elevated nitrate concentrations in shallow groundwater along the northern boundary of the site

Particularly high concentrations are predicted in soil drainage from maize crops. This prediction is supported by elevated observed nitrate concentrations to the south of a field known to be used for maize.

Simulation of potential mitigation methods against nitrate leaching in the catchment suggests that to achieve the target water quality of 9mg NO3/l would require changes in the use of all arable land (both maize and wheat) with a move to grazed grassland with low stocking rates.

1.4 Publishing details

This summary relates to information from project SC160010, reported in detail in the following output(s):

Report:

SC160010/R1 An approach to terrestrial nitrate source apportionment for wetlands

Report:

SC160010/R2 Nitrate modelling of Newbald Becksies SSSI wetland catchment

Report:

SC160010/R3 Nitrate modelling of Wybunbury Moss SSSI wetland catchment

February 2020

Project manager:

Natalie Phillips, Research, Analysis and Evaluation

Research contractor:

Wood Environment & Infrastructure Solutions, Shinfield Park, Reading, RG2 9FW.

This project was funded by the Environment Agency’s Research, Analysis and Evaluation group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk

© Environment Agency

Related reports (project SC130039)

Phase 1 report:

Farr, G. & Hall, J. 2014. Atmospheric deposition at groundwater dependent wetlands: implications for effective catchment management and Water Framework Directive groundwater classification in England and Wales. British Geological Survey, OR/14/047. http://nora.nerc.ac.uk/510750/

Phase 2 Report:

Farr, G., Hall, J., Jones, L., Whiteman, M., Haslam, A., Phillips, N., Tang, S., Williams, H., Davison, P. & Lapworth, D. 2018. Atmospheric deposition at groundwater dependant wetlands Phase 2 – nutrient source apportionment case studies from England and Wales. British Geological Survey, OR/17/021. http://nora.nerc.ac.uk/id/eprint/522392/