EVALUATION OF LONGITUDINAL DISPERSIVITY FROM NONUNIFORM FLOW TRACER TESTS

The importance of incorporating nonuniform flow effects in determining field-scale values of longitudinal dispersivity from tracer tests in aquifers is illustrated. We show that under conditions of Fickian disp ersive flux, where a tracer has traveled at least 10 times the longitu dinal dispersivity value, nonuniform flow effects account for the typi cal 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. S pecific cases evaluated include pulse and step inputs in diverging rad ial flow, a pulse input in converging radial flow, and doublet (two-we ll) tests with pulse and step inputs. The additional effect of longitu dinal dispersivity increasing linearly with distance for radial flow c ases is examined. Also presented is an analysis of the effect of injec tion borehole flushing represented by an exponentially decreasing trac er 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 longitudi nal dispersivity values which are up to a factor of three smaller than those previously reported, indicating a previous overprediction of pl ume dilution. We demonstrate that an understanding of the sensitivity of the mathematical solutions to tracer input conditions, nonuniform f low effects, and scale can lead to better tracer-test design.