|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.|
Many groundwater dependent terrestrial ecosystems (GWDTEs) in England and Wales are under pressure from nutrient enrichment, from both terrestrial (surface water — groundwater) and atmospheric sources. The Water Framework Directive classification has highlighted that there are up to 65 groundwater bodies in England and Wales at risk due to chemical (nutrient) pressures at a groundwater dependent terrestrial ecosystem (GWDTE). Groundwater however is just one pathway for nutrients. The need to better understand all of the sources and pathways for nutrients is essential to inform WFD programs of measures aimed at reducing nutrients at GWDTEs.
A critical review of available literature highlighted the following knowledge gaps
- There are very few studies (N budgets) that consider both inputs from the hydrological cycle and the atmosphere in sufficient detail
- There are 35 GWDTEs that have current or historic information relating to on site atmospheric deposition
- Over half of the atmospheric monitoring sites are within 10 km of a designated GWDTE and could provide information for future work
GIS study combining results of Critical Loads and Threshold Values
This study represents the first attempt to combine atmospheric nitrogen deposition loads (Critical Loads), terrestrial groundwater nitrate (Threshold Values) and condition assessments for all 3320 GWDTEs included within the Water Framework Directive classification process.
In England and Wales:
- ~64% (2129sites) exceed the nitrogen critical load for at least one feature habitat
- ~3% (107sites) exceed their groundwater Threshold Value for nitrate as N mg/l
- ~3% (87sites) exceed both their Critical Load and Threshold Value
The lower number of sites exceeding their threshold value compared to critical load is not representative of the true risks or potential pressures from groundwater mediated nitrate. The low percentage of sites exceeding their TV (3%) is a reflection of the lack of nutrient data from WFD monitoring considered to by hydrologically linked to a GWDTE. In addition localised pressures such as nutrient rich surface runoff from fields adjacent to GWDTE may be significant contributors to nitrate loading.
Implications of these results for the WFD classification
- There is a need for the collection of more water quality data at/or in WFD monitoring points considered to be hydrologically linked to GWDTEs, to provide data for the future classification of GWDTEs against groundwater threshold values
- The Critical Load information should be included within future WFD classification so that 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 levels of terrestrial nutrients, acidification, source and fate of nutrients in wetlands is needed before any such assessment can be undertaken
Implications of these results for effective catchment management
- The results clearly show that the majority of sites (64%) exceed their critical load, suggesting that effective catchment management and the regulation of emissions from industry and agriculture need to be considered together to help GWDTEs achieve favourable status
- It is possible to regulate point source emissions (e.g factories, poultry farms) however the contribution of diffuse, and perhaps unregulated, nitrogen to the loading at a GWDTE may be very difficult, if not impossible to regulate
- There still exists much uncertainty about the relative contribution and fate of terrestrial and atmospheric nutrient loading at GWDTEs, and with this uncertainty a reduced ability to successfully mitigate against these pressures
- Where localised nutrient enriched waters, e.g. agricultural surface runoff, are known to have significant impacts along the periphery of GWDTEs they may be controlled by local land management agreements. This localised management may offer substantial improvements to the GWDTE and be both cost and time effective
- 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
Future source apportionment studies: partnerships, risks and costs
It is hoped that future work will address source apportionment studies at a subset of preselected GWDTEs in England and Wales, that could be selected from site listed in Table 8. Utilising sites with existing groundwater monitoring networks, vegetation mapping and conceptual models we will use traditional (i.e water chemistry and vegetation mapping) and novel techniques (stable isotope and groundwater age dating) to better understand the source apportionment of nutrients.
A project board consisting of partners from EA, NE, NRW, CEH and BGS, with expertise in air quality, hydrogeology and wetland ecology should be formed. Risk to any potential source apportionment project can be reduced with effective planning, but could include: unrepresentative selection of GWDTEs, poor project management, inconclusive results or failure to secure sufficient funding and ‘buy in’ from partner organisations.
Possible research hypothesis, sources of existing data, costs of analysis and equipment are detailed within the report as are opportunities to share equipment and services with partners thus reducing the cost of the project.