THE GROUNDWATER GROUP  Grundvandsgruppen    Det Strategiske Miljøforskningsprogram  The Danish Environmental Research Program

Glacial till with fractures, Gedser Odde, Island of Falster, Denmark
 
The Groundwater Group is a centre funded from the Danish Environmental Research Programme. During the period from 1 July 1996 to 31 December 1999, the Centre's principal activity will be concerned with the subprogramme "Pesticides and Groundwater".

In Denmark, the use of pesticides to control plant diseases and weeds started shortly after 1950 and consumption grew rapidly until the mid-1980s. The total quantity of pesticides used in the years after 1970 was approximately four times as large as the quantity used in the preceding period. At present, approximately 440 different types of pesticides are known to have been applied or are still being used. In addition, a substantial number of degradation products (metabolites) have
been formed.

Until the first signs of pesticide contamination of groundwater emerged in the early 1990s, the general opinion was that the pesticides would remain in the upper strata (the root zone) long enough to be completely degraded by the existing microorganisms into naturally occurring chemical compounds.

The first pesticides found in groundwater were ascribed to point sources, e.g. the thoughtless cleaning of crop sprayers near well covers which were not tight, pesticide packaging which had been buried in the ground, etc. However, it soon became apparent that the finds could not be regarded as isolated phenomena. A rapidly increasing number of finds proved that the theory of complete degradation of pesticides in the root zone was incorrect.
 
Up to the end of 1996, Danish groundwater samples had been analysed for approximately 50 different pesticides and metabolites. At least 24 of the targetted substances have been definitely identified and there is every probability that the number will increase as the analysis programme is extended.
 
A substantial part of the cap rock over Danish aquifers consists of clay-dominated glacial deposits, mainly clayey till. Until quite recently, it was taken for granted that the clayey strata would act as an effective barrier eliminating the risk of groundwater contamination. However, it is clear that in many places, seepage through the root zone takes place at a much faster rate than previously assumed due to this barrier being broken by fractures which are often found in the upper 5-8 metres of clayey till. Sites with a potential for increased seepage and risk of contamination are also found in areas where the cap rock over the aquifers is dominated by sandy deposits.
 
The most recent results of the attempts made to date groundwater contaminated by pesticides suggest that the pesticides detected up to the present are from a period when the quantities and the number of products used were still relatively small. Therefore, it is to be expected that the amount of pesticides recorded in Danish groundwater will increase, not only because of the extension of the analysis programme but also because of a delayed effect. The delay may be due to deeper-seated cap rock where, however, conditions for pesticide-degrading processes are likely to be poorer than in the root zone.