Nitrate concentrations in groundwater in Northern Ireland

The Environment and Heritage Service (EHS) commissioned the British Geological Survey to review the available groundwater nitrate data in Northern Ireland and examine relationships between groundwater nitrate concentrations and landuse or aquifer vulnerability. This forms part of work EHS is undertaking to comply with EC Nitrates Directive (91/676/EEC) and the recently introduced EC Water Framework Directive (2000/60/EC). There are two main sources of information on groundwater nitrate concentrations in Northern Ireland. The first is the EHS regional groundwater quality monitoring network which was established in 2000. Currently, information is available for 91 sources. The second source of nitrate information is from the general survey of groundwater sources from which the EHS regional groundwater quality monitoring network was subsequently chosen. A total of 759 groundwater sources (mainly boreholes) were visited from 1992 to 1994. At 352 sites, groundwater samples were taken for analysis (including nitrate) under stringent field conditions. 1. Groundwater nitrate concentrations, where measured, are generally low: • 6.2% (22 sites) of the BGS 92-94 survey data exceeded 50 mg NO3 / L; 20.1% (71 sites) exceeded 25 mg NO3 / L (based on the average concentration at each monitoring point). • The current EHS regional monitoring network 91 sources have 2.2 % of sites greater than 50 mg NO3 / L and 8.8% greater than 25 mg NO3 / L. • These concentrations are considerably lower than those measured in England & Wales, and slightly lower than those measured throughout Scotland. The most likely causes of the low groundwater nitrate concentrations are: the limited extent of arable agriculture; the favourable denitrification conditions in the Northern Ireland soils; and the dilution from the high rainfall. 2. Analysis of twenty-seven sites where sampling was undertaken in both 1992-94 and 2000-02 indicate no consistent temporal trend from 1993 to 2001 in nitrate concentrations across Northern Ireland. More detailed data from two sites in Clogh Mills and Comber NVZs show a general rise from 1992 to 2002. 3. Nitrate concentrations in groundwater are highly correlated with land use. • Arable areas have approximately 50% of sites exceeding 25 mg NO3 / L • More than 85% of sites situated in areas with no agricultural activity have less than 10 mg NO3 / L. • Both the BGS Survey data and EHS monitoring data show a steady rise in groundwater nitrate concentrations as the agricultural activity becomes increasingly intensive. 4. The permeability of the superficial deposits (for example glacial till, or alluvium) affects the nitrate concentrations measured in groundwater. For example, in areas of good pasture, 34.2% of sites in high permeability areas have > 25 mg NO3 / L compared to 18.6% in moderate or low permeability areas. 5. Groundwater nitrate concentrations are higher in shallow (< 25 m) sources. 6. During the BGS 92-94 survey, data was collected on the presence of septic tanks and the practice of slurry spreading. The data indicate that septic tanks have no statistical effect on the nitrate concentrations measured in the sample sites. However, sites where slurry was spread had higher nitrate concentrations than sites where it was not (this reflects the correlation of groundwater nitrate concentrations and agricultural activity described above). The statistics and correlations highlighted above have several important implications for the protection of groundwater from nitrate contamination in Northern Ireland. Some recommendations are given below, in the understanding that the implementation of the EC Water Framework Directive in Northern Ireland will require a more detailed assessment of land use, groundwater quality and hydrogeological conditions over the next 2-3 years. • Since land use is an excellent predictor of nitrate concentrations in groundwater it could be used to indicate potential concentrations in areas with no monitoring data. • Low permeability superficial deposits offer protection to groundwater. A more detailed assessment of the permeability and thickness of superficial deposits in Northern Ireland would help to highlight areas most vulnerable to contamination. • Areas with intensive agriculture (for example arable farming, complex cultivation patterns or intensive livestock rearing) particularly where superficial deposits are absent or highly permeable are most at risk of nitrate contamination. An approach which combines these factors may be appropriate for predicting groundwater nitrate contamination in Northern Ireland. • Detailed information on the location of intensive livestock rearing and slurry applications across Northern Ireland would be helpful in estimating local risks to groundwater. • The role of the gley soils and rainfall in Northern Ireland in generally reducing groundwater nitrate concentrations could be investigated further. It would be beneficial to target a small number of catchments or sub-catchments with more intensive monitoring to establish a more comprehensive local dataset. • Ongoing monitoring of groundwater for nitrogen species is essential in Northern Ireland and any future review of the regional groundwater monitoring network should take into account this objective.