CALCULATION OF DISPERSIVITY AND EFFECTIVE POROSITY FROM SALT TRACER TEST RESULTS IN A BASALT AQUIFER

Sodium chloride and potassium chloride solutions were injected into the flow tops and dense interiors of the Roza Basalt formation and specific conductance measured at various locations in down-gradient wells. All tracer tests were passive and run under natural ground-water gradient conditions. Results of four tracer tests, ranging in duration from 29 to 369 days in wells separated by between 9 and 89 meters, were used to calculate tracer travel times, effective porosity, and longitudinal and transverse dispersion coefficients and dispersivities.

Data loggers were used to measure water levels and specific conductance in the injection and down-gradient wells. A method, using the theory of de Josselin de Jong, is given to estimate field-scale dispersion coefficients and effective porosity using the results of passive tracer tests. The tracer-test and analysis method is applicable to anisotropic media, especially fractured rock formations where the direction of the longitudinal dispersion is usually not parallel to the direction of ground-water flow.

A single point injection of tracer in an up-gradient well is required, with at least one down-gradient observation well to measure the change in concentration of the injected tracer. The mean tracer travel time will provide information on effective porosity; peak tracer concentration, standard deviation, and mean travel time will provide information needed to compute dispersion coefficients.