Evaluation of longitudinal dispersivity from nonuniform flow tracer tests

Abstract

The importance of incorporating nonuniform flow effects in determining field-scale values of long-itudinal dispersivity from tracer tests in aquifers is illustrated. We show that under conditions of Fickian dispersive flux, where a tracer has traveled at least 10 times the longitudinal dispersivity value, nonuniform flow effects account for the typical tailing exhibited by data from radial flow and doublet (two-well) tracer tests. Approximate analytical solutions for solute transport in nonuniform flow fields based on earlier general work are presented. Specific cases evaluated include pulse and step inputs in diverging radial flow, a pulse input in converging radial flow, and doublet (two-well) tests with pulse and step inputs. The additional effect of longitudinal dispersivity increasing linearly with distance for radial flow cases is examined. Also presented is an analysis of the effect of injection borehole flushing represented by an exponentially decreasing tracer input in the converging radial flow case. The solutions are applied to four sets of field data where previous analyses did not adequately address nonuniform flow effects. Some data reanalyses yield longitudinal dispersivity values which are up to a factor of three smaller than those previously reported, indicating a previous overprediction of plume dilution. We demonstrate that an understanding of the sensitivity of the mathematical solutions to tracer input conditions, nonuniform flow effects, and scale can lead to better tracer-test design.