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To apply the point‐dilution method of measuring the rate of ground water flow by tracers, the distortion of the flow field by the well must be known. The distortion can be calculated with sufficient accuracy if the permeabilities of the aquifer, k 3 , of the gravel pack, k 2 , and of the well screen, k 1 , fulfill the condition k 3 ≤ k 2 ≤ k 1 . The tracer must be adequately mixed during the dilution procedure. Experiences obtained from model tests are applied in investigations in the field. By using peak‐to‐peak and total count methods in wells where vertical flow occurred, we could determine the depths and the rates of inflow and outflow, which relate to seepage under dams and water exploration. To determine the direction of ground water flow, a radio tracer was injected into a well‐screen section and, after having left the well‐screen, was detected by a collimated detector. The use of tracers that are fixed on the soil is advisable; 198 Au, having been homogeneously distributed within the dilution volume initially, has proved most suitable. Strong vertical currents in the well represent a serious obstacle in field measurements of the rate and of the direction of ground water flow.
Drost et al. (Thu,) studied this question.