Artificial tracer experiments are generally considered to be the most direct methods available for evaluating ground-water flow and solute transport parameters. Experimental data generated from tracer tests can serve as input to solute transport models that reconstruct or predict ground-water flow system behavior or response. Such experiments have been carried out within the Columbia River basalts in south-central Washington State. The results of these experiments, while successful in terms of obtaining numerical estimates of the targeted parameters (effective thickness and dispersivity), also provided information on the general mechanics of basalt aquifers. Initial experiments were performed within a basalt flow top, approximately 3,450 ft (1,050 m) below land surface, using paired boreholes separated by about 55 ft (16.7 m) at depth. Transmissivity of the flow top was estimated to be 0.7 ft 2 /day (0.07 m 2 /day), and storativity was about 3 × 10 −5 . The tracer testing provided estimates of longitudinal dispersivity in the range of 1.5 to 2.8 ft (0.46 to 0.84 m) and effective thicknesses between 0.006 and 0.01 ft (0.002 and 0.003 m). Testing procedures involved closed-loop recirculation of the tracer while both monitoring in real time and sampling over discrete time intervals. Corroborative results were obtained from two individual experiments using iodine-131, and potassium thiocyanate (KSCN) tracers, respectively. Data reduction and interpretation involved type-curve matching. The type curves were specially developed to account for either single- or distributed-pulse inputs of the tracer into the formation. This aspect provided a means of evaluating the degree of tracer dispersion that can occur during transport from the surface to the test interval. Analysis of cation species collected during the KSCN tracer test also provided important information about tracer dynamics within the ground-water system.