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Scotland's groundwater monitoring network : its effectiveness for monitoring nitrate

By A.M. MacDonald, K.J. Griffiths, B.E. O Dochartaigh, A. Lilly and P.J. Chilton


Scotland has had a national groundwater quality monitoring network since the year 2000.\ud One of the main functions of this network is to monitor nitrate concentrations. Nitrate can be\ud elevated in the environment due to modern agricultural practice. Initially there were 150\ud monitoring sites, but these have been added to, and in 2005, the number of groundwater\ud monitoring points for nitrate was 219, comprising 139 boreholes, 51 springs and 27 wells;\ud 67% of these sites are in agricultural areas.\ud In order to have confidence in the interpretation of data gathered from the network it is\ud important to know the context of the sample points, and in particular whether any sites are\ud compromised by surface contamination or nearby point sources. Prior to this study, many of\ud the sites had not undergone a formal risk assessment and their condition was unclear. In order\ud to improve confidence in the network, and to help act as a baseline before improving the\ud network, the British Geological Survey and the Macaulay Institute were commissioned by the\ud Scottish Executive to carry out a review during the period February-July 2005.\ud The core of the project was to undertake field assessments for 151 sites where no formal\ud assessment had been made previously. Using criteria developed in this project, a judgement\ud was made as to whether the monitoring point was adequate, required improvement or further\ud assessment, or should be considered for removal. For all 219 sites on the network, a zone of\ud influence was estimated using a semi-quantitative method. These zones were used to help\ud focus the field surveys and also to characterise each site using national datasets; for example\ud the monitoring site would be assigned the land use that occupied more than 60% of the zone.\ud These data were then used to conduct an analysis of the factors controlling nitrate\ud concentrations across the network and to help evaluate how effective the network is at\ud monitoring nitrate in Scottish groundwater.\ud Below is a summary of the main results from the project:\ud 1. The fieldwork and analysis of the 151 previously unassessed sites indicated that:\ud • 61 of the 151 sites are adequate and can continue to be monitored with no\ud improvements.\ud • There are serious concerns about 29 of the 151 sites (19% of the sites assessed and\ud 13% of the total nitrate network). These sites should be considered for removal\ud from the network. The sources found to be least reliable were shallow large\ud diameter wells. However, there is little evidence to suggest that the data from\ud these sources collected from 2000 – 2005 has been seriously compromised by\ud point source pollution.\ud • 30 sites require further assessment before being judged suitable. Most of these\ud sites are springs and require additional work to identify the precise source.\ud • 31 sites require improvements to the monitoring points – the improvements range\ud from better sampling protocols to improving the headworks through simple\ud engineering.\ud 2. There is a clear difference between nitrate concentrations measured in the areas\ud designated as nitrate vulnerable zones (NVZs) and other areas. Within the NVZs, the\ud mean concentration is 25 mg-NO3 l-1 and the median 17 mg-NO3 l-1; outside the\ud NVZs, the mean concentration is 9 mg-NO3 l-1 and the median 4.4 mg-NO3 l-1.\ud The data from the network indicate that land use has a large influence on the nitrate\ud concentrations measured in the monitoring network: arable areas, mixed cultivation of\ud both arable and grassland, and areas where dairy, pigs and poultry are reared\ud contribute to the highest nitrate concentrations, with 18% of sites in these areas\ud exceeding 50 mg-NO3 l-1. The most significant control on nitrate concentrations in the\ud monitoring network is the presence of dairy, pigs or poultry within the zone of\ud influence.\ud 4. A considerable number of monitoring sites have lower nitrate concentrations than\ud would be expected from the nitrate pressure. This can be attributed to dilution from\ud rainfall, mixing with older low nitrate waters, denitrification, or the presence of low\ud permeability soil and superficial deposits which slow the movement of high nitrate\ud water into the aquifers.\ud 5. A “gaps” analysis which compared the current network with an idealised network\ud based on nitrate pressures across Scotland indicated that overall the distribution of the\ud current network is generally good. However, there are significant gaps in the improved\ud grassland areas of the Midland Valley and Ayrshire and in the arable areas of\ud Aberdeenshire, while Mid and East Lothian and the Borders are currently overrepresented.\ud The following recommendations are made for the nitrate groundwater quality network in\ud Scotland:\ud 1. Consideration should be given to removing or replacing 29 of the 219 sites, and\ud undertaking further assessments on 30 sites. A further 31 sites would benefit from\ud improvements to the headworks or sampling arrangements.\ud 2. Further statistical analysis should be undertaken to help understand the factors that\ud control the nitrate concentrations in groundwater – particularly the environmental\ud factors that help to reduce the measured nitrate.\ud 3. The network should continue to be concentrated on nitrate pressured areas in\ud Scotland, with approximately 75% of the network in high nitrate areas, and 25% used\ud to monitor background nitrate concentrations in less pressured areas.\ud 4. Any future sites added to the network must undergo a risk assessment similar to\ud the one developed for this study to ensure that the network remains of good quality.\ud 5. The network must continue to reflect the diverse hydrogeological, soil and land use\ud conditions in Scotland. Therefore, both bedrock and superficial aquifers should be\ud monitored in a variety of soil conditions. The network should continue to include\ud different types of sources, although less emphasis should be given to wells, which are\ud generally poor monitoring points.\ud 6. In the future, the data from the network will require to be actively interpreted: an\ud inevitable outcome from having a diverse network is that the results of the monitoring\ud must be interpreted not only in terms of agricultural practice, but in light of the other\ud factors such as geological and environmental conditions. In practice this could mean a\ud regular detailed review (maybe every 2-3 years) of the data from the network.\ud 7. The network will also need to be actively managed to account for various changes in\ud monitoring sites, for example the land use, the condition of the headworks and the\ud pumping rate. This will involve SEPA hydrogeologists having an overview of the\ud network; individual sources being periodically reviewed using a simple checklist; and additional new sources being sought, possibly through the ongoing BGS/SEPA study\ud of baseline groundwater chemistry across Scotland.\ud 8. Wellhead measurements should be taken periodically to help identify denitrification\ud or mixing with older waters. The limited samples taken during this study proved\ud invaluable for interpreting apparently anomalous nitrate concentrations. To undertake\ud this successfully, dedicated sample taps may have to be introduced.\ud 9. A separate programme of focussed monitoring should be developed in tandem with\ud the national groundwater monitoring network to give information on the effects of the\ud action programmes within the NVZs. These sites should be in a controlled\ud environment that will respond rapidly to changes in agricultural practice. The results\ud from these studies can then be upscaled to help interpret changes in the national\ud network as well as be used on their own to help understand the success of the Action\ud Programmes

Topics: Earth Sciences
Publisher: British Geological Survey
Year: 2005
OAI identifier:

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