In general tracer experiments made for the evaluation of waste water dilution in recipients have been of short-term type, with the tracer having an injection time that is markedly less than the time scale of flowpattern changing. In typical Finnish lakes and coastal waters, with complicated topography and frequently varying winds, the results from short term experiments have proved to be of rather limited value. In view of this, a study has been made of the possibility of measuring the dilution under such circumstances, by means of a long-term experiment with a tracer injection time extending over several weeks. Indium was considered to be the most suitable tracer for such an experiment; the complex form of indium is preferable to ionic solution to ensure reliability in the results.

The applicability of the method in the fields is illustrated by an example of a waste-water dilution study made for a pulp mill under construction. Two subsequent long-term experiments were conducted for measurement of the dilution of waste water from two alternative discharge points. The results obtained gave a very clear illustration of the local average dilution ratios, and allowed a reliable comparison being made between the two alternative discharge points.