Comparison of Conduit Volumes Obtained from Direct Measurements and Artificial Tracer Tests

Abstract

An isolated phreatic loop in a natural cave was used to test the reliability of artificial-tracer tests for estimating the volume of a flooded karst conduit. The volume of a phreatic tube was measured by filling a drained phreatic loop with a constant inflow over a known time period. The volume of the phreatic loop is 190 6 20 m 3 , and it was compared to independent calculations of conduit volumes based on values based on tracer breakthrough curves. The best results were for mean transit time, where tracer-test calculations yielded volumes very similar to the volume obtained by direct filling of the loop. On the other hand, using the first-arrival time or peak time in the volume calculation resulted in considerable underestimation of the phreatic tube's volume, and these methods should be avoided except when breakthrough curves are affected by molecular diffusion. This demonstrates that volume estimation by tracer tests may be quite precise for common natural conduits, but results are strongly affected by the breakthrough-curve parameter chosen by the experimenter.