This paper presents the numerical analysis results of a field tracer test. The objective of this test was to measure the dispersivity and the effective porosity of a permeable sedimentary layer. The tracer test was performed using two vertical bore holes 90 meters in depth and located 7 meters apart. The sedimentary layer, composed of sandstone, conglomerate, and weathered granite, was 10 meters thick and started 80 meters below the surface. One bore hole was used to inject groundwater with tracer and the other to recover to recirculate groundwater. We used a tracer solution containing Br. Before the tracer was injected, groundwater was circulated through the sedimentary layer between the two bore holes to establish a steady state of groundwater flow. The circulation flow rate and Br concentration initially injected were determined by using Gelhar's analytical solution so that the detectable tracer concentration (several ppm) could be recovered within one month. When the groundwater level at the recovery hole became stable, the tracer was injected into the injection hole and the concentration of Br was analyzed at the recovery hole. The circulation flow rate was 7800cm³/ min. The initially injected Br- tracer concentration was 8300 ppm. Thirty eight hours later, the concentration of Br at the recovery hole reached a maximum level of 10 ppm.
The convection-dispersion equation was used for the tracer test's numerical analysis. This differential equation was transformed by F. E. M. Half of the horizontal section of the analytical region was used for numerical analysis and meshes like a semi-circle were used for the circulation-line above the surface between the two bore holes. A dispersion coefficient depends upon flow velocity, dispersivity, and diffusion. Initially, the flow velocity distribution and the permeability of layer were analyzed using F. E. M. as a seepage problem. The analyzed permeability of the layer was 2.2×10-5cm/sec.
According to the F. E. M. analysis result, using the breakthrough curve of Br at the recovery hole and the matching technique, the effective porosity was determined to be 7% and the dispersivity 3.5 meters. The effective porosity was determined to be 4.5% and the dispersivity 3.5 meters by Gelhar's analytical solution. The 7% or 4.5% effective porosity value was smaller than the 30% porosity value with the boring core samples. This suggests that the measurement of effective porosity using a field tracer test is important in evaluating groundwater flow velocity and mass transport. The relationship between the dispersivity 3.5 meters and the distance 7 meters agreed well with those of Leonhart (1985).
