THE USE OF VELOCITY AND CONDUCTIVITY DATA FOR THE QUANTIFICATION OF HETEROGENEITY - A COMPARISON

Hydraulic conductivities of a portion of the Twin Lake aquifer are der ived from grain size analyses performed at 3-cm vertical sampling inte rvals in four boreholes, spaced 1 m apart. Correlation of conductivity logs between four boreholes leads to a geometrical model which corres ponds closely to the spatial structure derived from the velocity field of previous tracer tests. Histogram and correlogram analyses of condu ctivities agree closely with those of velocities, and the two data set s appear equally useful for characterizing the aquifer and its spatial variability. Using Darcy's law, flow velocities and conductivities ar e combined to calculate a theoretical, hydraulic gradient. Comparison to gradients observed at the site reveals agreement on a large scale. However, differences exist in detail because observed piezometric head s fail to reflect small-scale heterogeneity. Since these differences a re significant for contaminant transport prediction, it is recommended for future studies to directly measure flow velocities from a tracer experiment, rather than to reply on head measurements and on Darcy's l aw for the calculation of transport parameters.