Summary and conclusions

Undersøgelsesprogram for grundvandsforurening

In this report a concept is set up regarding the investigation of the extension of contaminant plumes beyond the site boundary. The concept is exemplified by contamination by MTBE and BTEX at gasoline stations. The statistical basis for the concept is comprised of data from an extensive survey carried out on 72 gasoline stations by the County of Fu nen.

The concept divides the contamination situations into two types of spills:

short larger spills
small spills of long duration,

since it has been shown that the intensity and the duration of the spill influences the length and width of the plume. In the report, directions are given on how to distinguish between the two types of spill, dependent on the measured contaminant concentration for instance.

A program is set up for a primary investigation, which has two goals. The first is to register if contamination is present or not, the other is to determine the main run of direction in the secondary groundwater. The type of spill is also estimated based on the primary investigation. A program is also described for the main investigation dependent of the type of spill. The aim of this investigation is to determine the presence of contamination beyond the site border. The statistical basis for the procedures are described in a set of annexes.

After the concept was first developed it was tested on 11 sites. The results of this test is reported in a separate report from the Danish Environmental Protection Agency. Based the experiences gained from the testing, minor adjustment were done to the concept and procedure. An overview of the concept is given in Table 3.3. In connection with the testing it has been pointed out that it is dependent on the existence of a relatively continuous secondary groundwater aquifer, if sufficiently reliable results are to be obtained. It should be noted that during the test, borings were almost exclusively carried out outside the site boundaries. This will increase the necessity of a continuous aquifer in order for the concept to be meaningful.

The report contains a chapter (chapter 4) about relevant investigation and analysis methods. Due to the testing period, the details in these descriptions has meanwhile been reported in other reports, and it has thus been decide not to update the chapter in this report, since it was first written.