Nitrate concentrations in Nithsdale groundwater, 2004

This report describes the results of a survey of groundwater nitrate concentrations in the Lower Nithsdale nitrate vulnerable zone (NVZ), southwest Scotland. The survey was carried out during September 2004. Rigorous field sampling techniques were used, and the samples analysed in the BGS laboratories at Wallingford. Shallow groundwater was targeted, where the effects of recent land use (within the past 10 years) would be reflected in the samples. Twenty-one sites were sampled comprising 5 springs and 16 boreholes; all the sites were rural – thirteen were located on dairy farms. None of the boreholes showed any signs of contamination from surface water. Most were located in huts with concrete plinths, others were under manhole covers in fenced areas. The springs were more vulnerable to surface water contamination. The median concentration of nitrate from the 21 samples was 33 mg NO3 L-1. Six samples had concentrations greater than 50 mg NO3 L-1, a further 3 sites had concentrations between 40 and 50 mg NO3 L-1. All sites with concentrations greater than 40 mg NO3 L-1 are located on dairy farms. There is broad agreement between nitrate concentrations measured in this survey and a previous survey in August 2002. This current survey indicates much higher nitrate concentrations than monitored by SEPA from their 12 monitoring sites in Nithsdale. This is due to the nature of the sampled sources. The SEPA monitoring network in the area comprises deep boreholes in the Permian bedrock; age dating has shown that much of the water sampled is pre-1950s. In contrast, much of the groundwater sampled during the current survey is young – recharged within the past decade. By making corrections for the dilution factor from the older water at the SEPA sites, it is possible to estimate nitrate concentrations in the recently recharged groundwater. These corrected concentrations are considerably higher, and consistent with those measured from the shallow groundwater sources in this current survey. In the light of this present survey, it is recommended that SEPA review the monitoring network in the Lower Nithsdale. Several shallow sources, which contain a high proportion of modern water, and are regularly pumped, should be adopted, and data from the existing deep sources should be interpreted in the context of the age of the groundwater sampled.