Arbejdsrapport fra Miljøstyrelsen 40/2006 – Modeller for revision af udpegningen af grundvandsforekomster i Danmark

Modeller for revision af udpegningen af grundvandsforekomster i Danmark

Summary and conclusions

The designation of groundwater bodies is a central part of the baseline analysis according to the Water Framework Directive (WFD). In Denmark, the baseline analysis according to WFD article 5 has been carried out by 12 water authorities at county level. The delineation of groundwater bodies has been based on hydogeological mapping, but due to geological variations, different levels of mapping and different methods of identifying groundwater bodies, the size and shape of groundwater bodies varies so much that they cannot serve as the basis of the future monitoring, assessment of status and establishment of the programme of measures, according to the WFD.

As a result of the municipal reform taking place in Denmark on January 1st 2007, the water authority will, from this date, be the state, and the number of River Basin Districts (RBDs) will be reduced to four. Hence, the state is responsible for the elaboration of the monitoring programme according to the WFD which has to be established before December 22, 2006. The Danish Environmental Agency has therefore initiated a project with the goal of revising the groundwater bodies to conform to the needs. The project has been carried out in close collaboration with representatives of the water authorities.

Phase one of the project aims to analyse and describe the groundwater bodies designated and to establish three different models for revising them. This report describes the results of the second part of phase one; the set up of three different models for revising the groundwater bodies.

The total number of groundwater bodies in Denmark is more than 1900, but the number in each RBD varies from only three to more than 1400. Accordingly, the average size of the groundwater bodies varies by two decades. Is also worth noting that the size of groundwater bodies varies remarkably within each RBD, and only two RBDs have not designated groundwater bodies less than 4 km².

The definition of groundwater bodies was handled in different ways in each river basin district, and although some have used methods that at first glance appear to be similar, the results may be very different. A major part of the differences can be explained by geological variations, as some parts of the country has primary aquifers in regional limestone aquifers or tertiary sand, whereas other parts only have aquifers in alluvial sand embedded in till.

Most districts have divided aquifers into two or three levels; shallow, medium or deep groundwater bodies, but the limits used are different; some have used geological criteria, others e.g. depth below the groundwater table. Some groundwater bodies were defined in three-dimensional geolgical models where the layers were defined by geological facies; others used gridded pixel models. This often results in lack of continuity across the borders.

Three models for revision of the groundwater bodies were defined as follows:

Model 1 is founded on the present administration of groundwater as a source of drinking water. All hydrogeological knowledge is to be utilized, and new tasks due to the WFD are added.
Model 2 is a minimum model where only the new requirements of the WFD are taken into account. This model is more similar to the models used in England and Germany.
Model 3 combines the two other models, where model 1 is used in drinking water areas and model 2 outside these.

The models will all result in a smaller number of groundwater bodies and a more uniform distribution of groundwater bodies across the country. The total area of groundwater bodies will be equivalent to the area of the present in all models.

Model 1 results in a relatively large number of groundwater bodies. This model is closest to the present delineation of groundwater bodies, and it will entail only small changes in most districts. A few districts with very large groundwater bodies will, however, experience a large need for change, resulting in an extensive amount of work.

It is suggested that a geological model, established by the Danish Geological Survey as the fundament of a new groundwater monitoring model, is used as a framework for the new designation of groundwater bodies. The greatest advantage of this model is the possibility of coordinating all groundwater tasks, referring to the same key; the groundwater body.

Model 2 largely reduces the number of groundwater bodies, although the groundwater bodies are still smaller and the number bigger than in other countries. The scale for sub-division of river catchments will be decisive for the resulting number and size of groundwater bodies. This model is close to the model used in Southern Jutland, but it will cause great changes in all other districts.

The model described uses different elements of landscape as a key, but this key will not lead to sub-division of groundwater bodies in large parts of the country, and division only by catchments will be the result in these areas. Deep aquifers without dependent surface waters or wetlands will be defined as groundwater bodies without further sub-division.

Model 3 results in a very large number of groundwater bodies, but in contrast to the present designation, they are distributed more uniformly across the country. The great number of groundwater bodies comes from dividing the area using to different sets of criteria which rarely coincide.

The model is very complicated and raises a number of questions that will have to be answered, and it will cause extensive work in all districts.