OR/14/047 Spatial analysis of critical loads and threshold values

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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.


For the first time the results of the WFD chemical classification, incorporating threshold values (TV), wetland habitat condition and atmospheric nitrogen critical loads (CLnutN) exceedances are considered together.

The agreed methodology is a spatial analysis of several key datasets (listed below). The aim was to see where CLnutN for atmospheric nitrogen deposition and TV for groundwater were both exceeded at the same sites, thus suggesting multiple pathways for nutrients to a GWDTE. We are also interested in areas where GWDTEs exceed their CLnutN from atmospheric deposition and the impact this may have on the WFD groundwater body classification. For instance could ‘we’ be pointing the finger at nutrient pressures in groundwater when actually atmospheric deposition is playing a key role in the significant damage of many GWDTE.

The following text describes the geospatial datasets used for the spatial analysis:

GWDTEs: The list of GWDTE was agreed for the WRD classification work by CCW (now NRW) and NE ecologists along with colleagues in the EA. Guidance in UKTAG, 2012a[1] was used to help delineate wetlands that could be considered to be groundwater dependent. Of the 3320 GWDTE in England and Wales 2508 have an EU designation (i.e SAC, RAMSAR, SPA) the remaining 812 are non EU designated sites (e.g. LNR, SSSI). It is useful to note that even when a site is classified as ‘non designated’ it can still support examples of Annex 1 habitats — which in turn are reported for the Habitats Directive Article 17.

WFD Classification results and threshold values: The results from the WFD chemical classification were supplied by the EA and NRW, with a final status (good or poor chemical status) attributed to every groundwater body in England and Wales. It was possible to query each of the 6 individual tests that are involved within the overall chemical assessment including the GWDTE test (see UKTAG, 2012b)[2]. The GWDTE test incorporated the recently defined ‘threshold values’ (UKTAG, 2012a[1]) with a score of 0-3 being applied to all 3320 GWDTE in this analysis.

The scores are as follows:

0 (NO) sites Threshold value has not been reached within the groundwater body. This applies to 1277
1 (NO) Threshold value has been exceeded as a groundwater body average and or at one monitoring point within 5km of the GWDTE, or a NEAP N loading assessment indicated a high nitrate loading. This applies to 1929 sites
2 (NO) Threshold predicted to exceed level by 2027. This applies to just 7 sites
3 (YES) Threshold exceeded a local monitoring point with high connectivity to the GWDTE. This applies to 107 sites

Wetland Condition and NVC mapping[edit]

All of the EU designated sites have condition data (favourable — unfavourable etc) in total 2084 sites have condition data and 1236 have no condition data.

Critical Loads[edit]

Critical Loads have been assigned to the designated feature habitats of the SSSIs that correspond to or are co-located with GWDTEs (See Section 4). N deposition (NOx + NHx) values have been assigned to each SSSI by two methods: (a) extracting the value for a single point within each SSSI (b) calculating an area-weighted average value for the SSSI. In both cases the CBED 5 x 5 km annual average deposition data for 2010–12 were used. Critical load exceedances were calculated for each feature habitat and the maximum exceedance per site derived for the results presented below. It should be noted that some features within the SSSIs may be less sensitive to nitrogen and have higher critical loads and lower exceedance values, or not be exceeded. The use of the maximum exceedance is a precautionary approach.

The abbreviations used in the GIS dataset are provided below for reference. Where a critical load has not been assigned, a value of -9999 is recorded for data analysis purposes. When deposition is below the critical load then a negative value is reported; positive exceedance values indicate the critical load has been exceeded by nitrogen deposition.

MaxOfRecCLexc_pts_kg: maximum exceedance of recommended nutrient N critical loads by N deposition for single point within each site in kg N ha-1year-1

MaxOfRecCLexc_awtd_kg: maximum exceedance of recommended nutrient N critical loads by area-weighted N deposition for each site in kg N ha-1year-1

-9999 This means no critical load value assigned to any features on the site. This applies to 965 sites
0 A score of zero means the nitrogen deposition is below the critical load(ie, critical load not exceeded). This is also represented by negative exceedance values <0.0 (and greater than - 9999). This applies to 226 sites.
1 A score of 1 indicates that the nitrogen deposition is greater than the critical load (ie, the critical load is exceeded). This is also represented by positive exceedance values. This applies to 2129 sites.

The scores for the TV and CL are combined to give an overall score.

Site Score Explanation WFD TV excedance CL excedance No of sites by site_score_pt No of sites by site_score_awtd
-9999 TV not exceeded and no CL assigned NO -9999
(i.e no CL)
949 949
0 Neither TV or CL exceeded NO 0 222 219
1 TV exceeded but CL not exceeded YES 0 4 4
1 TV exceeded but no CL assigned YES -9999
(i.e no CL)
16 16
2 Only CL exceeded NO 1 2042 2045
3 Both TV & CL exceeded YES 1 87 87

Results: comparison of cl, tv and habitat condition[edit]

Before any spatial analysis was performed a comparison was undertaken to quantify the number of sites that exceeded the CL or the TV and how this applied to their habitat condition (Table 4, Table 5 and Table 6), unfortunately habitat condition was only available for England.

The analysis does not show similar trends in England and Wales, this is due to the inclusion of a large number of non designated sites in Wales that have not been assigned a critical load. The majority of sites in England and Wales (if you consider the 872 non designated sites in Wales with no CL) have nitrogen deposition above the critical loads however far fewer exceed their groundwater threshold values.

From this analysis it would appear that excedance of CL are far more common than the excedance of a TV, this analysis should be interpreted with caution for the following reasons:

  • The CBED deposition provides national coverage and therefore every site can be assigned a deposition value, and exceedances calculated for each feature habitat (for which critical loads are available). In addition, as mentioned above, critical loads and exceedances may vary between features within an individual site, and while some may not be exceeded, the most sensitive feature habitats have lower critical loads and are widely exceeded by current levels of N deposition. The analysis presented here does not take into account which habitat features are exceeded or not, it simply uses the maximum exceedance per site
  • TVs rely upon NEAP N modelled data and real data collected from a monitoring point considered to be in hydraulic connection with the GWDTE. Critically the groundwater data has not been modelled across the country and thus it is only possible to assign values where there is ‘real’ data. The scoring for the WFD classification process is slightly less ‘black and white’ with four possible categories (see 9.1 Methodology). In this analysis only GWDTEs that score 3 i.e where the TV is exceeded at a local monitoring point, have been considered to be truly in excedance of their threshold value

However, it is worth noting (i) exceedance of critical loads does not necessarily equate with current damage or impacts; but does indicate that adverse impacts are expected to occur. (ii) CSM may substantially under report impacts due to N deposition as CSM was not designed for monitoring N deposition impacts (e.g. Williams, 2006)[3].

Table 4    Comparison of the CL and TV for (n2084) GWDTEs (SSSIs) in England only
No of GWDTE in England >CLV <CLV NO CLV >TV <TV >CLV & >TV
No of GWDTE 2084 1770 224 90 84 2000 79
% of total GWDTE in England 100 85 10 5 4 96 4
Table 5    Comparison of the CL and TV data for (n1236) GWDTEs (SSSIs) in Wales only.
No condition assessments were supplied
No of GWDTEs in Wales >CLV <CLV No CLV >TV <TV >CLV & >TV
No of GWDTE 1236 359 3 872 23 1213 8
% of total GWDTE in Wales 100 29 0.2 71 2 98 0.6
Table 6    Summary of excedance of CL and TV in both England and Wales separately and combined, for the list of (n 3320) GWDTEs
England Wales
85% 29% GWDTEs exceed their CLV
4% 2% GWDTEs exceed their TV
4% 0.6% GWDTEs exceeded both the CLV and TV
England and Wales
66.1% GWDTEs exceed their CLV
3% GWDTEs exceed their TV
3 % GWDTEs exceeded both the CLV and TV
Figure 14    Water Framework Directive ‘threshold value’ results (Exceeded = score of 3, not exceeded = score <3).
Contains OS data © Crown Copyright and database right [2015].
Figure 15    Exceedance of nutrient nitrogen critical loads by CBED total nitrogen deposition for 2010–12; results are based on the maximum exceedance for any feature habitat within each SSSI in England and Wales. (exceeded = score of 1, not exceeded = score 0).
Contains OS data © Crown Copyright and database right [2015].
Figure 16    Map showing excedance of nitrogen critical loads and Water Framework Directive threshold values. Exceedance of nitrogen critical loads based on CBED deposition for 2010–12 and maximum exceedance for any habitat feature per site (SSSI) in England and Wales.
Contains OS data © Crown Copyright and database right [2015].

Implications for WFD and effective catchment management[edit]

Water Framework Directive[edit]

The results show that 85% of GWDTEs in England and 29% in Wales assessed as part of the WFD classification exceed the nitrogen critical load for at least one feature habitat within a site (SSSI). This figure for Wales should be larger however the inclusion of a large amount of non designated sites without critical load values resulted in a lower percentage failing their critical load.

  • The low percentage of sites exceeding their groundwater TV (3%) is most likely a reflection of the lack of chemical nutrient data from WFD monitoring at or near GWDTE within this study.
  • There is a need for the collection of more water chemistry data at or in WFD monitoring points linked to GWDTEs. This would provide vital data for the future classification of GWDTEs in England and Wales against existing threshold values.
  • Localised nutrient enriched waters, e.g. agricultural surface runoff, are known to have significant impacts along the periphery of GWDTEs. This localised pressure is often not reflected in the WFD classification process. One positive is that land management agreements targeting adjacent fields may offer very effective solutions to tackle nutrient enrichment when the source and pathway of nutrients can be shown to be from adjacent fields.
  • The critical load information should be included within future WFD classification so that future assessments consider atmospheric loading in conjunction with terrestrial loading.
  • It is possible that many GWDTEs are in poor condition primarily due to atmospheric deposition, however a greater understanding of the source and fate of nutrients in wetlands is needed before any such conclusion can be drawn.
  • The widespread excedance of critical loads reported in this preliminary study supports the need for detailed source apportionment studies at GWDTEs. Defining the sources and pathways for nutrients will support regulatory bodies with implementing targeted and effective WFD programs of measures.

Effective catchment management[edit]

  • The results show that 64% (this includes the non designated sites with no critical load) of the GWDTEs in England and Wales exceed the critical load for at least one habitat feature (though habitat type not taken into account in this study), suggesting that effective catchment management, may in some cases only be part of the solution, and that regulation of emissions from industry and agriculture may be required to help GWDTEs achieve favourable status.
  • Regulation and management of nutrient application and water management within fields immediately adjacent to many GWDTEs (for example see Anglesey and L lyn Fens SACs) could also offer a simple and effective solution to reduce localised terrestrial sources and pathways of nutrients.

It is possible to regulate point source emissions (e.g factories) however the contribution of diffuse, and perhaps unregulated, nitrogen to the loading at a GWDTE may be very challenging to regulate.

  • There still exists much uncertainty about the relative contribution and fate of terrestrial and atmospheric nutrient loading at GWDTEs. With this uncertainty comes a reduced ability to successfully mitigate against these pressures.
  • There is a need to define a defensible methodology to quantify nitrogen loading (source apportionment) both at GWDTEs and potentially within entire groundwater bodies. Defining the sources and pathways for nutrients will allow targeted and effective WFD programs of measures to be undertaken.


  1. 1.0 1.1 UKTAG. 2012a. Technical report on groundwater dependent terrestrial ecosystem (GWDTE) threshold values. Version 8 March 2012. http://www.wfduk.org/resources%20/groundwater-dependent-terrestrial-ecosystem-threshold-values
  2. UKTAG. 2012b. Paper 11b(i) Groundwater Chemical Classification for the purpose of the Water Framework Directive and the Groundwater Directive. Version Feb 2012. http://www.wfduk.org/sites/default/files/Media/Assessing%20the%20status%20of%20the%20water%20environment/GWChemical%20Classification_FINAL_2802121.pdf
  3. WILLIAMS, J M. ED. 2006. Common Standards Monitoring for Designated Sites: First Six Year Report. Peterborough, JNCC.