OR/14/047 Nitrogen budgets and source attribution studies

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Farr, G, and Hall J. 2014. Atmospheric deposition and groundwater dependent wetlands: implications for effective catchment management and future Water Framework Directive groundwater classification in England and Wales. British Geological Survey Internal Report, OR/14/047.

Nitrogen Budget[edit]

Nitrogen budgets for GWDTE must include both atmospheric wet and dry deposition and terrestrial pathways such as groundwater and surface water. It may also be necessary to monitor fluxes from a wetland to quantify if the site is a source or sink for nitrogen (e.g. Lohila et al. 2010)[1]. Substantial monitoring programs are required to quantify the loading from a combination of nutrient pathways. Factors that influence the accumulation of nitrogen must also be considered such as vegetation and soil cover. It is important to have an advanced hydrological conceptual model of any GWDTE in order to quantify the loading of nitrate from groundwater and surface water pathways. A poor understanding of the hydrology of any given site will result in knowledge gaps when it comes to quantifying the input from the hydrological system against that from atmospheric deposition. Jones et al. (2005)[2] undertook a nitrogen budget for a dune site including humid dune slacks in South Wales (Merthyr Mawr) measuring atmospheric deposition of dry and wet atmospheric in puts, including dry gaseous deposition of NH3 and estimating inputs of groundwater NO3 entering the site via limestone streams over a 12 month period. However a limited understanding of the hydrogeological regime of the site was stated as a knowledge gap in understanding. A nitrogen budget has also been conducted for Newborough Warren, investigating the impact of NH3 emissions from the nearby poultry unit (Jones et al. 2013)[3]. The study showed that contributions from this point source caused critical load exceedance within the dune site, and contributed 30% of the atmospheric deposition load. The variety and quality of humid dune slacks comprise some of the key qualifying features for this SAC. Additional research at the nearby dune system of Aberffraw has specifically addressed groundwater N concentrations and impacts on the biological condition of the site (Rhymes et al. 2014)[4]. That study showed adverse impacts on vegetation composition at low levels of groundwater nitrate input, below current GWDTE guidelines for dune slacks. Atmospheric inputs have not been assessed, but could be derived from calculations at Newborough Warren. Environment Agency Wales (2005)[5] produced a source apportionment study at Crymlyn Bog, South Wales. The report noted that where regulated activities only contribute to a small percentage of total atmospheric deposition (i.e there are other sources that are not regulated) then regulatory action on its own is unlikely to succeed.

Source attribution[edit]

Source attribution is the estimation of the contribution by different sources (atmospheric or terrestrial) to pollution, in this example the nitrogen budget of a GWDTE. Very few studies have assessed impacts from atmospheric and surface or groundwater inputs at the same site in the UK and this presents a major knowledge gap. Source apportionment studies can be divided into two approaches: load orientated approach and the source orientated approach (EEA, 2005)[6]. The load orientated approach and the source orientated approach are similar but differ in their approach to estimating the input from diffuse sources.

Both the load orientated and source orientated approaches were used by the EEA to estimate nutrient inputs to river catchments and coastal areas, rather than individual wetland sites.

Existing source attribution model for atmospheric deposition in the UK[edit]

Source apportionment data (and concerns) is available for all of the UK SACs, SPAs and SSSIs through APIS (www.apis.ac.uk). The APIS website allows the user to look up national-scale nitrogen deposition for selected interest features at any given SAC, SPA or A/SSSI. Deposition data for 2005 based on the CBED methodology is used together with and a forecast for the year 2020 (UEP30 scenario) generated by the FRAME (Fine Resolution Atmospheric Multi-pollutant Exchange) model. This model was applied to assess the magnitude and spatial distribution of nitrogen and sulphur from 156 different point and background sources. The outputs from the APIS website are in a pie chart format (Figure 13) and can be produced for the emissions data year 2005 or for a future emissions scenario year (2020). The APIS website informs the user that both are now out of date.

Figure 13    Pie chart describing nitrogen source attribution for a wetland based on the 2005 dataset (www.apis.ac.uk).

Difficulties may arise when quantifying the input of diffuse and point sources into the total nitrogen budget of a site and a detailed understanding of the hydrology of the GWDTE would be required.

Although the EEA report focuses on source apportionment for the aquatic environment it has several recommendations that would be directly applicable to any studies at GWDTE including the need for more:

  • Data to quantify annual discharges from point sources (e.g sewage systems)
  • Data to quantify annual retention within the wider hydrological cycle
  • Information on groundwater residence time and degradation of nitrogen within aquifers
  • Information on agricultural practices to allow development of models for nutrient loss


  1. LOHILA, A, AURELA, M, HATAKKA, J, PIHLATIE, M, MINKKINEN, K, PENTTILÄ, T, and LAURILA, T. 2010. Responses of N2O fluxes to temperature, water table and N deposition in a northern boreal fen. European Journal of Soil Science, 10, Vol. 61, No. 5, pp.651–661 ISSN 13510754. DOI 10.1111/j.1365-2389.2010.01265.x.
  2. JONES, M L M, PILKINGTON, M G, HEALEY, M, NORRIS, D A, BRITTAIN, S A, TANG, S Y, JONES, M, and REYNOLDS, B. 2005. Determining a nitrogen budget for Merthyr Mawr sand dune system. Report for the Countryside Council for Wales. CCW Ref: FC 72-02-59, CEH Project Number: CO2352NEW.
  3. JONES L, NIZAM, M S, REYNOLDS, B, BAREHAM, S, OXLEY, E R B. 2013). Upwind impacts of ammonia from an intensive poultry unit. Environmental Pollution 180, 221–228.
  4. RHYMES, J, WALLACE, H, FENNER, N, and JONES, L. 2014. Evidence for sensitivity of dune wetlands to groundwater nutrients. Science of the Total Environment 490, 106–113.
  5. ENVIRONMENT AGENCY WALES. 2005. Summary of the assessment of levels and loads of air pollutants and the nutrient source apportionment at Crymlyn Bog candidate Special Areas of Conservation. Version 3. September 2005. pp.44.
  6. EUROPEAN ENVIRONMENT AGENCY. 2005. Source apportionment of nitrogen and phosphorus inputs into the aquatic environment. ISBN 92-9167-777-9