Groundwater nitrate vulnerable zones for Scotland

The Nitrates Directive (91/676/EEC) requires Member States to identify areas where groundwaters have nitrate concentrations of more than 50 mg/l nitrate or are thought to be at risk of nitrate contamination. Areas associated with such groundwaters are to be designated as Nitrate Vulnerable Zones (NVZs) within which Member States are required to establish Action Programmes in order to reduce and prevent further nitrate contamination. The British Geological Survey (BGS), with funding from the Scottish Executive SEERAD Agricultural and Biological Research Group Flexible Fund, has devised a methodology for the establishment of groundwater NVZs in Scotland. The work was carried out between August and December 2001. The methodology was based on an earlier phase of work carried out by the Macaulay Land Use Research Institute (MLURI) who produced a series of land use risk zones for the east of Scotland. These zones show where the highest risk of nitrate leaching occurs. The Scottish Executive then contracted BGS to identify aquifers associated with these risk zones. BGS have used GIS analysis to assist the work. Digital geological line work based on the Digital Geological Map of Great Britain (DiGMapGB-50) was used to develop and apply a High, Moderate and Low permeability classification to both solid and superficial geological formations between the Borders and the Black Isle. By means of combining the solid and drift classifications, a form of groundwater vulnerability map was then constructed. Most vulnerable aquifers are in low-lying areas. Databases containing information on the location of wells, springs and boreholes were included in the GIS. The SEPA groundwater monitoring network, the BGS groundwater database and other sources of information on private water supplies were also included to display the range of nitrate concentrations in groundwater across eastern Scotland. Areas where highly permeable aquifers coincided with MLURI high risk leaching zones were identified using the GIS. These are the zones where the greatest risk to groundwater from the leaching of nitrogen fertilisers is thought to be present. They are the also the main surface recharge areas for the major aquifers in eastern Scotland. The next stage in the process involved calculating surface water sub-catchments associated with these high risk aquifer/land use coincident zones. Those sub-catchments that include the coincident zones (called class 1 and 2 catchments) are distinguished from sub-catchments lying upstream, generally in upland areas, where there are no highly permeable aquifers or land risk zones (called class 3 catchments). In general the available groundwater nitrate data indicate elevated nitrate concentrations in class 1 and 2 catchments. SEPA surface water nitrate data and the available groundwater nitrate data indicate that upland catchments (class 3) are essentially low in nitrate. For the purposes of the report, the east of Scotland has been divided into 7 areas. Descriptions of the types of aquifers present, the location of coincident zones and the extent of source nitrate data coverage are provided for each area. Data from within the highly permeable aquifer zones show that the highest proportion of groundwater sources with high nitrate concentrations are under the highest risk land use zones. The results of the vulnerability/risk analysis are consistent with the available nitrate data for much of Scotland. There are significant discrepancies in only two catchments, Dumfries and the Upper Don (called class 4 catchments). Here, unverified private supply data show clusters of elevated nitrate. For Dumfries a separate study by BGS in 2000 also indicated rising nitrate concentrations within the main aquifer.